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Acid Rain

CHAPTER 5
ACID RAIN

WHAT IS ACID RAIN?

Acid rain is the common name for acidic deposits that fall to Earth from the atmosphere. The term was coined in 1872 by the Scottish chemist Robert Angus Smith (18171884) to describe the acidic precipitation in Manchester, England. In the twenty-first century scientists study both wet and dry acidic deposits. Even though there are natural sources of acid in the atmosphere, acid rain is primarily caused by emissions of sulfur dioxide (SO2) and nitrous oxide (N2 O) from electric utilities burning fossil fuels, especially coal. These chemicals are converted to sulfuric acid and nitric acid in the atmosphere and can be carried by the winds for many miles from where the original emissions took place. (See Figure 5.1.) Other chemicals contributing to acid rain include volatile organic compounds (VOCs). These are carbon-containing chemicals that easily become vapors or gases. VOC sources include paint thinners, degreasers, and other solvents and burning fuels such as coal, natural gas, gasoline, and wood.

Wet deposition occurs when the acid falls in rain, snow, or ice. Dry deposition is caused by tiny particles (or particulates) in combustion emissions. They may stay dry as they fall or pollute cloud water and precipitation. Moist deposition occurs when the acid is trapped in cloud or fog droplets. This is most common at high altitudes and in coastal areas. Whatever its form, acid rain can create dangerously high levels of acidic impurities in water, soil, and plants.

Measuring Acid Rain

The acidity of any solution is measured on a potential hydrogen (pH) scale numbered from zero to fourteen, with a pH value of seven considered neutral. (See Figure 5.2.) Values higher than seven are considered more alkaline or basic (the pH of baking soda is eight); values lower than seven are considered acidic (the pH of lemon juice is two). The pH scale is a logarithmic measure. This means that every pH change of one is a tenfold change in acid content. Therefore, a decrease from pH seven to pH six is a tenfold increase in acidity; a drop from pH seven to pH five is a one hundredfold increase in acidity; and a drop from pH seven to pH four is a one thousandfold increase.

Pure, distilled water has a neutral pH of seven. Normal rainfall has a pH value of about 5.6. It is slightly acidic because it accumulates naturally occurring sulfur oxides (SOx) and nitrogen oxides (NOx) as it passes through the atmosphere. Acid rain has a pH of less than 5.6.

Figure 5.3 shows the average rainfall pH measured during 2005 at various locations around the country by the National Atmospheric Deposition Program (NADP), a cooperative project between many state and federal government agencies and private entities. Rainfall was most acidic in the mid-Atlantic region and upper Southeast, particularly Ohio, Pennsylvania, West Virginia, Maryland, Delaware, Virginia, eastern Tennessee, and Kentucky. The areas with the lowest rainfall pH contain some of the country's most sensitive natural resources, such as the Appalachian Mountains, the Adirondack Mountains, Chesapeake Bay, and Great Smoky Mountains National Park. Overall, precipitation is much more acidic in the eastern United States than in the western United States because of a variety of natural and anthropogenic (human-caused) factors that are discussed below.

SOURCES OF SULFATE AND NITRATE IN THE ATMOSPHERE

Natural Sources

Natural sources of sulfate in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide, which is released from the decomposition of organic matter found in the Earth. Natural sources of nitrogen or nitrates include NOx produced by micro-organisms in soils, by lightning during thunderstorms,

FIGURE 5.1

and by forest fires. Scientists generally speculate that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources. (This is a rough estimate as there is no way to measure natural emissions as opposed to those that are manmade.)

Sources Caused by Human Activity

According to the U.S. Environmental Protection Agency (EPA), in "What Is Acid Rain?" (June 8, 2007, http://www.epa.gov/acidrain/what/index.html), the primary anthropogenic contributors to acid rain are SO2 and NOx, resulting from the burning of fossil fuels, such as coal, oil, and natural gas.

The EPA notes in "Clearinghouse for Inventories and Emissions Factors" (http://www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls) that approximately 70% of SO2 emissions produced in 2006 were because of fuel combustion by fossil-fueled electric utilities. Fuel combustion at industrial facilities contributed another 13%. Lesser sources included transportation vehicles and industrial processes. Highway vehicles were the primary source of NOx emissions, accounting for 36% of the total in 2006. Off-highway vehicles (such as bulldozers) contributed 22%. Fuel combustion in power plants was another major source,

FIGURE 5.2

accounting for 20% of the total. Lesser sources included industrial processes and waste disposal and recycling

NATURAL FACTORS THAT AFFECT ACID RAIN DEPOSITION

Major natural factors contributing to the impact of acid rain on an area include air movement, climate, and topography and geology. Transport systemsprimarily the movement of airdistribute acid emissions in definite patterns around the planet. The movement of air masses transports emitted pollutants many miles, during which the pollutants are transformed into sulfuric and nitric acid by mixing with clouds of water vapor.

FIGURE 5.3

In drier climates, such as those of the western United States, windblown alkaline dust moves more freely through the air and tends to neutralize atmospheric acidity. The effects of acid rain can be greatly reduced by the presence of basic (also called alkali) substances. Sodium, potassium, and calcium are examples of basic chemicals. When a basic and an acid chemical come into contact, they react chemically and neutralize each other. By contrast, in more humid climates where there is less dust, such as along the eastern seaboard, precipitation is more acidic.

Areas most sensitive to acid rain contain hard, crystalline bedrock and thin surface soils. When no alkaline-buffering particles are in the soil, runoff from rainfall directly affects surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity, such as flat land, neutralizes acid rain better. Lakes tend to be most susceptible to acid rain because of low alkaline content in lake beds. A lake's depth, its watershed (the area draining into the lake), and the amount of time the water has been in the lake are also factors.

EFFECTS OF ACID RAIN ON THE ENVIRONMENT

In nature the combination of rain and oxides is part of a natural balance that nourishes plants and aquatic life. However, when the balance is upset by acid rain, the results to the environment can be harmful and destructive. (See Table 5.1.)

Aquatic Systems

Even though pH levels vary considerably from one body of water to another, a typical pH range for the lakes and rivers in the United States is six to eight. Low pH levels kill fish, their eggs, and fish food organisms. The

TABLE 5.1

Effects of acid rain on human health and selected ecosystems and anticipated recovery benefits
Human health and ecosystem Effects Recovery benefits
SOURCE: "Appendix I. Effect of Acid Rain on Human Health and Selected Ecosystems and Anticipated Recovery Benefits," in Acid Rain: Emissions Trends and Effects in the Eastern United States, U.S. General Accounting Office, March 2000, http://www.gao.gov/archive/2000/rc00047.pdf (accessed July 27, 2007)
Human health In the atmosphere, sulfur dioxide and nitrogen oxides become sulfate and nitrate aerosols, which increase morbidity and mortality from lung disorders, such as asthma and bronchitis, and impacts to the cardiovascular system. Decrease emergency room visits, hospital admissions, and deaths.
Surface waters Acidic surface waters decrease the survivability of animal life in lakes and streams and in the more severe instances eliminate some or all types of fish and other organisms. Reduce the acidic levels of surface waters and restore animal life to the more severely damaged lakes and streams.
Forests Acid deposition contributes to forest degradation by impairing trees' growth and increasing their susceptibility to winter injury, insect infestation, and drought. It also causes leaching and depletion of natural nutrients in forest soil. Reduce stress on trees, thereby reducing the effects of winter injury, insect infestation, and drought, and reduce the leaching of soil nutrients, thereby improving overall forest health.
Materials Acid deposition contributes to the corrosion and deterioration of buildings, cultural objects, and cars, which decreases their value and increases costs of correcting and repairing damage. Reduce the damage to buildings, cultural objects, and cars, and reduce the costs of correcting and repairing future damage.
Visibility In the atmosphere, sulfur dioxide and nitrogen oxides form sulfate and nitrate particles, which impair visibility and affect the enjoyment of national parks and other scenic views. Extend the distance and increase the clarity at which scenery can be viewed, thus reducing limited and hazy scenes and increasing the enjoyment of national parks and other vistas.

degree of damage depends on several factors, one of which is the buffering capacity of the watershed soilthe higher the alkalinity, the more slowly the lakes and streams acidify. The exposure of fish to acidified freshwater lakes and streams has been intensely studied since the 1970s. Scientists distinguish between sudden shocks and chronic (long-term) exposure to low pH levels.

Sudden, short-term shifts in pH levels result from snowmelts, which release acidic materials accumulated during the winter, or sudden rainstorms that can wash residual acid into streams and lakes. The resulting acid shock can be devastating to fish and their ecosystems. At pH levels below 4.9, fish eggs are damaged. At acid levels below 4.5, some species of fish die. Below pH 3.5, most fish die within hours. (See Table 5.2.)

TABLE 5.2

Generalized short-term effects of acidity on fish
pH range Effect
SOURCE: "Generalized Short-Term Effects of Acidity on Fish," in National Water Quality Inventory: 1998 Report to Congress, U.S. Environmental Protection Agency, June 2000
6.59 No effect
6.06.4 Unlikely to be harmful except when carbon dioxide levels are very high (1,000 mg l1)
5.05.9 Not especially harmful except when carbon dioxide levels are high (20 mg I1) or ferric ions are present
4.54.9 Harmful to the eggs of salmon and trout species (salmonids) and to adult fish when levels of Ca2, Na+and Clare low
4.04.4 Harmful to adult fish of many types which have not been progressively acclimated to low pH
3.53.9 Lethal to salmonids, although acclimated roach can survive for longer
3.03.4 Most fish are killed within hours at these levels

Because many species of fish hatch in the spring, even mild increases in acidity can harm or kill the new life. Temporary increases in acidity also affect insects and other invertebrates, such as snails and crayfish, on which the fish feed.

Gradual decreases of pH levels over time affect fish reproduction and spawning. Moderate levels of acidity in water can confuse a salmon's sense of smell, which it uses to find the stream from which it came. Atlantic salmon are unable to find their home streams and rivers because of acid rain. In addition, excessive acid levels in female fish cause low amounts of calcium, thereby preventing the production of eggs. Even if eggs are produced, their development is often abnormal.

Increased acidity can also cause the release of aluminum and manganese particles that are stored in a lake or river bottom. High concentrations of these metals are toxic to fish.

Soil and Vegetation

Acid rain is believed to harm vegetation by changing soil chemistry. Soils exposed to acid rain can gradually lose valuable nutrients, such as calcium, magnesium, and potassium and become too concentrated with dissolved inorganic aluminum, which is toxic to vegetation. Long-term changes in soil chemistry may have already affected sensitive soils, particularly in forests. Forest soils saturated in nitrogen cannot retain other nutrients required for healthy vegetation. Subsequently, these nutrients are washed away. Nutrient-poor trees are more vulnerable to climatic extremes, pest invasion, and the effects of other air pollutants, such as ozone.

FIGURE 5.4

Some researchers believe that acid rain disrupts soil regeneration, which is the recycling of chemical and mineral nutrients through plants and animals back to the Earth. They also believe acids suppress decay of organic matter, a natural process needed to enrich the soils. Valuable nutrients such as calcium and magnesium are normally bound to soil particles and are, therefore, protected from being rapidly washed into groundwater. Acid rain, however, may accelerate the process of breaking these bonds to rob the soil of these nutrients. This, in turn, decreases plant uptake of vital nutrients. (See Figure 5.4.)

Acid deposition can cause leafy plants such as lettuce to hold increased amounts of potentially toxic substances such as the mineral cadmium. Research also finds a decrease in carbohydrate production in the photosynthesis process of some plants exposed to acid conditions. Research is under way to determine whether acid rain could ultimately lead to a permanent reduction in tree growth, food crop production, and soil quality. Effects on soils, forests, and crops are difficult to measure because of the many species of plants and animals, the slow rate at which ecological changes occur, and the complex interrelationships between plants and their environment.

trees

trees. The effect of acid rain on trees is influenced by many factors. Some trees adapt to environmental stress better than others; the type of tree, its height, and its leaf structure (deciduous or evergreen) influence how well it will adapt to acid rain. Scientists believe that acid rain directly harms trees by leaching calcium from their foliage and indirectly harms them by lowering their tolerance to other stresses.

According to the EPA, acid rain has also been implicated in impairing the winter hardening process of some trees, making them more susceptible to cold-weather damage. In some trees the roots are prone to damage because the movement of acidic rain through the soil releases aluminum ions, which are toxic to plants.

One area in which acid rain has been linked to direct effects on trees is from moist deposition via acidic fogs and clouds. The concentrations of acid and SOx in fog droplets are much greater than in rainfall. In areas of frequent fog, such as London, significant damage has occurred to trees and other vegetation because the fog condenses directly on the leaves.

Birds

Increased freshwater acidity harms some species of migratory birds. Experts believe the dramatic decline of the North American black duck population since the 1950s is because of decreased food supplies in acidified wetlands. Acid rain leaches calcium out of the soil and robs snails of the calcium they need to form shells. Because titmice and other species of songbirds get most of their calcium from the shells of snails, the birds are also perishing. The eggs they lay are defectivethin and fragile. The chicks either do not hatch or have bone malformations and die.

In "Adverse Effects of Acid Rain on the Distribution of the Wood Thrush Hylocichla mustelina in North America" (Proceedings of the National Academy of Sciences, August 12, 2002), Ralph S. Hames et al. discuss the results of their large-scale study, which shows a clear link between acid rain and widespread population declines in the wood thrush, a type of songbird. Hames and his colleagues believe that calcium depletion has had a negative impact on this bird's food source, mainly snails, earthworms, and centipedes. The bird may also be ingesting high levels of metals that are more likely to leach out of overly acidic soils. Declining wood thrush populations are most pronounced in the higher elevations of the Adirondack, Great Smoky, and Appalachian mountains. Hames and his cohorts warn that acid rain may also be contributing to population declines in other songbird species.

Materials

Acid rain can also be harmful to materials, such as building stones, marble statues, metals, and paints. Elaine McGee of the U.S. Geological Service reports in Acid Rain and Our Nation's Capital (1997, http://pubs.usgs.gov/gip/acidrain/contents.html) that limestone and marble are particularly vulnerable to acid rain. Historical monuments and buildings composed of these materials in the eastern United States have been hit hard by acid rain.

Human Health

Acid rain has several direct and indirect effects on humans. Particulates are extremely small pollutant particles that can threaten human health. Particulates related to acid rain include fine particles of SOx and nitrates. These particles can travel long distances and, when inhaled, penetrate deep into the lungs. Acid rain and the pollutants that cause it can lead to the development of bronchitis and asthma in children. Acid rain is also believed to be responsible for increasing health risks for those over the age of sixty-five; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

THE POLITICS OF ACID RAIN

Scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation in North America was not identified until 1972, when scientists found that precipitation was acidic in eastern North America, especially in northeastern and eastern Canada. In 1975 the First International Symposium on Acid Precipitation and the Forest Ecosystem convened in Columbus, Ohio, to define the acid rain problem. Scientists used the meeting to propose a precipitation-monitoring network in the United States that would cooperate with the European and Scandinavian networks and to set up protocols for collecting and testing precipitation.

In 1977 the Council on Environmental Quality was asked to develop a national acid rain research program. Several scientists drafted a report that eventually became the basis for the National Acid Precipitation Assessment Program (NAPAP). This initiative eventually translated into legislative action with the Energy Security Act of 1980. Title VII (Acid Precipitation Act of 1980) of the act produced a formal proposal that created NAPAP and authorized federally financed support.

The first international treaty aimed at limiting air pollution was the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Trans-boundary Air Pollution, which went into effect in 1983. It was ratified by thirty-eight of the fifty-four UNECE members, which included not only European countries but also Canada and the United States. The treaty targeted sulfur emissions, requiring that countries reduce emissions 30% from 1980 levelsthe so-called Thirty Percent Club.

The early acid rain debate centered almost exclusively on the eastern United States and Canada. The controversy was often defined as a problem of property rights. The highly valued production of electricity in coal-fired utilities in the Ohio River Valley caused acid rain to fall on land in the Northeast and Canada. An important part of the acid rain controversy in the 1980s was the adversarial relationship between U.S. and Canadian government officials over emission controls of SO2 and NO2. More of these pollutants crossed the border into Canada than the reverse. Canadian officials very quickly came to a consensus over the need for more stringent controls, whereas this consensus was lacking in the United States.

Throughout the 1980s the major lawsuits involving acid rain all came from eastern states, and the states that passed their own acid rain legislation were those in the eastern part of the United States.

Legislative attempts to restrict emissions of pollutants were often defeated after strong lobbying by the coal industry and utility companies. These industries advocated further research for pollution-control technology rather than placing restrictions on utility company emissions.

The NAPAP Controversy

In 1980 Congress established NAPAP to study the causes and effects of acid deposition and recommend policy approaches for controlling acid rain effects. About two thousand scientists worked on this unique inter-agency program, which ultimately cost more than $500 million. Even though its first report was due in 1985, the program was plagued by problems that resulted in numerous delays. In 1985 the first executive director, Christopher Bernabo, resigned and was replaced by Lawrence Kulp. In 1987 the study group released to Congress Interim Assessment: The Causes and Effects of Acidic Deposition, a massive four-volume preliminary report that caused a storm of controversy. The report contained detailed scientific information in its technical chapters about acid rain. The executive summary, written by Kulp, was released to the public and widely criticized for mis-representing the scientific findings of the report and downplaying the negative effects of acid rain. Philip Shabecoff notes in "Acid Rain Report Unleashes a Torrent of Controversy" (New York Times, March 20, 1990) that critics claimed Kulp had slanted the summary to match the political agenda of the administration of President Ronald Reagan (19112004), which advocated minimum regulation of business and industry.

Some of the scientific findings in the 1987 report included:

  • Acid rain had adversely affected aquatic life in about 10% of eastern lakes and streams.
  • Acid rain had contributed to the decline of red spruce at high elevations by reducing this species' cold tolerance.
  • Acid rain had contributed to erosion and corrosion of buildings and materials.
  • Acid rain and related pollutants had reduced visibility throughout the Northeast and in parts of the West.

The report concluded, however, that the incidence of serious acidification was more limited than originally feared. At that time the Adirondacks area of New York was the only region showing widespread, significant damage from acid. Furthermore, results indicated that electricity-generating power plants were responsible for two-thirds of SO2 emissions and one-third of NOx emissions.

Controversy over Kulp's role led to him being replaced by James Mulhoney. The new director ordered reassessments and revisions of the interim report. This was completed in 1991. However, by that time President George H. W. Bush (1924) was in power, and he had made acid rain legislation a component of his election campaign. As a result, political forces, rather than NAPAP, largely drove the nation's emerging policy toward acid rain.

THE ACID RAIN PROGRAMCLEAN AIR ACT AMENDMENTS, TITLE IV

Congress created the Acid Rain Program under Title IV (Acid Deposition Control) of the 1990 Clean Air Act Amendments. The goal of the program is to reduce annual emissions of SO2 and NOx from electric power plants nationwide. The program set a permanent cap on the total amount of SO2 that could be emitted by these power plants. According to the EPA, in Acid Rain Program: 2005 Progress Report (October 2006, http://www.epa.gov/airmarkets/progress/docs/2005report.pdf), this cap was set at 8.9 million tons (approximately half the number of tons of SO2 emitted by these plants during 1980). The program also established NOx emissions limitations for certain coal-fired electric utility plants. The objective of these limitations was to achieve and maintain a two-million-ton reduction in NOx emission levels by 2000 compared with the emissions that would have occurred in 2000 if the limitations had not been implemented.

In the 1999 Compliance Report: Acid Rain Program (July 2000, http://www.epa.gov/airmarkets/progress/docs/1999compreport.pdf), the EPA indicates that the reduction was implemented in two phases. Phase I began in 1995 and covered 263 units at 110 utility plants in 21 states with the highest levels of emissions. Most of these units were at coal-burning plants located in eastern and midwestern states. They were mandated to reduce their annual SO2 emissions by 3.5 million tons. An additional 182 units joined Phase I voluntarily, bringing the total of Phase I units to 445.

Phase II began in 2000. It tightened annual emission limits on the Phase I group and set new limits for more than two thousand cleaner and smaller units in all forty-eight contiguous states and the District of Columbia.

A New Flexibility in Meeting Regulations

Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amount, and what technology to use. Title IV, however, gave utilities flexibility in choosing how to achieve these reductions. For example, utilities could reduce emissions by switching to low-sulfur coal, installing pollution-control devices called scrubbers, or shutting down plants.

Utilities took advantage of their flexibility under Title IV to choose less costly ways to reduce emissionsmany switching from high- to low-sulfur coaland as a result, they have been achieving sizable reductions in their SO2 emissions.

Allowance Trading

Title IV also allows electric utilities to trade allowances to emit SO2. Utilities that reduce their emissions below the required levels can sell their extra allowances to other utilities to help them meet their requirements.

Title IV allows companies to buy, sell, trade, and bank pollution rights. Utility units are allocated allowances based on their historic fuel consumption and a specific emissions rate. Each allowance permits a unit to emit one ton of SO2 during or after a specific year. For each ton of SO2 discharged in a given year, one allowance is retired and can no longer be used. Companies that pollute less than the set standards will have allowances left over. They can then sell the difference to companies that pollute more than they are allowed, bringing them into compliance with overall standards. Companies that clean up their pollution would recover some of their costs by selling their pollution rights to other companies.

The EPA holds an allowance auction each year. The sale offers allowances at a fixed price. This use of market-based incentives under Title IV is regarded by many as a major new method for controlling pollution.

From 1995 to 1998 there was considerable buying and selling of allowances among utilities. Because the utilities that participated in Phase I reduced their sulfur emissions more than the minimum required, they did not use as many allowances as they were allocated for the first four years of the program. Those unused allowances could be used to offset SO2 emissions in future years. In Acid Rain: Emissions Trends and Effects in the Eastern United States (March 2000, http://www.gao.gov/archive/2000/rc00047.pdf), the U.S. General Accounting Office (now the U.S. Government Accountability Office) notes that from 1995 to 1998 a total of 30.2 million allowances were allocated to utilities nationwide; almost 8.7 million,

FIGURE 5.5

or 29%, of the allowances were not used but were carried over (banked) for subsequent years.

Figure 5.5 shows the status of the allowance bank from 1995 through 2005. Banked allowances increased dramatically in 2000 due to the addition of the Phase II sources to the Acid Rain Program. Over the next five years the allowance bank steadily decreased in size. The EPA reports in Acid Rain Program: 2005 Progress Report that in 2005 a total of 9.5 million allowances were allocated. Another 6.9 million banked allowances were carried over from previous years. The EPA expects that the allowance bank will eventually be depleted as SO2 emissions are further restricted by the implementation of the Clean Air Interstate Rule.

PERFORMANCE RESULTS OF THE ACID RAIN PROGRAM

There are three quantitative measures that environmental regulators use to gauge the performance of the Acid Rain Program: emissions, atmospheric concentrations, and deposition amounts.

U.S. Progress Report

The following information comes from the EPA's Acid Rain Program: 2005 Progress Report.

sources and emissions

sources and emissions. The report notes that in 2005 there were 3,456 electric generating units subject to the SO2 provisions of the Acid Rain Program. Most emissions were associated with approximately 1,100 coal-fired units making up the total. In all, program sources emitted 10.2 million tons of SO2 into the air. (See Figure 5.6.) The EPA expects that the 8.9-million-ton annual cap on emissions will be achieved by 2010. SO2 emissions from sources covered by the program decreased by 41% between 1980 and 2005.

In 2005 the NOx portion of the Acid Rain Program applied to a subset of the 3,456 units mentioned earlier, specifically 982 operating coal-fired units generating at least 25 megawatts. Between 1990 and 2005 NOx emissions from power plants subject to the Acid Rain Program decreased from 5.5 million tons per year to 3.3 million tons per year. (See Figure 5.7.)

According to the report, in 2000 the program first achieved its goal of reducing emissions by at least 2 million tons; 8.1 million tons were originally predicted in 1990 to be emitted in 2000 without the program in place.

The report indicates that the SO2 and NOx emission reductions were achieved even though the amount of fuel used to produce electricity in the United States increased by more than 30% between 1990 and 2005. Coal was the

FIGURE 5.6

FIGURE 5.7

single-largest fuel source for U.S. electric generating plants in 2005, accounting for 50% of the total.

atmospheric concentrations and deposition amounts

atmospheric concentrations and deposition amounts. The EPA's Acid Rain Program uses two complementary monitoring networks to track trends in regional air quality and acid deposition: the Clean Air Status and Trends Network and the NADP's National Trends Network. Additional monitoring data are provided by national, state, and local ambient monitoring systems.

As shown in Figure 2.14 and Figure 2.6 in Chapter 2, atmospheric levels of SO2 and NO2 averaged nationwide since 1990 have been well below the national standards for these pollutants.

Table 5.3 shows trends in atmospheric concentrations and deposition for four key regions in the Acid Rain Program: mid-Atlantic, Midwest, Northeast, and Southeast. Overall, concentrations of ambient SO2 and wet sulfates averaged over the period 200305 declined in all these regions, compared with the period 198991. The most dramatic differences are evident in the Northeast, where ambient SO2 concentrations decreased by more than 50%. The results for nitrogen and nitrate compound concentrations are mixed, with decreases in some areas and increases in others. The same is true for wet inorganic nitrogen deposition, which decreased in the mid-Atlantic, Midwest, and Northeast, but increased slightly in the Southeast.

Canadian Progress Report

In November 2006 Environment Canada released a report on progress made by Canada and the United States on cross-border air pollution. The study, CanadaUnited States Air Quality Agreement: 2006 Progress Report (http://www.ec.gc.ca/cleanair-airpur/caol/canus/report/2006canus/toc_e.cfm), is the eighth biennial report related to the 1991 agreement between the two countries. The report states that Canada has been successful at reducing SO2 emissions below its national cap. Canada's total SO2 emissions were 2.3 million tonnes (metric tons) in 2004, which is 28% below the national cap of 3.2 million tonnes. However, Environment Canada notes that the reductions have not been sufficient to reduce acid deposition below the levels needed to ensure the recovery of ecosystems damaged by excess acidity in its eastern provinces.

ARE ECOSYSTEMS RECOVERING?

Monitoring data clearly indicate decreased emissions and atmospheric concentrations of SO2 and NOx and some reductions in deposition amounts. These improvements have not necessarily resulted in recovery of sensitive aquatic and terrestrial ecosystems. This is due, in part, to the long recovery times required to reverse damage done by acidification. The EPA reports that ecosystems

TABLE 5.3

Regional changes in air quality and deposition of sulfur and nitrogen, 198991 and 200305
Average
Measurement Unit Region 19891991 20032005 Percent change*
SOURCE: "Table 4. Regional Changes in Air Quality and Deposition of Sulfur and Nitrogen, 19891991 Versus 20032005," in Acid Rain Program: 2005 Progress Report, U.S. Environmental Protection Agency, October 2006, http://www.epa.gov/airmarkets/progress/docs/2005report.pdf (accessed June 19, 2007)
*Percent change is estimated from raw measurement data, not rounded; some of the measurement data used to calculate percentages may be at or below detection limits.
Notes: kg=kilogram. ha=hectare. mg=milligram. L=liter. μg=microgram. m3=cubic meter.
Wet sulfate deposition kg/ha Mid-Atlantic 27 20 24
Wet sulfate concentration mg/L Mid-Atlantic 2.4 1.6 33
Midwest 2.3 1.6 30
Northeast 1.9 1.1 40
Southeast 1.3 1.1 21
Ambient sulfur dioxide concentration μg/m3 Mid-Atlantic 13 8.4 34
Midwest 10 5.8 44
Northeast 6.8 3.1 54
Southeast 5.2 3.4 35
Ambient sulfate concentration μg/m3 Mid-Atlantic 6.4 4.5 30
Midwest 5.6 3.8 33
Northeast 3.9 2.5 36
Southeast 5.4 4.1 24
Wet inorganic nitrogen deposition kg/ha Mid-Atlantic 5.9 5.5 8
Midwest 6.0 5.5 8
Northeast 5.3 4.1 23
Southeast 4.3 4.4 +2
Wet nitrate concentration mg/L Mid-Atlantic 1.5 1.0 29
Midwest 1.4 1.2 14
Northeast 1.3 0.9 33
Southeast 0.8 0.7 9
Ambient nitrate concentration μg/m3 Mid-Atlantic 0.9 1.0 +5
Midwest 2.1 1.8 14
Northeast 0.4 0.5 +20
Southeast 0.6 0.7 +17
Total ambient nitrate concentration (nitrate + nitric acid) μg/m3 Mid-Atlantic 3.5 3.0 14
Midwest 4.0 3.5 12
Northeast 2.0 1.7 13
Southeast 2.2 2.1 5

harmed by acid rain deposition can take a long time to fully recover even after harmful emissions cease. The most chronic aquatic problems can take years to be resolved. Forest health is even slower to improve following decreases in emissions, taking decades to recover. Finally, soil nutrient reserves (such as calcium) can take centuries to replenish.

The most recent comprehensive analysis of acidified ecosystems was presented by NAPAP in the National Acid Precipitation Assessment Program Report to Congress: An Integrated Assessment (2003, http://www.cleartheair.org/documents/NAPAP_FINAL_print.pdf). The report presents a literature review summarizing findings from various government and academic studies. Overall, NAPAP finds that some ecosystems affected by acid deposition are showing limited signs of recovery. For example, one study shows that more than 25% of affected lakes and streams studied in the Adirondacks and northern Appalachians are no longer acidic. However, little to no improvement has been seen in examined water bodies in other regions, including New England and portions of Virginia. The report notes that even though chemical recovery has begun in some waterways, complete recovery for aquatic life forms, such as fish, is expected to take "significantly longer."

In regards to terrestrial ecosystems, NAPAP reports that forests are under many stresses besides acid rain, such as global warming, land use changes, and air pollution from urban, agricultural, and industrial sources. The combined effect of these stressors has greatly limited forest recovery from acidification. According to NAPAP, "There are as yet no forests in the U.S. where research indicates recovery from acid deposition is occurring." However, it is expected that reduced emissions under the Acid Rain Program will benefit forests in the long term.

The report acknowledges the future benefits of continued implementation of the Acid Rain Program, but it concludes that "the emission reductions achieved by Title IV are not sufficient to allow recovery of acid-sensitive ecosystems." Recent studies support the idea that additional emission cuts 40% to 80% beyond those of the existing program will be needed to protect acid-sensitive ecosystems. NAPAP modeling indicates that even virtual elimination of SO2 emissions from power plants will be insufficient to provide this protection. It is believed that emission reductions from other sources (such as the industrial and transport sectors) will be necessary.

The Next Step: The Clean Air Interstate Rule

In 2005 the EPA issued the Clean Air Interstate Rule (CAIR; April 5, 2007, http://www.epa.gov/cair/) to address the transport of air pollutants across state lines in the eastern United States. CAIR puts permanent caps on emissions of SO2 and NOx in twenty-eight eastern states and the District of Columbia. It is expected to reduce SO2 emissions by more than 70% and reduce NOx emissions by more than 60% compared with 2003 levels. These measures should reduce the formation of acid rain and other pollutants, such as fine particulate matter and ground-level ozone.

The CAIR program will use a cap-and-trade system similar to that used in the SO2 portion of the acid rain program. The EPA projects that complete implementation of CAIR in 2015 will result in up to $100 billion in annual health benefits and a substantial reduction in premature deaths because of air pollution in the eastern United States. It should also improve visibility in southeastern national parks that have been plagued by smog in recent years.

PUBLIC OPINION ABOUT ACID RAIN

Every year the Gallup Organization polls Americans about their attitudes regarding environmental issues. The most recent poll to assess acid rain was conducted in March 2007. Participants were asked to express their level of personal concern about various environmental issues, including acid rain, water pollution, soil contamination, air pollution, plant and animal extinctions, loss of tropical rain forests, damage to the ozone layer, and global warming. The results showed that acid rain ranked last among these environmental problems.

Analysis of historical Gallup poll results shows a dramatic decline in concern about acid rain since the late 1980s. (See Table 5.4.) In 1989 Gallup found that 41% of respondents felt a great deal of concern about acid rain and 11% felt none at all. By 2007 only 25% of people polled were concerned a great deal about acid rain and 20% expressed no concern about the acid rain issue.

TABLE 5.4

Public concern about acid rain, 19892007
Great deal Fair amount Only a little Not at all No opinion
SOURCE: "I'm going to read you a list of environmental problems. As I read each one, please tell me if you personally worry about this problem a great deal, a fair amount, only a little, or not at all. First, how much do you personally worry aboutAcid Rain," in Environment, The Gallup Organization, 2007, http://www.galluppoll.com/content/?ci=1615&pg=1 (accessed June 19, 2007). Copyright © 2007 by The Gallup Organization. Reproduced by permission of The Gallup Organization.
% % % % %
2007 Mar 1114 25 25 28 20 1
2006 Mar 1316 24 28 24 23 1
2004 Mar 811 20 26 27 26 1
2003 Mar 35 24 26 27 21 2
2002 Mar 47 25 23 31 19 2
2001 Mar 57 28 28 26 16 2
2000 Apr 39 34 31 19 15 1
1999 Apr 1314 29 35 23 11 2
1991 Apr 1114 34 30 20 14 3
1990 Apr 58 34 30 18 14 4
1989 May 47 41 27 19 11 3

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Acid Rain

Acid Rain

In October 1998, U.S. Senator Daniel Patrick Moynihan testified before Congress on acid rain. A longtime champion of the issue, Moynihan stated that "As far back as the 1960s, fishermen in the Adirondacks began to complain about more than 'the big one that got away.' Fish, once abundant in the pristine, remote Adirondack lakes, were not just getting harder to catch. They were gone."

The issue of acid rain emerged in the United States in the mid-1970s. At the time, little was known about the magnitude and distribution of acid rain or about its impacts on terrestrial (land-based) and aquatic ecosystems . However, many believed that acid rain and the air pollutants that caused it posed a threat to forests, aquatic life, crops, structures (e.g., buildings), cultural artifacts (e.g., statues and monuments), and human health.

Since the 1970s, acid rain has been addressed in the United States through hundreds of millions of dollars of research, passage of laws, and implementation of regulatory programs. However, Senator Moynihan's 1998 remark is stark testimony to the fact that acid rain continues to have a negative effect on natural resources, and addressing the problem is an enduring public policy dilemma.

Sources and Forms of Acid Rain

Rain, snow, sleet, and other forms of precipitation are naturally slightly acidic because of chemical reactions with carbon dioxide and other naturally occurring substances in the atmosphere. But this natural acidity can be increased by human-induced air pollution. Acid deposition or "acid rain" occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx ) in the atmosphere react with water, oxygen, and oxidants to form mild solutions of sulfuric acid or nitric acid. Sunlight increases the rate of most of these reactions. These compounds fall to Earth and are deposited in either wet form (e.g., rain, snow, sleet, and hail), known as wet deposition, or dry form (e.g., particles, gases, and vapor), known as dry deposition. Cloud or fog deposition, a form of wet deposition, occurs at high elevations and in coastal areas.

In the United States, nearly two-thirds of annual SO2 emissions and just over one-fifth of NOx emissions are produced by electric utility plants that burn fossil fuels . Transportation sources (e.g., cars, trucks, and other vehicles) account for more than half of NOx emissions. Ammonia emissions derive largely from livestock waste and fertilized soil. Industrial combustion and industrial processes are the other major categories of emission sources. Acid rain is a regional problem because prevailing winds can transport SO2 and NOx emissions over hundreds of kilometers, sometimes crossing state, national, and international borders.

Wet Deposition.

Wet deposition of sulfur and nitrogen compounds is commonly known as acid rain, although it also takes the form of snow, sleet, clouds, or fog. Wet deposition is intermittent because acids reach the Earth only when precipitation falls. Nevertheless, it can be the primary pathway for acid deposition in areas with heavy precipitation.

The eastern United States receives more acidic precipitation than the rest of the country, with the greatest rates occurring in Ohio, West Virginia, western Pennsylvania, upstate New York, New England, and other northeastern areas. Because nitrogen compounds can remain stored in snow until it melts, nitrate concentrations in lakes and streams can increase dramatically during seasonal or episodic acidification, particularly in the Northeast, resulting in toxic impacts on aquatic organisms.

Acidic compounds can reach plants, soil, and water from contact with acidic clouds as well. Although cloud deposition affects only a limited number of locations, it can provide a relatively steady source of acids in comparison with wet deposition, particularly at high altitudes. As a result, trees such as the red spruce have declined in areas of significant cloud deposition.

Dry Deposition.

Dry deposition occurs when acidic gases and particles in the atmosphere are deposited directly onto surfaces when precipitation is not occurring. Dry-deposited gases and particles can also be washed from trees and other surfaces by rainstorms, making the combination more acidic than the falling rain alone. Dry deposition is the primary acid deposition pathway in arid regions of the West, such as Joshua Tree National Park.

Effects on Aquatic Ecosystems

The ecological effects of acid rain are most clearly seen in aquatic environments, particularly streams and lakes. Acid rain mainly affects sensitive bodies of water that are located in watersheds whose soils have limited ability to neutralize acidic compounds. The ability of forest soils to neutralize acidity, referred to as buffering capacity, results from chemicals in the soil that neutralize some or all of the acidity in rainwater. Buffering capacity depends on the thickness and composition of the soil as well as the type of bedrock beneath the forest floor.

Lakes and streams become acidic (pH decreases) when the water itself and its surrounding soil cannot neutralize the acidity in the rain. Differences in soil buffering capacity are an important reason that some areas receiving acid rain show damage, whereas other areas receiving about the same amount of acid rain do not appear to be harmed.

Several regions in the United States contain many of the surface waters sensitive to acidification. They include the Adirondacks and Catskill Mountains in New York State, the mid-Appalachian highlands, the upper Midwest, and mountainous areas of the western United States. In areas such as the northeastern United States, where soil buffering capacity is low, some lakes have a pH value of less than 5. With a pH of 4.2, Little Echo Pond in Franklin, New York was one of the most acidic lakes reported as of 2002.

Ecosystem Impacts.

Acid rain is not the sole cause of low pH in lakes and streams. There are many natural sources of acidity that can drive down pH to low levels (as low as 4) even in the absence of acid rain: for example, organic acid inputs or mineral veins in underlying geologic materials. Similarly, natural sources of buffering capacity such as limestone bedrock can push pH to as high as 8. Notwithstanding these natural influences in specific locations, lakes and streams generally have pH values from 6 to 8. Hence, reductions in pH due to human-induced acid rain create an imbalance in the chemistry and ultimately the entire ecosystem of a lake or stream.

Acid rain causes a cascade of effects that harm or kill individual fish, reduce fish populations, completely eliminate fish species from a waterbody, and decrease biodiversity . As acid rain flows through soils in a watershed, aluminum and other metals are released from soils into the lakes and streams located in that watershed. Thus, as a lake or stream becomes more acidic (has lower pH), aluminum levels increase. Both low pH and increased aluminum levels are directly toxic to fish. In addition, low pH and increased aluminum levels cause chronic stress that may not kill individual fish but may make fish less able to compete for food and habitat.

The impact of declining pH varies because not all aquatic organisms can tolerate the same amount of acid. For example, frogs are better able than trout to tolerate somewhat acidified water. Generally, the young of most species are more sensitive to environmental conditions than adults.

As pH levels decline, acid-sensitive species may attempt to migrate to better habitat, or, if blocked from migration, will likely die. At pH 5 and below, most fish species disappear, and ecosystem-level processes are affected. Some acid lakes and streams contain no fish.

Effects on Forests and Soils

Acid rain has been implicated in forest and soil degradation in many areas of the eastern United States, particularly high elevation forests of the Appalachian Mountains from Maine to Georgia. Acid rain does not usually kill trees directly. Instead, it weakens trees by damaging their foliage, limiting the nutrients available to them, or exposing them to toxic substances slowly released from the soil. Quite often, injury or death is a result of acid rain in combination with other environmental stressors, such as insects, disease, drought, or very cold weather.

Chemicals in watershed soils that provide buffering capacity (such as calcium and magnesium) are also important nutrients for many species of trees. As forest soils receive year after year of acid rain, these chemicals are washed away, depriving trees and other plants of essential soil nutrients. At the same time, acid rain causes the release of dissolved aluminum into the soil water, which can be toxic to trees and plants. The chemicals that provide buffering capacity take many decades to replenish through gradual natural processes, such as the weathering of limestone bedrock.

Trees also can be damaged by acid rain even if the soil is well buffered. Mountainous forests often are exposed to greater amounts of acidity because they tend to be surrounded by acidic clouds and fog. Essential nutrients in foliage are stripped away when leaves and needles are frequently bathed in acid fog, causing discoloration and increasing the potential for damage by other environmental factors, especially cold weather.

Effects on Human Health and Human Environments

The pollutants that cause acid rain also damage human health. These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be inhaled deep into the lungs. Scientific studies show relationships between elevated levels of fine particles and increased illness and premature death from heart disease and lung disorders, such as bronchitis. In addition, nitrogen oxides react in the atmosphere to form ozone , increasing risks associated with lung inflammation, such as asthma.

Sulfates and nitrates in the atmosphere also contribute to reductions in visibility. Sulfate particles account for 50 to 70 percent of decreased visibility in eastern U.S. national parks, such as the Shenandoah and the Great Smoky Mountains. In the western United States, nitrates and carbon also play roles, but sulfates have been implicated as an important source of visibility impairment in some national parks, such as the Grand Canyon.

Wet and dry acid deposition contribute to the corrosion of metals (such as bronze) and the deterioration of paint and stone (such as marble and limestone). These effects seriously reduce the value to society of buildings, bridges, cultural objects (such as statues, monuments, and tombstones), and automobiles.

1990 Clean Air Act Amendments: Title IV

In 1990, the U.S. Congress took action intended to address acid rain issues, passing the Clean Air Act Amendments (CAAA) (42 U.S.C. 7651). The purpose of the Acid Rain Program (Title IV of the 1990 amendments) was to address the adverse effects of acid rain by reducing annual emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx )the main air pollutants that cause the problemsfrom stationary power generation sources.

Implemented by the U.S. Environmental Protection Agency starting in 1995, the program consists of two major components. The SO2 emission reduction program employs a two-phase cap-and-trade approach to reduce total annual SO2 emissions by 10 million tons below 1980 levels by 2010 (roughly a 40-percent reduction in total emissions). When the SO2 emission reduction is fully implemented in approximately 2010, electric utility emissions will be capped at 8.95 million tons per year (representing approximately a 50-percent reduction in emissions from this sector).

The NOx emission reduction program aims to reduce annual NOx emissions from coal-fired electric utility boilers by 2 million tons below what they would have been without Title IV. The NOx component of the program does not include a cap on NOx emissions or any emissions trading provisions.

Emissions Trading.

In establishing the Acid Rain Program, Congress chose to utilize an innovative environmental management approach known as capand-trade, or emissions trading, to reduce SO2 emissions. Emissions trading is a departure from more traditional "command and control" regulatory approaches in which the government commands industry to install particular control technologies at specific plants in order to reduce pollution. Because emissions trading allows industry the flexibility to reduce pollution from sources that can achieve reductions least expensively, large amounts of emissions are reduced at lower costs, with less administrative burden and fewer lengthy lawsuits, than if sources were regulated individually.

The approach first sets an overall cap (maximum amount of emissions) that policymakers believe will achieve the desired environmental effects. Affected sources are then allocated emission allowances that permit them to emit a certain amount of a pollutant. The total number of allowances given to all sources cannot exceed the cap.

Sources are not told how to reach the emissions goal established by the number of allowances they are given. They may reach their goal through various means, including buying allowances from sources that are able to reduce emissions more cost effectively and so have excess allowances to sell. The only requirements are that sources completely and accurately measure and report all emissions and then turn in the same number of allowances as emissions at the end of the yearly compliance period. If emissions exceed allowances, a source faces expensive fines and other penalties.

Cap-and-trade is effective for the following reasons:

  • The mandatory cap always protects the environment. Even as the economy grows, or as new sources enter the industry, total emissions cannot exceed the cap.
  • Complete and consistent emissions measurement and reporting by all sources guarantee that total emissions do not exceed the cap and that individual sources' emissions are no higher than their allowances.
  • The design and operation of the program is simple, which helps keep compliance and administrative costs low.

Effectiveness of the Acid Rain Program

In terms of SO2 emissions reductions, the results of the Acid Rain Program have been dramaticand unprecedented. From its 1995 inception to 1999 (completion of Phase I), annual SO2 emissions from the largest, highestemitting sources dropped by nearly 5 million tons from 1980 levels. These reductions were an average of 25 percent below required emission levels, resulting in early achievement of human health and environmental benefits. In 2001, SO2 emissions from power generation were more than 6.7 million tons below 1980 levels.

Emissions of NOx have been reduced by 1.5 million tons from 1990 levels (about 3 million tons lower than projected growth). Because the NOx component of the program includes no cap, there is no guarantee that NOx emissions will stay at these low levels; without a cap, emissions may increase as power generation increases.

Because of the reduction in SO2 emissions, acidity of rainfall in the eastern United States has dropped by up to 25 percent. As a consequence, some sensitive lakes and streams in New England are showing signs of recovery. Further, sulfate concentrations in the air have decreased, leading to improved air quality and associated benefits to public health, such as fewer irritations or aggravations to respiratory conditions (e.g., asthma and chronic bronchitis). Finally, visibility has improved in some parts of the eastern United States, including areas with scenic vistas, such as Acadia National Park in coastal Maine.

Although the Clean Air Act has had positive effects, emissions and acid deposition remain high compared to background conditions. The rate and extent of ecosystem recovery from acid deposition are directly related to the timing and degree of emissions reductions. Research suggests that deeper emissions cuts will lead to greater and faster recovery from acid deposition in the northeastern United States.

see also Amphibian Population Declines; Cavern Development; Chemical Analysis of Water; Ecology, Fresh-Water; Fresh Water, Natural Composition of; Fresh Water, Physics and Chemistry of; Karst Hydrology; Lakes: Chemical Processes.

Richard Haeuber

Bibliography

Dehayes, Donald et al. "Acid Rain Impacts on Calcium Nutrition and Forest Health." Bioscience 49 (October 1999):789800.

Driscoll, Charles T. et al. Acid Rain Revisited: Advances in Scientific Understanding Since the Passage of the 1970 and 1990 Clean Air Act Amendments. Hanover, NH: Hubbard Brook Research Foundation, 2001. Available online at <http://www.hubbardbrook.org/hbfound/report.pdf>.

Driscoll, Charles T. et al. "Acidic Deposition in the Northeastern U.S.: Sources andInputs, Ecosystem Effects, and Management Strategies." Bioscience 51, no. 3 (March 2001):180198.

Driscoll, Charles T. et al. "The Response of Lake Water in the Adirondack Region of New York State to Changes in Acid Deposition." Environmental Science and Policy 1 (1998):185198.

Ellerman, A. Denny et al. Markets for Clean Air: The U.S. Acid Rain Program. New York: Cambridge University Press, 2000.

Kosobud, Richard F., ed. Emissions Trading: Environmental Policy's New Approach. NewYork: John Wiley & Sons, 2000.

Likens, Gene E., Charles T. Driscoll, and D. C. Buso. 1996. "Long-Term Effects ofAcid Rain: Response and Recovery of a Forest Ecosystem." Science 272 (12 Apr. 1996):244246.

Lovett, Gary. "Atmospheric Deposition of Nutrients and Pollutants in North America: An Ecological Perspective." Ecological Applications 4, no. 4 (1994):629650.

Stoddard, John L. et al. "Regional Trends in Aquatic Recovery from Acidification inNorth America and Europe." Nature 401 (7 Oct. 1999):575578.

U.S. Environmental Protection Agency, Clean Air Markets Division. EPA Acid Rain Program2001 Progress Report. EPA-430-R-02-009. Washington, D.C.: U.S. Environmental Protection Agency. Available online at <http://www.epa.gov/airmarkets/cmprpt/arp01/index.html>.

Internet Resources

Clean Air Markets. U.S. Environmental Protection Agency. <http://www.epa.gov/airmarkets>.

National Acid Precipitation Assessment Program. <http://www.oar.noaa.gov/organization/napap.html>.

National Atmospheric Deposition Program. <http://nadp.sws.uiuc.edu/>.

Nilles, Mark A. Atmospheric Deposition Program of the U.S. Geological Survey. U.S. Geological Survey. <http://bqs.usgs.gov/acidrain/Program.pdf>.

ACID RAIN AND THE U.S. CAPITOL BUILDING

The buildings and monuments of Washington, D.C. use many types of stone. Marble and limestone structures are the most likely to show damage caused by acid precipitation and urban pollution. They are vulnerable to accelerated deterioration because they are composed primarily of the mineral calcite (calcium carbonate), which dissolves readily in weak acid.

The United States Capitol building shows evidence of stone deterioration. For example, preferential dissolution of calcite (where the silicate mineral inclusions remain) has caused pockmarks in marble columns and balustrades and their square bases. Although stone deterioration has many causes, both natural and human-induced, it is almost certain that some deterioration can be attributed to acid rain.

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Acid Rain

Acid rain

Acid rain is a popular phrase used to describe rain, snow, fog, or other precipitation that is full of acids that collect in the atmosphere due to the burning of fuels such as coal, petroleum, and gasoline. Acid rain was first recognized in Europe in the late 1800s but did not come to widespread public attention until about 1970, when its harmful effects on the environment were publicized. Research has shown that in many parts of the world, lakes, streams, and soils have become increasingly acidic, prompting a corresponding decline in fish populations.

Acid rain occurs when polluted gases become trapped in clouds that drift for hundredseven thousandsof miles and are finally released as acidic precipitation. Trees, lakes, animals, and even buildings are vulnerable to the slow, corrosive (wearing away) effects of acid rain.

Acid deposition

Acidification (the process of making acid) is not just caused by deposits of acidic rain but also by chemicals in snow and fog and by gases and particulates (small particles) when precipitation is not occurring.

The major human-made causes of acid deposition are (1) emissions of sulfur dioxide from power plants that burn coal and oil and (2) emissions of nitrogen oxides from automobiles. These emissions are transformed into sulfuric acid and nitric acid in the atmosphere, where they accumulate in cloud droplets and fall to Earth in rain and snow. (This is called wet deposition.) Other sources of acid deposition are gases like sulfur dioxide and nitrogen oxides, as well as very small particulates (such as ammonium sulfate and ammonium nitrate). These gases and particulates are usually deposited when it is not raining or snowing. (This is called dry deposition.)

Areas affected by acid deposition. Large areas of Europe and North America are exposed to these acidifying depositions. However, only certain types of ecosystems (all the animals, plants, and bacteria that make up a particular community living in a certain environment) are affected by these depositions. The most vulnerable ecosystems usually have a thin cover of soil, containing little calcium and sitting upon solid rock made up of hard minerals such as granite or quartz. Many freshwater lakes, streams, and rivers have become acidic, resulting in the decline or local

destruction of some plant and animal populations. It is not yet certain that land-based ecosystems have been affected by acidic deposition.

Words to Know

Acidification: An increase over time in the content of acidity in a system, accompanied by a decrease in the acid-neutralizing capacity of that system.

Acidifying substance: Any substance that causes acidification, either directly or indirectly, as a result of chemical changes.

Acidity: The quality, state, or degree of being acidic. Acidity is usually measured as the concentration of hydrogen ions in a solution using the pH scale. A greater concentration of hydrogen ions means a more acidic solution and a lower corresponding pH number. Strictly speaking, an acidic solution has a pH less than 7.0.

Leaching: The movement of dissolved chemicals with water that is percolating, or oozing, downward through the soil.

Neutralization: A chemical reaction in which the mixing of an acidic solution with a basic (alkaline) solution results in a solution that has the properties of neither an acid nor a base.

Oxide: A compound containing oxygen and one other element.

pH: A measure of acidity or alkalinity of a solution referring to the concentration of hydrogen ions present in a liter of a given fluid. The pH scale ranges from 0 (greatest concentration of hydrogen ions and therefore most acidic) to 14 (least concentration of hydrogen ions and therefore most alkaline), with 7 representing a neutral solution, such as pure water.

After acid rain was discovered in Europe, scientists began measuring the acidity of rain in North America. Initially, they found that the problem was concentrated in the northeastern states of New York and Pennsylvania because the type of coal burned there was more sulfuric. Yet by 1980, most of the states east of the Mississippi, as well as areas in southeastern Canada, were also receiving acidic rainfall. Acid rain falls in the West as well, although the problem is not as severe. Acid rain in Los Angeles, California, is caused primarily by automobile emissions.

How is acid rain measured?

Acid rain is measured through pH tests that determine the concentration of hydrogen ions in a liter of fluid. The pH (potential for hydrogen) scale is used to measure acidity or alkalinity. It runs from 0 to 14. Water has a neutral pH of 7. (The greater the concentration of hydrogen ions and the lower the pH number, the more acidic a substance is; the lower the concentration of hydrogen ions and the higher the pH number, the more alkalineor basica substance is.) So a pH greater than 7 indicates an alkaline substance while a pH less than 7 indicates an acidic substance.

It is important to note that a change of only one unit in pH equals a tenfold change in the concentration of hydrogen ions. For example, a solution of pH 3 is 10 times more acidic than a solution of pH 4.

Normal rain and snow measure about pH 5.60. In environmental science, the definition of acid precipitation refers to a pH less than 5.65.

Measured values of acid rain vary according to geographical area. Eastern Europe and parts of Scandinavia have rain with pH 4.3 to 4.5; rain in the rest of Europe ranges from pH 4.5 to 5.1; rain in the eastern United States and Canada ranges from pH 4.2 to 4.6, and the Mississippi Valley has a range of pH 4.6 to 4.8. The worst North American area, analyzed at pH 4.2, is centered around Lake Erie and Lake Ontario.

When pH levels are drastically upset in soil and water, entire lakes and forests are endangered. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to disease. Also, high acid levels in soil causes leaching (loss) of other valuable minerals such as calcium, magnesium, and potassium.

Small marine organisms cannot survive in acidic lakes and rivers, and their depletion (reduced numbers) affects the larger fish who usually feed on them, and, ultimately, the entire marine-life food chain. Snow from acid rain is also damaging; snowmelt has been known to cause massive, instant death for many kinds of fish. Some lakes in Scandinavia and New York's Adirondack Mountains are completely devoid of fish life. Acid rain also eats away at buildings and metal structures. From the Acropolis in Greece to Renaissance buildings in Italy, ancient structures are showing signs of corrosion from acid rain. In some industrialized parts of Poland, trains cannot exceed 40 miles (65 kilometers) per hour because the iron railway tracks have been weakened from acidic air pollution.

Treatment of water bodies affected by acid rain

Usually, waters affected by acid rain are treated by adding limestone or lime, an alkaline substance (base) that reduces acidity. Fishery biologists especially are interested in liming acidic lakes to make them more habitable (capable of being lived in) for sport fish. In some parts of Scandinavia, for instance, liming is used extensively to make the biological damage of acidification less severe.

Avoiding acid rain

Neutralizing (returning closer to pH 7) ecosystems that have become acidic treats the symptoms, but not the sources, of acidification. Although exact sources of acid rain are difficult to pinpoint and the actual amount of damage caused by acid deposition is uncertain, it is agreed that acid rain levels need to be reduced. Scientific evidence supports the notion that what goes up must come down, and because of public awareness and concerns about acid rain in many countries, politicians have begun to act decisively in controlling or eliminating human causes of such pollution. Emissions of sulfur dioxide and nitrogen oxides are being reduced, especially in western Europe and North America. For example, in 1992 the governments of the United States and Canada signed an air-quality agreement aimed at reducing acidifying depositions in both countries.

While countries in western Europe and North American have actively carried out actions to reduce emissions of gases leading to acid deposition for a number of years, countries in other parts of the world have only recently addressed the issue. In eastern Europe, Russia, China, India, southeast Asia, Mexico, and various developing nations, acid rain and other pollution problems are finally gaining notice. For example, in 1999, scientists identified a haze of air pollution that hovers over the Indian Ocean near Asia during the winter. The 3.8 million-square-mile haze (about the size of the combined area of all fifty American states) is made up of small by-products from the burning of fossil fuels. Such a cloud has the potential to cool Earth, harming both marine and terrestrial life.

[See also Acids and bases; Forests; Pollution ]

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Acid Rain

Acid rain

Acid rain is rain with a pH (a logarithmic measurement of acidity or alkalinity) of less than 5.7. Acid rain usually results from elevated levels of nitric and sulfuric acids in air pollution. Acidic pollutants that can lead to acid rain are common by-products from burning fossil fuels (e.g., oil, coal , etc.) and are found in high levels in exhaust from internal combustion engines (e.g., automobile exhaust). Acidic precipitation may also occur in other forms such as snow.

Acid rain occurs when polluted gasses become trapped in clouds . The clouds may drift for hundreds, even thousands, of miles before finally releasing acidic precipitation. Trees, lakes , animals, and even buildings are vulnerable to the slow corrosive effects of acid rain, whose damaging components are emitted by power plants and factories, especially those burning low grades of coal and oil.

Acid rain was first recognized in 1872, approximately 100 years after the start of the Industrial Revolution in England, when an English scientist, Robert Angus Smith (18171884), pointed out the problem. Almost another century passed, however, before the public became aware of the damaging effects of acid rain. In 1962, the Swedish scientist Svante Oden brought the acid rain quandary to the attention of the press, instead of the less popular scientific journals. He compiled records from the 1950s indicating that acid rain came from air masses moving out of central and western Europe into Scandinavia.

After acid rain was discovered in Europe, scientists began measuring the acidity of rain in North America . Initially, they found that the problem was concentrated in the northeastern states of New York and Pennsylvania because the type of coal burned there was more sulfuric. By 1980, most of the states east of the Mississippi, as well as southeastern Canada, were receiving acidic rainfall. Acid rain falls in the West also, although the problem is not as severe. Acid rain in Los Angeles, California is caused primarily by local traffic emissions. Car emissions contain nitrogen oxide, the second highest problematic gas in acid rain after sulfur dioxide.

Acid rain is measured through pH tests that determine the concentration of hydrogen ions. Pure water has a neutral pH of approximately 7.0. When the pH is greater than 7, the material is thought to be alkaline. At a pH of 5.7, rain is slightly acidic, but when its pH is further reduced, the rain becomes an increasingly stronger acid rain. In the worst cases, acid rain has shown a pH of 2.4 (about as acidic as vinegar). When pH levels are drastically tipped in soil and water, entire lakes and forests are jeopardized. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to other dangers. High acid levels in soil causes leaching of other valuable minerals such as calcium, magnesium, and potassium. According to the World Watch Institute, in the late 1980s and early 1990s forest damage in Europe ranged from a low of 4% in Portugal to a high of 71% in Czechoslovakia, averaging 35% overall.

Small marine organisms cannot survive in acidic lakes and rivers , and their depletion affects larger fish and ultimately the entire marine life food chain. Snow from acid rain is also damaging; snowmelt has been known to cause massive, instant death for many kinds of fish. Some lakes in Scandinavia, for example, are completely devoid of fish. Acid rain also eats away at buildings and metal structures. From the Acropolis in Greece to Renaissance buildings in Italy, ancient structures are showing signs of slow corrosion from acid rain. In some industrialized parts of Poland, trains cannot exceed 40 miles (65 km) per hour because the iron railway tracks have been weakened from acidic air pollution.

New power plants in the United States are being built with strict emissions standards, but retrofitting older plants is difficult and expensive. Nevertheless, the United States Environmental Protection Agency requires most of the older and dirtier power plants to install electrostatic precipitators and baghouse filtersdevices designed to remove solid particulates. Such devices are required in Canada, in industrialized countries in Western Europe, and in Japan. Scrubbers, or flue-gas desulfurization technology, are also being used because of their effectiveness in removing as much as 95% of a power plant's sulfur dioxide emissions. These devices are expensive, however, and there are clauses in pollution control laws that allow older plants to continue operation at higher pollution levels. Another way to reduce acid rain is for power plants to burn cleaner coal in their plants. This does not require retrofitting but it does increase transportation costs since coal containing less sulfur is mined in the western part of the United States, far away from where it is needed in the midwest and eastern part of the country.

See also Atmospheric pollution; Erosion; Global warming; Groundwater; Petroleum, economic uses of; Rate factors in geologic processes; Weathering and weathering series

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Acid Rain

ACID RAIN

"Acid rain" is the common term for a complex process more appropriately referred to as acid deposition. It includes the deposition of acidic compounds onto the ground and onto surface waters when it rains (wet deposition), and at other times as well (dry deposition). The acid compounds include both acidic gases, such as sulfur dioxide (SO2) and nitrogen dioxide (NO2), and acidic particles, such as sulfate and nitrate compounds. Acid deposition is believed to have adversely affected lakes and forests in the northeastern United States, Canada, and Europe, and to have caused material damage as well.

The primary anthropogenic source of airborne acidity is the burning of fossil fuels. Coal-and oil-fired electric utilities and industries emit gaseous SO2 and nitrogen oxides (NO and NO2) into the atmosphere. Automobiles and other mobile sources also contribute significant amounts of nitrogen oxides.

As these primary pollutants are transported by the wind, sometimes over long distances, they are slowly transformed through a variety of atmospheric reactions to secondary pollutants, such as nitric acid vapor and sulfuric acid droplets, which are strongly acidic. With further transport and reactions with ammonia gas (NH3) from biological decay processes at the ground level, they are transformed to less strongly acidic sulfate and nitrate particles. These atmospheric reaction products can remain suspended, impairing visibility, reducing air quality, and causing adverse human health effects or these products can be deposited directly onto surfaces at ground level.

The area affected by the emission sources is determined to a large extent by the time that pollutants stay in the atmosphere before removal through deposition.

Sulfur and nitrogen deposition have caused adverse impacts on highly sensitive forest ecosystems in the United States and northern Europe, such as high-elevation spruce and fir forests in the eastern United States. On the other hand, most U.S. forest ecosystems are not currently known to be adversely impacted. The gradual leaching of soil nutrients from sustained acid deposition can impede forest nutrition and growth. Potential risk depends on numerous factors, including rate of cation (positively charged ion) deposition, soil cation reserves, age of forest, weathering rates, species composition, and disturbance history. Dry deposition is now considered to be more damaging to stone than wet deposition.

Since sulfate significantly contributes to visibility-reducing particles in the eastern United States, reduced SO2 emissions will reduce sulfate concentrations and, in turn, their contribution to haze. In the 1990 U.S. Clean Air Act Amendments, Congress mandated reductions in annual emissions of SO2 by 1995 and nitrogen oxides from utilities burning fossil fuels starting in 1995.

As a result, statistically significant reductions in the acidity (represented by hydrogen ion content) and sulfate concentrations in precipitation were reported at deposition-monitoring sites in the Midwest, Mid-Atlantic, and northeast United States. Although utilities have significantly reduced their emissions, observable responses will lag due to inherent time lags between changes in emissions and responses by sensitive receptors, especially within ecosystems.

It is still too early to determine whether changes in aquatic ecosystems have resulted from emission reductions. Over the last fifteen years, lakes and streams throughout many areas of the United States have experienced decreases in sulfate concentrations in response to decreased emissions and deposition of sulfur, and there is evidence of recovery from acidification in New England lakes. In contrast, the acidity levels of the majority of Adirondack lakes have remained fairly constant, while the most sensitive Adirondack lakes have continued to acidify.

The kind of damages seen in forests and lakes in the northeastern United States have also been witnessed in Scandinavia and other parts of northern Europe.

Morton Lippmann

(see also: Airborne Particles; Ambient Air Quality [Air Pollution]; Clean Air Act; Environmental Determinants of Health; Inhalable Particles [Sulfates]; Total Suspended Particles [TSP] )

Bibliography

National Acid Precipitation Assessment Program (1998). National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment. Silver Spring, MD: Author.

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Acid Rain

ACID RAIN

ACID RAIN, precipitation whose acidity has increased on account of some human activity. Dust storms, volcanic eruptions, and biological decay can affect the acid level of rain or snow, but industrial pollutants may raise the acidity of a region's precipitation more than tenfold. Certain pollutants mix with atmospheric water vapor to form acids, which may then fall to the ground in a process called dry disposition, or fall in combination with rain or snow, called wet disposition.

The term "acid rain" was coined in 1872 by Robert Angus Smith, an English chemist who studied the chemical content of rain near Manchester, England. In retrospect it is clear that U.S. cities such as Chicago, Pittsburgh, and St. Louis, heavy consumers of bituminous coal, also suffered from acid precipitation. Nevertheless the first large-scale effort to monitor the chemistry of precipitation did not occur until the late 1940s with the work of Hans Egner of Sweden. In the 1960s, European researchers began publicizing the effects of acidic precipitation on soils, vegetation, aquatic ecology, and human-made structures. In the 1970s, the discovery that several Canadian lakes had high acid levels (pH levels between 4 and 5) increased public awareness of the issue. By that time problems as diverse as crumbling monuments, fish kills, and dying forests were linked to acid precipitation.

The acid rain issue transcends political boundaries. Power plants in the Midwest of the United States, for example, may create acid rain that falls to the ground in eastern Canada. Indeed pressure from Canada, Sweden, and Norway, net receivers of atmospheric sulfur dioxide, led to a series of international acid rain conferences beginning in 1979. Acid rain debates seriously strained relations between the United States and Canada in the 1980s. The Canadian government expressed anger when the Ronald Reagan administration deferred action pending further study of the issue.

Efforts to abate acid rain have focused on two pollutants, sulfur dioxide, a by-product of burning coal or fuel oil, and nitrogen oxides generated largely by automobiles and power plants. In the United States the federal Clean Air Act (CAA) of 1970 restricted both pollutants. However, acid rain was not the motivating factor behind the CAA, and studies later suggested the law may have worsened the problem. When monitoring devices were

placed near factories, many firms simply built taller smokestacks to disperse pollutants higher into the atmosphere, away from the monitors. Consequently acid rain spread even wider. In 1977 amendments to the CAA required that utilities install scrubbers in each new coal-fired power plant. Implementation of these and additional amendments in the 1980s are credited for decreasing annual sulfur dioxide emissions in the United States from 26 to 21 million metric tons by 1989. Similarly nitrogen oxide emissions, which peaked at 22 million metric tons in 1981, fell to 19 million tons by 1990.

In 1990 additional amendments to the CAA imposed stricter air pollution standards on vehicles and set a cap on national sulfur emissions governed by a market-based system of emission allowances. These regulations, along with a provision allowing eastern utilities to use more low-sulfur western coal, apparently helped reduce acid precipitation in the Northeast by up to 25 percent. But acidified water and soil continued to imperil lake and forest ecosystems. A major study sponsored by the U.S. Environmental Protection Agency released in late 1999 found that sulfate levels had fallen sharply in most lakes of the Northeast and the Midwest, but that acidity levels had not fallen along with them, perhaps because prolonged acid precipitation had weakened the lakes' natural buffering capacity.

BIBLIOGRAPHY

Bryner, Gary C. Blue Skies, Green Politics: The Clean Air Act of 1990. Washington, D.C.: CQ Press, 1993.

Schmandt, Jurgen, Judith Clarkson, and Hilliard Roderick, eds. Acid Rain and Friendly Neighbors: The Policy Dispute Between Canada and the United States. Rev. ed. Durham, N.C.: Duke University Press, 1988.

Regens, James L., and Robert W. Rycroft. The Acid Rain Controversy. Pittsburgh, Pa.: University of Pittsburgh Press, 1988.

HughGorman/w. p.

See alsoCanada, Relations with ; Conservation ; Electric Power and Light Industry ; Energy Industry ; Energy, Renewable ; Water Pollution .

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Acid Rain

Acid Rain


Acid rain is any form of atmospherically deposited acidic substance containing strong mineral acids of anthropogenic origin. It was reportedly first described in England by Robert Angus Smith in 1872. Acid rain is more properly called acidic deposition, which occurs in both wet and dry forms. Wet deposition usually exists in the form of rain, snow, or sleet but also may occur as fog, dew, or cloud water condensed on plants or the earth's surface. Dry deposition includes solid particles (aerosols) that fall to the earth's surface. Condensation of fog, dew, or cloud water is referred to as occult deposition.

The most common acidic substances are compounds containing hydrogen (H+), sulfates (SO = 4 ), and nitrates (NO 3 ). The chief source of these compounds is the combustion of fossil fuels such as coal, petroleum, and petroleum by-products, primarily gasoline. Agriculture is also a major source of nitrates. Power plants that burn coal contribute over 50 percent of sulfates to the atmosphere and 25 percent of nitrates.

Prior to the Clean Air Act of 1970, acid deposition was mostly a local problem confined to the immediate vicinity of the pollution source. After 1970, emitters of acidifying pollutants increased the height of smokestacks to reduce local pollution by diluting pollutants in larger volumes of air. The result was the regional transport of acid deposition to remote locations. Acid rain has adversely affected large areas of the mountainous regions of the eastern United States and Canada, Scandinavia, central and Eastern Europe, and parts of China. Areas that are downwind of heavy concentrations of power plants receive the most deposition.

Acid rain acidifies soils with low calcium carbonate levels, which results in the acidification of water passing through the soil to streams and lakes. Calcium carbonate soil-buffering capacity is related to soil origin. Soils weathered from rocks high in calcium carbonate have high calcium carbonate buffer capacity. Fish and other aquatic life have been eliminated from streams and lakes by acid deposition. Continued acid deposition leaches calcium and magnesium from the soil and results in the increased mobility of aluminum, which is toxic to both animals and plants. Aluminum is always present in soils, but it is innocuous until mobilized into soil water by acidic deposition. Its presence in water in small amounts will cause the outright death of fish and other aquatic life, disrupt normal fish spawning, and reduce populations of many species of aquatic insects.

Acid forest soils are thought to cause forests to decline and grow more slowly. Soil acidity causes nutrient deficiencies in trees and other plants and predisposes them to attack by pathogens such as insects and fungi. Soil acidity also increases photo-oxidant stress in plants. Monuments and buildings made of marble or other forms of calcium carbonate and statuary made of certain metals such as copper are also damaged by acid deposition. The acidification of waters leads to increases in mercury uptake by fish, causing them to be unsafe to eat.

The governments of the European Economic Community, Canada, and the United States have taken steps to reduce the emissions of sulfate and nitrates. The Clean Air Act Amendments of 1990 were designed to reduce U.S. emissions of sulfate by about 40 percent through a program of emissions trading between emissions generators, use of low-sulfur coals (fuel switching), and controls on power plant smokestack emissions. Although this program has significantly reduced acidic deposition in many parts of the northeastern United States, many scientists agree that additional reductions will be required to prevent continued damage and allow for meaningful recovery of affected lakes and streams.

see also Air Pollution; Coal; Electric Power; NOx (Nitrogen Oxides); Petroleum; Sulfur Dioxide; Vehicular Pollution.

Internet Resource

Environment Canada Web site. Available from http://www.ec.gc.ca/acidrain.

William E. Sharpe

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acid rain

acid rain or acid deposition, form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids (pH below 5.5–5.6). Produced when sulfur dioxide and various nitrogen oxides combine with atmospheric moisture, acid rain can contaminate drinking water, damage vegetation and aquatic life, and erode buildings and monuments. Automobile exhausts and the burning of high-sulfur industrial fuels are thought to be the main causes, but natural sources, such as volcanic gases and forest fires, may also be significant. It has been an increasingly serious problem since the 1950s, particularly in the NE United States, Canada, and W Europe, especially Scandinavia.

Acid rain became a political issue in the 1980s, when Canada claimed that pollutants from the United States were contaminating its forests and waters. Since then regulations have been enacted in North America and Europe to curb sulfur dioxide emissions from power plants; these include the U.S. Clean Air Act (as reauthorized and expanded in 1990) and the Helsinki protocol (1985), in which 21 European nations promised to reduce emissions by specified amounts. To assess the effectiveness of reductions a comprehensive study, comparing data from lakes and rivers across N Europe and North America, was conducted by an international team of scientists in 1999. The results they reported were mixed: while sulfates (the main acidifying water pollutant from acid rain) were lower, only some areas showed a decrease in overall acidity. It remained to be determined whether more time or a greater reduction in sulfur emissions was needed to reduce freshwater acidity in all areas. See air pollution; forest; pollution.

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acid rain

acid rain Precipitation having a pH value of less than about 5.0, which has adverse effects on the fauna and flora on which it falls. Rainwater typically has a pH value of 5.6, due to the presence of dissolved carbon dioxide (forming carbonic acid). Acid rain results from the emission into the atmosphere of various pollutant gases, in particular sulphur dioxide and various oxides of nitrogen, which originate from the burning of fossil fuels and from car exhaust fumes, respectively. These gases dissolve in atmospheric water to form sulphuric and nitric acids in rain, snow, or hail (wet deposition). Alternatively, the pollutants are deposited as gases or minute particles (dry deposition). Both types of acid deposition affect plant growth – by damaging the leaves and impairing photosynthesis and by increasing the acidity of the soil, which results in the leaching of essential nutrients. This acid pollution of the soil also leads to acidification of water draining from the soil into lakes and rivers, which become unable to support fish life. Lichens are particularly sensitive to changes in pH and can be used as indicators of acid pollution (see indicator species).

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Acid Rain

Acid Rain

Acid rain can be defined as rain that has a pH less than 5.6, formed primarily through the chemical transformation of sulfur and nitrogen compounds emitted by anthropogenic sources. In addition, acidic compounds can be deposited as aerosols and particulates (dry deposition), and mists, fogs, snow, and clouds (wet deposition). Most scientists agree that the phrase acidic deposition is more appropriate when characterizing the overall problem, but acid rain is the most widely used term.

Robert Angus Smith (1817-1884), a Scottish chemist, first used the expression "acid rain" in 1872 when describing the acidic nature of rain deposited around Manchester, England. The problem was believed to be localized and confined to urban areas until reports appeared during the 1970s and 1980s describing widespread acidification of lakes in the northeastern United States, eastern Canada, and Europe. Additional reports surfaced regarding declines in growth and vigor of forested ecosystems throughout the world, with acid rain as the possible culprit. These findings resulted in several large research initiatives, including the U.S. government-funded National Atmospheric Precipitation Assessment Program.

Results indicated that pH in rainfall, mists, clouds, snow, and fog in the United States, especially the East, was generally below normal, and was due to an increase in industrial emissions of sulfur and nitrogen compounds transported to rural areas. Some lakes and streams were acidified and their productivity reduced by acid rain. Most lakes and streams that were acidified were located in the northeastern United States. The majority of forested and agricultural ecosystems were found not to be directly affected by acid rain. Certain high-elevation systems, such as red spruce in the northeastern United States, were reported as possibly being affected by acid rain, but many other factors were involved. Research findings resulted in increased environmental legislation, including the 1990 Clean Air Act Amendments enacted by the U.S. Congress to significantly reduce sulfur emissions.

Since 1990, sulfur dioxide emissions have decreased 25 percent, resulting in a significant reduction in sulfate in rain and surface waters in some areas of the United States. Nitrogen compounds, however, have not decreased. The role nitrogen plays in acidification is currently of concern to the scientific community. Several forested ecosystems have been found to be nitrogen saturated . Also, it is hypothesized that acid rain has caused a depletion in base cations , mainly calcium, potassium, and magnesium, in the soils of several forested ecosystems making uptake of these essential minerals more difficult. Research is underway to investigate the effects of these problems on ecosystem function.

see also Atmosphere and Plants.

Arthur H. Chappelka

Bibliography

Irving, P. M. "Acid Deposition: State of Science and Technology." In National Acid Precipitation Assessment Program, 1990 Integrated Assessment Report. Washington, DC: 1991.

Krupa, S. V. Air Pollution, People, and Plants: An Introduction. St. Paul, MN: American Phytopathological Society Press, 1997.

NAPAP Biennial Report to Congress: An Integrated Assessment. Washington, DC: National Science and Technology Council, 1998.

Wellburn, A. Air Pollution and Acid Rain: The Biological Impact. New York: John Wiley & Sons, 1988.

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acid rain

acid rain Rain that is highly acidic because of sulphur oxides, nitrogen oxides, hydrocarbons, and other air pollutants dissolved in it. Acid rain can severely damage both plant and animal life; certain lakes have lost all fish and plant life because of acid rain. The major causes of acid rain are motor vehicle emissions, industrial processes, and the burning of fossil fuels in power-stations.

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acid rain

acid rain Precipitation with a pH of less than about 5.0, which is the value produced when naturally occurring carbon dioxide, sulphate, and nitrogen oxides dissolve into cloud droplets. The effects of increased acidity on surface waters, soils, and vegetation are complex.

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acid rain

ac·id rain • n. rainfall made sufficiently acidic by atmospheric pollution that it causes environmental harm, typically to forests and lakes.

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acid rain

acid rain See acid precipitation.

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Acid Rain

Acid rain

Acid rain is the term used in the popular press that is equivalent to acidic deposition as used in the scientific literature. Acid deposition results from the deposition of airborne acidic pollutants on land and in bodies of water. These pollutants can cause damage to forests as well as to lakes and streams.

The major pollutants that cause acidic deposition are sulfur dioxide (SO2) and nitrogen oxides (NOx) produced during the combustion of fossil fuels . In the atmosphere these gases oxidize to sulfuric acid (H2SO4) and nitric acid (HNO3) that can be transported long distances before being returned to the earth dissolved in rain drops (wet deposition), deposited on the surfaces of plants as cloud droplets, or directly on plant surfaces (dry deposition ). Electrical utilities contribute 70% of the 20 million tons (21 million metric tons) of SO2 that are annually added to the atmosphere. Most of this is from the combustion of coal . Electric utilities also contribute 30% of the 19 million tons of NOx added to the atmosphere, and internal combustion engines used in automobiles, trucks, and buses contribute more than 40%. Natural sources such as forest fires, swamp gases, and volcanoes only contribute 15% of atmospheric SO2. Forest fires, lightning, and microbial processes in soils contribute about 11% to atmospheric NOx. In response to air quality regulations, electrical utilities have switched to coal with lower sulfur content and installed scrubbing systems to remove SO2. This has resulted in a steady decrease in SO2 emissions in the United States since 1970, with a 1820% decrease between 1975 and 1988. Emissions of NOx have also decreased from the peak in 1975, with a 915% decrease from 1975 to 1988.

A commonly used indicator of the intensity of acid rain is the pH of this rainfall. The pH of non-polluted rainfall in forested regions is in the range 5.05.6. The upper limit is 5.6, not neutral (7.0), because of carbonic acid that results from the dissolution of atmospheric carbon dioxide . The contribution of naturally occurring nitric and sulfuric acid, as well as organic acids, reduces the pH somewhat to less than 5.6. In arid and semi-arid regions, rainfall pH values can be greater than 5.6 due the effect of alkaline soil dust in the air. Nitric and sulfuric acids in acidic rainfall (wet deposition) can result in pH values for individual rainfall events of less than 4.0.

In North America, the lowest acid rainfall is in the northeastern United States and southeastern Canada. The lowest mean pH in this region is 4.15. Even lower pH values are observed in central and northern Europe. Generally, the greater the population density and density of industrialization the lower the rainfall pH. Long distance transport, however, can result in low pH rainfall even in areas with low population and low density of industries, as in parts of New England, eastern Canada, and in Scandinavia.

A very significant portion of acid deposition occurs in the dry form. In the United States, it is estimated that 3060% of acidic deposition occurs as dry fall. This material is deposited as sulfur dioxide gas and very finely divided particles (aerosols) directly on the surfaces of plants (needles and leaves). The rate of deposition depends not only on the concentration of acid materials suspended in the air, but on the nature and density of plant surfaces exposed to the atmosphere and the atmospheric conditions(e.g., wind speed and humidity).

Direct deposition of acid cloud droplets can be very important especially in some high altitude forests. Acid cloud droplets can have acid concentrations of five to 20 times that in wet deposition. In some high elevation sites that are frequently shrouded in clouds, direct droplet deposition is three times that of wet deposition from rainfall.

Acid deposition has the potential to adversely affect sensitive forests as well as lakes and streams. Agriculture is generally not included in the assessment of the effects of acidic deposition because experimental evidence indicates that even the most severe episodes of acid deposition do not adversely affect the growth of agricultural crops, and any long-term soil acidification can readily be managed by addition of agricultural lime. In fact, the acidifying potential of the fertilizers normally added to cropland is much greater than that of acidic deposition. In forests, however, long-term acidic deposition on sensitive soils can result in the depletion of important nutrient elements (e.g., calcium, magnesium, and potassium) and in soil acidification. Also, acidic pollutants can interact with other pollutants (e.g., ozone ) to cause more immediate problems for tree growth. Acid deposition can also result in the acidification of sensitive lakes and with the loss of biological productivity.

Long-term exposure of acid sensitive materials used in building construction and in monuments (e.g., zinc, marble, limestone, and some sandstone) can result in surface corrosion and deterioration. Monuments tend to be the most vulnerable because they are usually not as protected from rainfall as most building materials. Good data on the impact of acidic deposition on monuments and building material is lacking.

Nutrient depletion due to acid deposition on sensitive soils is a long-term (decades to centuries) consequence of acidic deposition. Acidic deposition greatly accelerates the very slow depletion of soil nutrients due to natural weathering processes. Soils that contain less plant-available calcium, magnesium and potassium are less buffered with respect to degradation due to acidic deposition. The most sensitive soils are shallow sandy soils over hard bedrock. The least vulnerable soils are the deep clay soils that are highly buffered against changes due to acidic deposition.

The more immediate possible threat to forests is the forest decline phenomenon that has been observed in forests in northern Europe and North America. Acidic deposition in combination with other stress factors such as ozone, disease and adverse weather conditions can lead to decline in forest productivity and, in certain cases, to dieback . Acid deposition alone cannot account for the observed forest decline, and acid deposition probably plays a minor role in the areas where forest decline has occurred. Ozone is a much more serious threat to forests, and it is a key factor in the decline of forests in the Sierra Nevada and San Bernardino mountains in California.

The greatest concern for adverse effects of acidic deposition is the decline in biological productivity in lakes. When a lake has a pH less than 6.0, several species of minnows, as well as other species that are part of the food chain for many fish, cannot survive. At pH values less than about 5.3, lake trout, walleye, and smallmouth bass cannot survive. At pH less than about 4.5, most fish cannot survive (largemouth bass are an exception).

Many small lakes are naturally acidic due to organic acids produced in acid soils and acid bogs. These lakes have chemistries dominated by organic acids, and many have brown colored waters due to the organic acid content. These lakes can be distinguished from lakes acidified by acidic deposition, because lakes strongly affected by acidic deposition are dominated by sulfate.

Lakes that are adversely affected by acidic deposition tend to be in steep terrain with thin soils. In these settings the path of rainwater movement into a lake is not influenced greatly by soil materials. This contrasts to most lakes where much of the water that collects in a lake flows first into the groundwater before entering the lake via subsurface flow. Due to the contact with soil materials, acidity is neutralized and the capacity to neutralize acidity is added to the water in the form of bicarbonate ions (bicarbonate alkalinity). If more than 5% of the water that reaches a lake is in the form of groundwater, a lake is not sensitive to acid deposition.

An estimated 24% of the lakes in the Adirondack region of New York are devoid of fish. In one third to one half of these lakes this is due to acidic deposition. Approximately 16% of the lakes in this region may have lost one or more species of fish due to acidification. In Ontario, Canada, 115 lakes are estimated to have lost populations of lake trout. Acidification of lakes, by acidic deposition, extends as far west as Upper Michigan and northeastern Wisconsin, where many sensitive lakes occur and there is some evidence for acidification. However, the extent of acidification is quite limited.

[Paul R. Bloom ]


RESOURCES

BOOKS

Bresser, A. H., ed. Acid Precipitation. New York: Springer-Verlag, 1990.

Mellanby, K., ed. Air Pollution, Acid Rain and the Environment. New York: Elsevier, 1989.

Turck, M. Acid Rain. New York: Macmillan, 1990.

Wellburn, A. Air Pollution and Acid Rain: The Biological Impact. New York: Wiley, 1988.

Young, P. M. Acidic Deposition: State of Science and Technology. Summary Report of the U. S. National Acid Precipitation Program. Washington, DC: U. S. Government Printing Office, 1991.

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Acid rain

Acid rain

Atmospheric deposition

Chemistry of precipitation

Dry deposition of acidifying substances

Resources

Acid rain is a popularly used phrase that refers to the deposition of acidifying substances from the atmosphere and the environmental damage that this causes. Acid rain became a prominent issue around 1970, and since then research has demonstrated that the deposition of atmospheric chemicals is causing widespread acidification of lakes, streams, and soil. The resulting biological effects include the disruption or even localized extinction of many populations of plants and fish.

Atmospheric deposition

Strictly speaking, the term acid rain should only refer to rainfall. However, acidification is not just caused by acidic rain, but also by chemicals in snow and fog and by inputs of gases and particulates when precipitation is not occurring.

Of the many chemicals that are deposited from the atmosphere, the most important in terms of causing acidity in soil and surface waters (such as lakes and streams) are dilute solutions of sulfuric and nitric acids (H2 SO4 and HNO3, respectively) deposited as acidic rain or snow; gases that include sulfur dioxide (SO2) and oxides of nitrogen (NO and NO2, together

called NOx); and, tiny particulates such as ammonium sulfate ([NH4]2 SO4) and ammonium nitrate (NH4NO3).

Depositions of the gases and particulates primarily occur when it is not raining or snowing and so are known as dry deposition. Large regions of Europe and North America are exposed to these acidifying depositions. However, only certain types of ecosystems are vulnerable to becoming acidified by these atmospheric inputs. These usually have a thin cover of soil that contains little calcium and sit upon bedrock of hard minerals such as granite or quartz. Atmospheric depositions have caused an acidification of freshwater ecosystems in such areas. Many lakes, streams, and rivers have become acidic, resulting in depleted populations of some plants and animals.

Chemistry of precipitation

The acidity of an aqueous solution is measured as its concentration of hydrogen ions (H+). The pH scale expresses this concentration in logarithmic units, ranging from very acidic solutions of pH 0, through the neutral value of pH 7, to very alkaline (or basic) solutions of pH 14. It is important to recognize that a one-unit difference in pH (for example, from pH 3 to pH 4) represents a ten-fold difference in the concentration of hydrogen ions.

Large regions are affected by acidic precipitation in North America, Europe, and elsewhere. A relatively small region of eastern North America is known to have experienced acidic precipitation before 1955, but this has since expanded so that most of the eastern United States and southeastern Canada is now affected.

Interestingly, the acidity of precipitation is not usually greater close to large point-sources of emission of important gaseous precursors of acidity, such as smelters or power plants that emit SO2 and NOx. This observation emphasizes the fact that acid rain is a regional phenomenon, not a local one. For instance, the acidity of precipitation is not appreciably influenced by distance from the worlds largest point-source of SO2 emissions, a smelter in Sudbury, Ontario. Furthermore, when that smelter was temporarily shut down by a labor dispute, the precipitation averaged pH 4.49, not significantly different from the pH 4.52 when there were large emissions of SO2.

Dry deposition of acidifying substances

Dry deposition occurs in the intervals of time between precipitation events. Dry deposition includes inputs of tiny particulates from the atmosphere, as well as the uptake of gaseous SO2 and NOx by plants, soil, and water. Unlike wet deposition, the rates of dry deposition can be much larger close to point-sources of emission, compared with further away.

Once they are dry deposited, certain chemicals can generate important quantities of acidity when they are chemically transformed in the receiving ecosystem. For example, SO2 gas can dissolve into the water of lakes or streams, or it can be absorbed by the foliage of plants. This dry-deposited SO2 is then oxidized to SO42, which is electrochemically balanced by H+, so that acidity results. Dry-deposited NOx gas can similarly be oxidized to NO3 and also balanced by H+.

In relatively polluted environments close to emissions sources, the total input of acidifying substances (i.e., wet and dry depositions) is dominated by the dry deposition of acidic substances and their acid-forming precursors. The dry deposition is mostly associated with gaseous SO2 and NOx, because wet deposition is little influenced by distance from sources of emission.

For example, within a 25 mi (40 km) radius of the large smelter at Sudbury, about 55% of the total input of sulfur from the atmosphere is due to dry deposition, especially SO2. However, less than 1% of the SO 2 emission from the smelter is deposited in that area, because the tall smokestack is so effective at widely dispersing the emissions.

Because they have such a large surface area of foliage and bark, forests are especially effective at absorbing atmospheric gases and particles. Consequently, dry inputs accounted for about 33% of the total sulfur deposition to a hardwood forest in New Hampshire, 5663% of the inputs of S and N to a hardwood forest in Tennessee, and 55% of their inputs to a conifer forest in Sweden.

In any forest, leaves and bark are usually the first surfaces encountered by precipitation. Most rainwater penetrates the foliar canopy and then reaches the forest floor as so-called throughfall, while a smaller amount runs down tree trunks as stemflow. Throughfall and stemflow have a different chemistry than the original precipitation. Because potassium is easily leached out of leaves, its concentration is especially changed. In a study of several types of forest in Nova Scotia, the concentration of potassium (K+) was about 10 times larger in throughfall and stemflow than in rain, while calcium (Ca2+) and magnesium (Mg2+) were three to four times more concentrated. There was less of a change in the concentration of H+; the rainwater pH was 4.4, but in throughfall and stemflow of hardwood stands pH averaged 4.7, and it was 4.44.5 in conifer stands. The decreases in acidity were associated with ion-exchange reactions occurring on foliage and bark surfaces, in which H+ is removed from solution in exchange for Ca2+, Mg2+, and K+. Overall, the consumption of hydrogen ions accounted for 4266% of the input of H+ by precipitation to these forests.

In areas polluted by SO2 there can be large increases in the sulfate concentration of throughfall and stemflow, compared with ambient precipitation. This is caused by previously dry-deposited SO2 and SO4 washing off the canopy. At Hubbard Brook this SO4 enhancement is about four times larger than ambient precipitation, while in central Germany it is about two to three times greater. These are both regions with relatively large concentrations of particulate SO4 and gaseous SO2 in the atmosphere.

Once precipitation reaches the forest floor, it percolates into the soil. Important chemical changes take place as microbes and plants selectively absorb, release, and metabolize chemicals; as ions are exchanged at the surfaces of particles of clay and organic matter; as minerals are made soluble by so-called acid-weathering reactions; and as secondary minerals such as certain clays and metal oxides are formed through chemical precipitation of soluble ions of aluminum, iron, and other metals. These various chemical changes can contribute to: soil acidification, the leaching of important chemicals such as calcium and magnesium, and the mobilization of toxic ions of aluminum, especially Al3+. These are all natural, closely-linked processes, occurring wherever there is well-established vegetation, and where water inputs by precipitation are greater than evapotranspiration (i.e., evaporation from vegetation and non-living surfaces). A potential influence of acid rain is to increase the rates of some of these processes, such as the leaching of toxic H+ and Al3+ to lakes and other surface waters.

Some of these effects have been examined by experiments in which simulated rainwater of various pHs was added to soil contained in plastic tubes. These experiments have shown that very acidic solutions can cause a number of effects including soil acidification; decreased soil fertility due to increased leaching of calcium, magnesium, and potassium ions from the soil; increased solubilization of toxic ions of metals such as aluminum, iron, manganese, lead, and zinc; and, loss of sulfate.

Soil acidification can occur naturally. This fact can be illustrated by studies of ecological succession on newly exposed parent materials of soil. At Glacier Bay, Alaska, the melting of glaciers exposes a mineral substrate with a pH of about 8.0, with up to 7-10% carbonate minerals. As this material is colonized and modified by vegetation and climate, its acidity increases, reaching about pH 4.8 after 70 years when a conifer forest is established. Accompanying this acidification is a reduction of carbonates to less than 1%, caused by leaching and uptake by plants.

Several studies have attempted to determine whether naturally occurring soil acidification has been intensified as a result of acid rain and associated atmospheric depositions. So far, there is no conclusive evidence that this has occurred on a wide scale. It appears that soil acidification is a potential, longer-term risk associated with acid rain.

Compared with the water of precipitation, that of lakes, ponds, streams, and rivers is relatively concentrated in ions, especially in calcium, magnesium, potassium, sodium, sulfate, and chloride. These chemicals have been mobilized from the terrestrial part of the watersheds of the surface waters. In addition, some surface waters are colored brown because of their high concentrations of dissolved organic compounds, usually leached out of nearby bogs. Brown-water lakes are often naturally acidic, with a pH of about 4 to 5.

Seasonal variations in the chemistry of surface waters are important. Where a snowpack accumulates, meltwater in the springtime can be quite acidic. This happens because soils are frozen and/or saturated during snowmelt, so there is little possibility to neutralize the acidity of meltwater. So-called acid shock events in streams have been linked to the first melt-waters of the snowpack, which are generally more acidic than later fractions.

A widespread acidification of weakly-buffered waters has affected the northeastern United States, eastern Canada, Scandinavia, and elsewhere. In 1941, for example, the average pH of 21 lakes in central Norway was 7.5, but only 5.4-6.3 in the 1970s. Before 1950 the average pH of 14 Swedish water bodies was 6.6, but 5.5 in 1971. In New Yorks Adirondack Mountains, 4% of 320 lakes had pH less than 5 in the 1930s, compared with 51% of 217 lakes in that area in 1975 (90% were also devoid of fish). The Environmental Protection Agency sampled a large number of lakes and streams in the United States in the early 1990s. Out of 10,400 lakes, 11% were acidic, mostly in the eastern United States. Atmospheric deposition was attributed as the cause of acidification of 75% of the lakes, while 3% had been affected by acidic drainage from coal mines and 22% by organic acids from bogs. Of the 4,670 streams considered acidic, 47% had been acidified by atmospheric deposition, 26% by acid-mine drainage, and 27% by bogs.

Surface waters that are vulnerable to acidification generally have a small acid-neutralizing capacity. Usually, H+ is absorbed until a buffering threshold is exceeded, and there is then a rapid decrease in pH until another buffering system comes into play. Within the pH range of 6 to 8, bicarbonate alkalinity is the natural buffering system that can be depleted by acidic deposition. The amount of bicarbonate in water is determined by geochemical factors, especially the presence of mineral carbonates such as calcite (CaCO3) or dolomite (CaMgCO3) in the soil, bedrock, or aquatic sediment of the watershed. Small pockets of these minerals are sufficient to supply enough acid-neutralizing capacity to prevent acidification, even in regions where acid rain is severe. In contrast, where bedrock, soil, and sediment are composed of hard minerals such as granite and quartz, the acid-neutralizing capacity is small and acidification can occur readily. Vulnerable watersheds have little alkalinity and are subject to large depositions of acidifying substances; these are especially common in glaciated regions of eastern North America and Scandinavia and at high altitude in more southern mountains (such as the Appalachians) where crustal granite has been exposed by erosion.

High-altitude, headwater lakes and streams are often at risk because they usually have small watersheds. Because there is little opportunity for rainwater to interact with the thin soil and bedrock typical of headwater systems, little of the acidity of precipitation is neutralized before it reaches surface water.

Acidification of freshwaters can be described as a titration of a dilute bicarbonate solution with sulfuric and nitric acids derived from atmospheric deposition. In waters with little alkalinity, and where the watershed provides large fluxes of sulfate accompanied by hydrogen and aluminum ions, the body of water is vulnerable to acidification.

Few studies have demonstrated injury to terrestrial plants caused by an exposure to ambient acid rain. Although many experiments have demonstrated injury to plants after treatment with artificial acid rain solutions, the toxic thresholds are usually at substantially more acidic pHs than normally occur in nature.

For example, some Norwegian experiments involved treating young forests with simulated acid rain. Lodgepole pine watered for three years grew 15-20% more quickly at pHs 4 and 3, compared with a control treatment of pH 5.6-6.1. The height growth of spruce was not affected over the pH range 5.6 to 2.5, while Scotch pine was stimulated by up to 15% at pHs of 2.5 to 3.0, compared with pH 5.6-6.1. Birch trees were also stimulated by the acid treatments. However, the feather mosses that dominated the ground vegetation were negatively affected by acid treatments.

Because laboratory experiments can be well controlled, they are useful for the determination of dose-response effects of acidic solutions on plants. In general, growth reductions are not observed unless treatment the pH is more acidic than about 3.0, and some species are stimulated by a more acidic pH than this. In one experiment, the growth of white pine seedlings was greater after treatment at pH levels from 2.3 to 4.0 than at pH 5.6. In another experiment, seedlings of 11 tree species were treated over the pH range of 2.6 to 5.6. Injuries to foliage occurred at pH 2.6, but only after a week of treatment with this very acidic pH.

Overall, it appears that trees and other vascular plants are rather tolerant of acidic rain, and they may not be at risk of suffering direct, short-term injury from ambient acidic precipitation. It remains possible, however, that even in the absence of obvious injuries, stresses associated with acid rain could decrease plant growth. Because acid rain is regional in character, these yield decreases could occur over large areas, and this would have important economic implications. This potential problem is most relevant to forests and other natural vegetation. This is because agricultural land is regularly treated with liming agents to reduce soil acidity, and because acid production by cropping and fertilization is much larger than that caused by atmospheric depositions.

Studies in western Europe and eastern North America have examined the possible effects of acid rain on forest productivity. Recent decreases in productivity have been shown for various tree species and in various areas. However, progressive decreases in productivity are natural as the canopy closes and competition intensifies in developing forests. So far, research has not separated clear effects of regional acid rain from those caused by ecological succession, insect defoliation, or climate change.

The community of microscopic algae (or phytoplankton) of lakes is quite diverse in species. Non-acidic, oligotrophic (i.e., unproductive) lakes in a temperate climate are usually dominated by golden-brown algae and diatoms, while acidic lakes are typically dominated by dinoflagellates, cryptomonads, and green algae.

An important experiment was performed in a remote lake in Ontario, in which sulfuric acid was added to slowly acidify the entire lake, ultimately to about pH 5.0 from the original pH of 6.5. During this whole-lake acidification, the phytoplankton community changed from an initial domination by golden-brown algae to dominance by green algae. There was no change in the total number of species, but there was a small increase in algal biomass after acidification because of an increased clarity of the water.

In some acidified lakes, the abundance of larger plants (called macrophytes) has decreased, sometimes accompanied by increased abundance of a moss known as Sphagnum. In itself, proliferation of Sphagnum can cause acidification, because these plants efficiently remove cations from the water in exchange for H+, and their mats interfere with acid neutralizing processes in the sediment.

Zooplankton are small crustaceans living in the water column of lakes. These animals can be affected by acidification through the toxicity of H+ and associated metals ions, especially Al3+; changes in their phytoplankton food; and changes in predation, especially if plankton-eating fish become extirpated by acidification. Surveys have demonstrated that some zooplankton species are sensitive to acidity, while others are more tolerant.

Fish are the best-known victims of acidification. Loss of populations of trout, salmon, and other species has occurred in many acidified freshwaters. A survey of 700 Norwegian lakes, for example, found that brown trout were absent from 40% of the water bodies and sparse in another 40%, even though almost all of the lakes had supported healthy fish populations prior to the 1950s. Surveys during the 1930s in the Adirondack Mountains of New York found brook trout in 82% of the lakes. However, in the 1970s fish did not occur in 43% of 215 lakes in the same area, including 26 definite extirpations of brook trout in resurveyed lakes. This dramatic change paralleled the known acidification of these lakes. Other studies documented the loss of fish populations from lakes in the Killarney region of Ontario, where there are known extirpations of lake trout in 17 lakes, while small-mouth bass have disappeared from 12 lakes, large-mouth bass and walleye from four, and yellow perch and rock bass from two.

Many studies have been made of the physiological effects of acidification on fish. Younger life-history stages are generally more sensitive than adults, and most losses of fish populations can be attributed to reproductive failure, rather than mortality of adults (although adults have sometimes been killed by acid-shock episodes in the springtime).

There are large increases in concentration of certain toxic metals in acidic waters, most notably ions of aluminum. In many acidic waters aluminum ions can be sufficient to kill fish, regardless of any direct effect of H+. In general, survival and growth of larvae and older stages of fish are reduced if dissolved aluminum concentrations are larger than 0.1 ppm, an exposure regularly exceeded in acidic waters. The most toxic ions of aluminum are Al3+ and AlOH2+.

Although direct effects of acidification on aquatic birds have not been demonstrated, changes in their habitat could indirectly affect their populations. Losses of fish populations would be detrimental to fish-eating waterbirds such as loons, mergansers, and osprey. In contrast, an increased abundance of aquatic insects and zooplankton, resulting from decreased predation by fish, could be beneficial to diving ducks such as common goldeneye and hooded merganser, and to dabbling ducks such as the mallard and black duck.

Fishery biologists especially are interested in liming acidic lakes to create habitat for sport fish. Usually, acidic waters are treated by adding limestone (CaCO3) or lime (Ca[OH]2), a process analogous to a whole-lake titration to raise pH. In some parts of Scandinavia liming has been used extensively to mitigate the biological damages of acidification. By 1988 about 5,000 water bodies had been limed in Sweden, mostly with limestone, along with another several hundred lakes in southern Norway. In the early 1980s there was a program to lime 800 acidic lakes in the Adirondack region of New York.

Although liming rapidly decreases the acidity of a lake, the water later re-acidifies at a rate determined by size of the drainage basin, the rate of flushing of the lake, and continued atmospheric inputs. Therefore, small headwater lakes have to be re-limed more frequently. In addition, liming initially stresses the acid-adapted biota of the lake, causing changes in species dominance until a new, steady-state ecosystem is achieved. It is important to recognize that liming is a temporary management strategy, and not a long-term solution to acidification.

Neutralization of acidic ecosystems treats the symptoms, but not the sources of acidification. Clearly, large reductions in emissions of the acid-forming gases SO2 and NOx are the ultimate solution to this widespread environmental problem. However, there is controversy over the amount that the emissions must be reduced in order to alleviate acidic deposition and about how to pursue the reduction of emissions. For example, should large point sources such as power plants and smelters be targeted, with less attention paid to smaller sources such as automobiles and residential furnaces? Not surprisingly, industries and regions that are copious emitters of these gases lobby against emission controls, which they argue do not have adequate scientific justification.

In spite of many uncertainties about the causes and magnitudes of the damage associated with acid rain and related atmospheric depositions, it is intuitively clear that what goes up (that is, the acid-precursor gases) must come down (as acidifying depositions). This common-sense notion is supported by a great deal of scientific evidence, and, because of public awareness and concerns about acid rain in many countries, politicians have began to act more effectively. Emissions of sulfur dioxide and nitrogen oxides are being reduced, especially in western Europe and North America. For example, in 1992 the governments of the

KEY TERMS

Acid mine drainage Surface water or groundwater that has been acidified by the oxidation of pyrite and other reduced-sulfur minerals that occur in coal and metal mines and their wastes.

Acid shock A short-term event of great acidity. This phenomenon regularly occurs in freshwater systems that receive intense pulses of acidic water when an accumulated snowpack melts rapidly in the spring.

Acidic rain (acidic precipitation) (1) Rain, snow, sleet, or fog water having a pH less than 5.65. (2) The deposition of acidifying substances from the atmosphere during a precipitation event.

Acidification An increase over time in the content of acidity in a system, accompanied by a decrease in acid-neutralizing capacity.

Acidifying substance Any substance that causes acidification. The substance may have an acidic character and therefore act directly, or it may initially be non-acidic but generate acidity as a result of its chemical transformation, as happens when ammonium is nitrified to nitrate, and when sulfides are oxidized to sulfate.

Acidity The ability of a solution to neutralize an input of hydroxide ion (OH). Acidity is usually measured as the concentration of hydrogen ion (H+), in logarithmic pH units (see also pH). Strictly speaking, an acidic solution has a pH less than 7.0.

Acidophilous Refers to organisms that only occur in acidic habitats and are tolerant of the chemical stresses of acidity.

Conservation of electrochemical neutrality Refers to an aqueous solution, in which the number of cation equivalents equals the number of anion equivalents, so that the solution does not have a net electrical charge.

Equivalent Abbreviation for mole-equivalent, calculated as the molecular or atomic weight multiplied by the number of charges of the ion. Equivalent units are necessary for a charge-balance calculation, related to the conservation of electrochemical neutrality (above).

Leaching The movement of dissolved chemicals with water percolating through soil.

pH The negative logarithm to the base 10 of the aqueous concentration of hydrogen ions in units of moles per liter. An acidic solution has pH less than 7, while an alkaline solution has pH greater than 7. Note that a one-unit difference in pH implies a ten-fold difference in the concentration of hydrogen ions.

United States and Canada signed an air-quality agreement aimed at reducing acidifying depositions in both countries. This agreement called for large expenditures by government and industry to achieve substantial reductions in the emissions of air pollutants during the 1990s. Eventually, these actions should improve environmental conditions related to damage caused by acid rain, but as of 2006 no long-term studies have definitively attributed specific changes (generally reductions in acid rain levels) to policy changes rather than improved technologies that also reduce levels of acidic rain.

So far, the actions to reduce emissions of the precursor gases of acidifying deposition have only been vigorous in western Europe and North America. Actions are also needed in other, less wealthy regions where the political focus is on industrial growth and not on control of air pollution and other environmental damages that are used to subsidize that growth. In the coming years, much more attention will have to be paid to acid rain and other pollution problems in eastern Europe, Russia, China, India, southeast Asia, Mexico, and other so-called developing nations. Emissions of air pollutants are rampant in these places, and are increasing rapidly.

A concept that has gained popularity since the 1990s is referred to as emissions trading. In this scheme, a polluting installation essentially pays a license to emit pollution. However, this sum is reimbursed if the pollution is cut or stopped. Thus, there is an economic incentive to reduce emissions.

See also Forests; Sulfur dioxide.

Resources

BOOKS

Morgan, Sally. Acid Rain. London: Watts Publishing Group, 2005.

Petheram, Louise. Acid Rain (Our Planet in Peril). Mankato, MI: Capstone Press, 2006.

Slade, John. Acid Rain, Acid Snow. Woodgate, NY: Woodgate International, 2001.

PERIODICALS

Galloway, James N. Acidification of the World: Natural and Anthropogenic. Water, Air, and Soil Pollution 130, no. 1-4 (2001): 17-24.

Krajick, K. Acid Rain: Long-term Data Show Lingering Effects from Acid Rain. Science 292, no. 5515 (2001): 195-196.

OTHER

The United Nations. The Conference and Kyoto Protocol, homepage (accessed October 29, 2006). <http://unfccc.int/resource/convkp.html>.

United Stated Geological Survey. What is Acid Rain? (accessed October 29, 2006). <http://pubs.usgs.gov/gip/acidrain/2.html>.

Bill Freedman

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Acid Rain

Acid Rain

Introduction

Rain is normally slightly acidic due to the presence of some carbon dioxide in the atmosphere. That normal acidity is increased significantly with the addition of sulfur and nitrogen oxides from burning fossil fuels. The amounts of these compounds entering the atmosphere as emissions from the burning of fossil fuels far exceed the amount released by natural sources into the atmosphere. Winds carry the damaging substances to regions far from their source, where the acid rain damages the ecosystem and corrodes buildings.

Historical Background and Scientific Foundations

Sulfur and nitrogen oxides released by the burning of fossil fuels react with water in the atmosphere to form strong acids. The acidity of solutions is measured in the

pH scale. The pH scale goes from 0 to 14; a pH of 7 is neutral. Any solution with a pH above 7 is described as basic, while a solution with a pH below 7 is acidic. The lower the pH number, the more acidic the solution. With the normal solubility of carbon dioxide in atmospheric water, a mildly acidic, unpolluted rainfall has a pH of 5.6. In the regions that are seriously impacted by acid rain from the sulfur and nitrogen oxides in the atmosphere, the precipitation may have a pH of 4.1 to 5.1.

In the early 1970s, a serious depletion of aquatic life was observed in water bodies in Scandinavia, Scotland, northern England, the northeastern United States, and Quebec, Canada. The cause of the problem was found to be high levels of acidity in these ecosystems. Scientists soon traced the source of this increased acidity to acid rain. The damage was related to air pollution from industrial development long distances down wind from the freshwater ecosystems affected. For example, industries located in Central Europe were found to be the cause of acid rain damage in Scandinavia.

Studies of lakes near major industrial areas showed that they were not as seriously impacted as those far away in the direction of the prevailing weather systems. By the 1980s, the effects of acid rain were also being observed in the forests of Germany. The impact of acid rain continued to be seen in the northern regions where aquatic ecosystems were already in trouble. By this time, salmon were extinct in some regions. Salmon are very sensitive to changes in pH and will die if the pH falls to 5.5 or below.

The way in which acid rain damages forests is complex and multi-faceted. It starts with soils that are too acidic. Important nutrients in the soil disappear as soil acidity increases. This increase in soil acidity is followed by the release of a soluble form of aluminum, which is very harmful to vegetation, into the soil. Especially at higher elevations, forests also can be damaged when acidic precipitation falls directly on tree needles or leaves.

Impacts and Issues

The ecological damage that results from acid rain was recognized as an international problem that required international cooperation to develop solutions. Since 1986, the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests has studied the impact of the release of sulfur and nitrogen oxides into the atmosphere on forests. Thirty-nine European countries have participated in these studies in close cooperation with the European Commission.

The European program has been extended to include the effects of climate change and carbon sequestering.

There has been some increase in nitrogen oxide, but also a great increase in the amount of carbon dioxide that is entering the atmosphere from the use of gasoline in vehicles for transportation and from industrial development. What started out as a program to address the acid rain problem has expanded to include consideration of the adverse environmental effects of all polluting gases.

WORDS TO KNOW

ACID: Substance that when dissolved in water is capable of reacting with a base to form salts and release hydrogen ions.

BASE: One of the four chemicals (nitrogenous bases) that are a part of the deoxyribonucleic (DNA) molcule. The sequence of the bases determines the genetic code (the information contained within DNA). The first letters of each base provide the alphabet for the DNA code (A, C, G, and T [short for adenine, cytosine, guanine, and thymine]). Adenine and guanine are also classified as purine bases and are found in both DNA and RNA. Cytosine and thymine are classified as pyrimidine bases and while cytosine also occurs both in DNA and RNA (Ribonucleic acid), in RNA molecules another base (uracil) substitutes for the thymine found in DNA molecules. The bases bind together in very restricted and specific ways to form base pairs between DNA strands. In DNA, adenine always bonds with thymine (A-T bond) on the opposite strand and cytosine always bonds with guanine to form a (C-G) base pair between stands of the DNA helix. In electronics a base is also the middle slice of a transistor.

CARBON SEQUESTERING: Storage or fixation of carbon in such a way that it is isolated from the atmosphere and cannot contribute to climate change. Sequestration may occur naturally (e.g., forest growth) or artificially (e.g., injection of CO2 into underground reservoirs).

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

PH: Measures the acidity of a solution. It is the negative log of the concentration of the hydrogen ions in a substance.

IN CONTEXT: ACID RAIN AND THE PH SCALE

The scale that is used to measure the acidity of a substance is called the pH scale. The pH scale runs from 0 to 14. If a material has a pH of 7, it is neutral, meaning that it is neither acidic nor alkaline (basic). Substances with pH values less than 7 are acidic and substances with pH values greater than 7 are alkaline. Distilled water is neutral, with a pH of 7.

The most important compounds that produce acids in the atmosphere are sulfur dioxide (SO2) and nitrogen oxides, like nitrogen oxide (NO) and nitrogen dioxide (NO2). These compounds combine with water in the atmosphere to form sulfuric acid (H2SO4), and nitric acid (HNO3).

When describing acid rain, scientists actually refer to a more precise term, acid deposition. Scientists distinguish between two types of acid deposition: wet and dry. Dry deposition includes acidic gases and solid particles containing sulfuric and nitric acid that settle out of the air and land on the ground or other surfaces. Dry deposition usually occurs very close to the point where the air pollution occurs. Wet deposition occurs when precipitation—such as rain, sleet, fog, or snow—becomes acidic and falls to the ground. Wet deposition can occur hundreds of miles from the place where the air pollution originates.

Some carbon dioxide is normal and necessary in the atmosphere, but excess carbon dioxide is a greenhouse gas that is contributing to global warming. Regulations have been passed to reduce the emissions of sulfur and nitrogen oxides and also to capture, or sequester, some of the extra carbon dioxide that is considered to be a prime offender in the issues of climate change beyond acid rain.

See Also Climate Change; Climate Engineering; Environmental Policy; Environmental Pollution; Greenhouse Gases.

BIBLIOGRAPHY

Periodicals

Sliggers, Johan, and Willem Kakebeeke, eds. “Clearing the Air: 25 Years of the Convention on Long-range Transboundary Air Pollution.” New York: United Nations, 2005.

Web Sites

“Convention on Long-range Transboundary Air Pollution.” United Nations Economic Commission for Europe. < http://www.unece.org/env/lrtap/> (accessed August 16, 2007).

“What Is Acid Rain?” U.S. Environmental Protection Agency, 2007 < http://www.epa.gov/acidrain/what/index.html> (accessed August 16, 2007).

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Acid Rain

Acid Rain

Introduction

Rain is normally slightly acidic due to the presence of carbon dioxide in the atmosphere. This normal acidity is increased significantly with the addition of sulfur and nitrogen oxides from burning fossil fuels. The amounts of these compounds entering the atmosphere as emissions from the burning of fossil fuels far exceeds the amount released by natural sources into the atmosphere. Winds carry these damaging substances to regions far from their source, where acid rain damages ecosystems and corrodes buildings.

Historical Background and Scientific Foundations

Sulfur and nitrogen oxides released by the burning of fossil fuels react with water in the atmosphere to form strong acids. The acidity of water solutions is expressed using the pH scale. The pH scale goes from 0 to 14. Any solution with a pH above 7 is described as basic; any solution with a pH below 7 is acidic; a pH of 7 is neutral. The smaller the pH number, the more acidic the solution.

As carbon dioxide (CO2) is a normal component of the atmosphere, and dissolving carbon dioxide in water produces carbonic acid, normal rainfall is weakly acidic. Given the normal solubility of CO2 in atmospheric water, a mildly acidic, unpolluted rainfall has a pH of 5.6. In the regions that are seriously impacted by acid rain from the sulfur and nitrogen oxides in the atmosphere, the precipitation may have a pH of 4.1-5.1, which is significantly more acidic than the natural norm.

In the early 1970s, serious loss of aquatic life was observed in water bodies in Scandinavia, Scotland, northern England, the northeastern United States, and Quebec, Canada. The cause of the problem was found to be high levels of acidity in these ecosystems. Scientists soon traced the source of this increased acidity to acid rain. The damage was related to air pollution from industrial development long distances down wind from the freshwater ecosystems affected. For example, industries located in Central Europe were found to be the cause of acid rain damage in Scandinavia.

Studies of lakes near major industrial areas showed that they were not as seriously impacted as those far away in the direction of the prevailing weather systems. By the 1980s, the effects of acid rain were also being observed in the forests of Germany. The impact of acid rain continued to be seen in the northern regions where aquatic ecosystems were already in trouble. By this time, salmon were extinct in some regions. Salmon are sensitive to changes in pH and will die if the pH falls to 5.5 or below.

The way that acid rain damages forests is complex. Damage occurs first in soils that are already acidic. Important nutrients in the soil disappear as soil acidity increases. This increase in soil acidity is followed by the release of a soluble form of aluminum, which is very harmful to vegetation, into the soil. Especially at higher elevations, forests also can be damaged when acidic precipitation falls directly on tree needles or leaves.

Impacts and Issues

The ecological damage that results from acid rain was recognized as an international problem that required international cooperation to develop solutions. Since 1986, the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests has studied the impact of the release of sulfur and nitrogen oxides into the atmosphere on forests. As of 2007, 41 European countries were participating in these studies in cooperation with the European Commission.

The European program has been extended to include the effects of climate change and carbon sequestering. There has been some increase in nitrogen oxide,

WORDS TO KNOW

ACID: A substance that when dissolved in water is capable of reacting with a base to form a salt.

CARBON SEQUESTERING: Storage or fixation of carbon in such a way that it is isolated from the atmosphere and cannot contribute to climate change.

FOSSIL FUELS: Non-renewable fuels formed by biological processes and transformed into solid or fluid minerals over geologic time. Fossil fuels include coal, petroleum, and natural gas.

GREENHOUSE GASES: Gases whose accumulation in the atmosphere increase heat retention.

pH: The measure of the amount of dissolved hydrogen ions in solution.

but also a great increase in the amount of carbon dioxide that is entering the atmosphere from deforestation and the burning of fossil fuels (for example, the burning of gasoline in vehicles). What started out as a program to address the acid rain problem has expanded to include consideration of the adverse environmental effects of all polluting gases. Acid rain and the pollutants that cause it are not an isolated problem.

Some carbon dioxide (CO2) is normal in the atmosphere, creating a greenhouse effect that prevents Earth from being frozen over. However, increasing the amount of CO2 in the atmosphere warms the global climate further, producing global warming. Regulations have been passed to reduce the emissions of sulfur and nitrogen oxides that cause acid rain, but so far few efforts have been made to capture, or sequester, some of the extra CO2 that is the primary cause of global climate change. As of 2008, world emissions of CO2 continued to increase rapidly (about 3% per year).

Human industrial activities have increased the amount of CO2 in the atmosphere by over a third since the beginning of the Industrial Revolution in the late 1700s. Although acid rain can only acidify lakes, ponds, and streams—there is not enough of it to acidify the oceans—increased atmospheric CO2 has been making the world’s oceans more acidic, imperiling coral reefs and some other forms of marine life. The pH of the oceans has already been lowered by 0.1 by human-released CO2, and it may be reduced by 0.5 by the end of this century. This could make the oceans more acidic than they have likely been for several hundred thousand years.

Primary Source Connection

The following news article reports on the record settlement between the U.S. Environmental Protection Agency (EPA) and American Electric Power to decrease smokestack pollution, including sulfur dioxide, SO2 (shown in the article as S02) by 79%, and nitrogen oxides, NOx (shown as NOx), by 69%. Many legal analysts concur that this agreement will provoke other utility companies to lessen their carbon footprints as well. Many companies have already followed suit by adjusting their business plants with the proper emissions-control devices.

EPA’S RECORD SETTLEMENT WITH UTILITY COULD LEAD TO OTHER DEALS

A utility’s dramatic agreement this week to trim smokestack pollution may do more than help clear the nation’s skies. It may clear the legal logjam that has kept other

large utilities from cutting similar deals that could trigger reductions in harmful power-plant emissions.

By one estimate, US power plants could cut their emissions of pollutants linked to acid rain and smog by 20 percent.

The agreement, announced Tuesday by the US Environmental Protection Agency (EPA), sends a powerful, though not necessarily decisive, signal to other utilities, legal analysts say.

In settling an EPA lawsuit, the nation’s largest utility, American Electric Power, agreed to spend $4.6 billion to reduce its emissions of sulfur dioxide (S02) by 79 percent and nitrogen oxides (NOx) by 69 percent. The EPA called the settlement its largest pollution-enforcement victory ever.

The agreement could have an even larger impact if it persuades other big power companies to settle their own pending legal cases, legal analysts say.

“When you have an eye-popping settlement of this magnitude, it sends a strong message to other litigants that at least one player has decided to fold his tent and move on, says Lynn Bergeson, a founding partner of Bergeson and Campbell, a Washington, D.C., law firm specializing in air-pollution litigation. If an election year brings Democratic appointees to the EPA and the Department of Justice, “it may seem safer to some of these companies to deal now with the devil you know, rather than the one you don’t,” she adds.

Duke Energy and partner Cinergy, the nation’s third-and fourth-largest utilities before they merged last year, are fighting EPA pollution charges similar to the ones AEP faced. Another suit against a subsidiary of Southern Company, the nation’s second-largest utility, is also pending.

If those three utilities alone agreed to clean up their emissions as AEP now has, all four would eliminate more than 2 million tons of SO2 and NOx emissions a year—roughly one-fifth of the annual output of those pollutants from all U.S. power plants, says John Walke, clean-air program director for the Natural Resources Defense Council. “We think this deal could be even more significant down the road if the other utilities go ahead and settle.”

The utilities themselves say the AEP settlement will have no effect on their legal strategy.

“Basically, the AEP decision to settle has no impact on the Alabama Power case,” says Mike Tyndall, a spokesman for Atlanta-based Southern Company, which owns Alabama Power. “We won the lower court decision and the government has appealed and is challenging that decision.”

Indeed, the companies say they are already equipping their power plants with pollution-control equipment.

IN CONTEXT: ACID RAIN THREATENS CULTURAL TREASURES

The Taj Mahal, located in northern India, was built between 1631 and 1653 by the Mughal Emperor Shah Jahanas as a tomb for his wife. It is considered one of the great architectural treasures of the world. Some three million tourists visit the Taj Mahal each year. It is also surrounded by pollution sources, including vehicular traffic, private electrical generators (which are run to compensate for frequent grid blackouts), brick kilns, glass-making units, and iron foundries. Air pollution emitted by these industries is slowly destroying the monument, whose primary building material is white marble. Marble, like limestone, consists mostly of calcite (calcium carbonate, CO3), which is readily broken down by acids.

This makes the Taj Mahal, like all buildings and monuments built from limestone and marble, vulnerable to acid rain—precipitation in which sulfur dioxide and nitrogen oxides from burning fossil fuels have been dissolved to produce sulfuric acid and nitric acid. Although the acidity of any given raindrop is too low to inflict visible damage on a structure such as the Taj Mahal, over decades the damage accumulates.

Finely detailed carvings, such as the flowers are the first features to be damaged or destroyed. Even inside the building, where rain cannot reach, airborne sulfur dioxide can settle directly onto the stone, reacting with humidity to form a fun-guslike crust called “marble cancer.”

The government of India began monitoring atmospheric sulfur dioxide levels at the Taj Mahal in 1981. In 1996, the Indian Supreme Court ordered 292 coal-based industries inside a 4,000 square mile (10,400 square kilometer) area containing the Taj Mahal and several other historic monuments, the “Taj Trapezium Zone,” to either switch to natural gas (a low-polluting fossil fuel) or to relocate outside the zone.

“We are already installing scrubbers on the vast majority of our units,” says Tom Williams, spokesman for Duke Energy, based in Charlotte, N.C. While it’s company policy not to comment on any discussions in ongoing litigation, he adds, “We still think we have a solid defense against the government claims” in the Duke Energy and Cinergy cases.

Tuesday’s AEP deal stems from a 1999 lawsuit that the EPA filed against the company, based in Columbus, Ohio. It charged AEP with rebuilding coal-fired plants without installing pollution controls—a violation, they said, of a provision of the Clean Air Act known as New Source Review.

Companies had been reluctant to settle with the government because, under President Bush, the EPA in concert with the White House had signaled a preference for the industry-favored interpretation of the pollution rule, Mr. Walke says. Power companies have long claimed that the New Source Review does not require additional pollution controls after new plant equipment is installed, even if annual pollution goes up, as long as hourly pollution levels don’t rise.

But in June, the US Supreme Court ruled 9 to 0 in favor of a stricter interpretation of that key provision.

In a statement, AEP officials said the company had done nothing wrong and had already spent $2.6 billion since 2004 to reduce the pollutants. But for business reasons, the officials added, it was best to get the dispute resolved.

Besides cutting SO2 and NOx emissions from 16 coal-fired plants in five states, the company also agreed to pay a $15 million civil penalty as well as $60 million to help fix environmental damage to Maryland’s Chesapeake Bay and the Shenandoah National Park in Virginia.

Mark Clayton

CLAYTON, MARK. “EPA’S RECORD SETTLEMENT WITH UTILITY COULD LEAD TO OTHER DEALS.” CHRISTIAN SCIENCE MONITOR (OCTOBER 11, 2007).

See Also Air Pollution; Carbon Dioxide (CO2); Carbon Dioxide (CO2) Emissions

BIBLIOGRAPHY

Books

Sliggers, Johan, and Willem Kakebeeke, eds. Clearing the Air: 25 Years of the Convention on Long-range Transboundary Air Pollution. New York: United Nations, 2005.

Web Sites

United Nations Economic Commission for Europe. “Convention on Long-range Transboundary Air Pollution.” 2008. http://www.unece.org/env/lrtap/ (accessed March 21, 2008).

U.S. Environmental Protection Agency. “What Is Acid Rain?” 2007. http://www.epa.gov/acidrain/what/index.html (accessed August 16, 2007).

Miriam C. Nagel

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Acid Rain

ACID RAIN

"Acid rain" is a popular term that can include all forms of precipitation (as well as fog and cloudwater) that is more acidic than expected from natural causes. Measurement of precipitation acidity at several remote sites around the world show natural background levels of acidity to be around pH of 5.1 to 5.2 (8 to 6 ueq/l H + or hydrogen in concentration, respectively). This compares with present annual average values of pH 4.3 to 4.4 (50 to 40 ueq/l H +) for most of the northeastern and midwestern United States. Note that as pH decreases, H + concentration or acidity increases exponentially. Individual storms, especially in the summer, can often produce pH values below 3.5 (>300 ueq/l H +). Cloudwater and fog often show even higher concentrations of acidity and this has major implications for high-elevation ecosystems such as mountain forests and water bodies. A more appropriate term than acid rain is "acid deposition," which includes both wet and dry deposition of acidic sulfur and nitrogen compounds to the earth's surface from the atmosphere.

Acid deposition is of greatest concern wherever there are large amounts of fossil fuel combustion upwind of an area. Eastern North America, large areas of Europe, and eastern Asia all receive acidic deposition. Acidic deposition is especially a concern when poorly buffered soils, with little acid-neutralizing capacity, are impacted. In North America, large areas of eastern Canada, the Adirondack Mountains of upstate New York, and sections of New England all are considered "acid sensitive " areas, where resistant bedrocks and thin soils prevent significant neutralization of acidity.

HISTORICAL PERSPECTIVE

Acidic deposition is not a new phenomena, as E. B. Cowling (1982) has noted. In 1872, the term "acid rain" was first known to be used by Angus Smith to describe the precipitation around Manchester, England. Smith analyzed the chemistry of the rain and attributed this acid rain to combustion of coal. He also noted damage from acid rain to plants and materials. C. Crowther and H. G. Ruston (1911) demonstrated gradients in rainfall acidity decreasing from the center of Leeds, England and associated the acidity with coal combustion. E. Gorham (1957, 1958) established that acid precipitation affects the acid-neutralizing capacity of lakes and bogs. A. Dannevig (1959) of Norway recognized the relationship between acid precipitation, lake and stream acidity, and the disappearance of fish. S. Oden (1968) used trajectory analysis to demonstrate that acid precipitation in Sweden was the result of long-range transport and transformation of sulfur emissions from England and central Europe. In 1972, Likens et al. identified acid precipitation in eastern North America. G. E. Likens and F. H. Bormann (1974) demonstrated its regional distribution in the eastern United States and indicated that the transformation of nitrogen oxides (NO x), as well as sulfur dioxide (SO 2), adds to precipitation acidity. D. W. Schindler and his colleagues (1985) performed a whole lake acidification in Canada and documented the adverse decline of the lake food web at pH levels as high as 5.8. The issue of acid rain or deposition has generated a vast amount of knowledge and understanding of atmospheric and watershed processes, and research in the field continues today.

FORMATION AND TRENDS

The formation of acidic deposition is largely from the combustion of fossil fuels and the smelting of sulfide ores. Minor natural sources exist such as the formation of hydrochloric and sulfuric acid from gaseous volcanic eruptions.

There are well over 100 gaseous and aqueous phase reactions that can lead to acid formation and more than fifty oxidizing agents and catalysts may be involved. However, in the simplest terms sulfur in fuels is oxidized to SO 2, and SO 2 in the atmosphere is further oxidized and hydrolyzed to sulfuric acid. Most nitric acid is formed by the fixation of atmospheric nitrogen gas (N 2) to NO x (NO and NO 2) during high temperature combustion, followed by further oxidation and hydrolysis that produces nitric acid in the atmosphere. These materials can be dry-deposited onto surfaces, or be removed from the atmosphere by precipitation. The acid-generating reactions can take from hours to days depending on a wide range of atmospheric parameters such as temperature, humidity, and the presence of oxidizing agents such as hydroxl (OH) radicals, ozone (O 3) and hydrogen peroxide (H 2O2). Depending on these conditions, and other factors such as height of release and wind speed, sulfur and nitrogen oxides can be transformed and deposited as acid deposition anywhere from a few kilometers to thousands of kilometers from their original source. Figure 1 shows the geographic distribution of acid deposition from precipitation for the United States.

The U.S. trends in emissions SO 2 and NO x from 1900 to 1997 are shown in Figure 2. The pattern for SO 2 emissions since 1900 has shown three peaks. From 1900 to the 1920s there was a general increase followed by a sharp decline during the Great Depression of the 1930s. World War II produced another peak, followed by a significant decline at the end of World War II. SO 2 emissions steadily rose from the early 1950s to the early 1970s. The Clean Air Act of 1970, which was directed at other air pollution concerns and not directly at acid depositon, was largely responsible for the decline in SO 2 emissions. At the time there was essentially no awareness that an "acid rain" problem existed in North America. By 1995, the implementation of the Clean Air Act Amendments (CAAA) of which specifically targeted SO 2 reductins to reduce acid deposition began to further decrease SO 2 emissions in a large part of the eastern United States, where acid deposition is most acute. NO x emissions rose steadily until the 1970s when emissions leveled off and then showed a very slight decline.

SOURCES OF ACID DEPOSITION

Major sources for emissions of SO 2 and NO x in the United States are presented in Figures 3 and 4 respectively. Approximately two-thirds of the SO 2 emissions are from electric utilities. Efforts to reduce SO 2 emissions both nationally and regionally have focused on electric utilities. The CAAA of 1990 have stipulated a reduction of 9.1 million metric tons (10 million short tons) of SO 2 below 1980 levels, with most of this reduction coming from coal-fired power plants. Implementation of Phase I reductions (1995–2000) has been successful and has resulted in an 18 percent decline in SO 2 emissions from electric utilities, compared with 1990 SO 2 emissions. There has been a 16 percent decline in SO 2 for this time period when all sources are considered. Phase 2 of the CAAA, which is designed to reduce SO 2 emissions from electric utilities by another 20 percent (compared to 1990), will go into effect from 2000 to 2005.

In the United States, recent reductions in emissions of SO 2 have been achieved by a shift to burning low sulfur coal and by the introduction of SO 2 scrubbers that remove SO 2 gases from power plant stacks. Most of the reductions in SO 2, mandated by the CAAA have come from the shift to burning low sulfur coal from the western United States. Electric utilities account for about one-fourth of the NO x emissions. However, the largest single sources of NO x emissions are "on-road vehicles," mainly cars and trucks, which account for 30 percent of the NO x emissions (Figure 4). Control of NO x emissions from vehicles is technically more difficult to achieve. Utilities can meet these targets for coal-fired boilers by using low NO x burner technology (LNBT) or by "emissions averaging." Emissions averaging for a utility requires over-control by one boiler to make up for higher emissions at another boiler.

EFFECTS OF ACID DEPOSITION

Acid deposition and the associated particulate nitrates and sulfates are implicated in the deterioration of certain sensitive ecosystems, decreased visibility, negative human health effects, and increased degradation of certain stone building materials and cultural resources, especially those made of limestone and marble. Fine particulate nitrate and sulfate particles associated with acid deposition are implicated in aggravating cardiorespiratory diseases such as asthma and chronic bronchitis, especially in urban areas.

In many cases estimating the impact of acid deposition on various ecosystems can be a difficult process because acid deposition is only one of many impacts that can effect a response. However, wet and dry acid deposition has been documented as a major factor in the following ecosystem responses.

Aquatic Effects

In both Europe and eastern North America the negative impacts of acid deposition were first documented in lakes and streams found in acid-sensitive areas. In the early 1970s the loss of fish populations and increasing acidity in rural lakes and streams were documented both in Scandinavia and North America. In the United States, studies showed increasing acidification of lakes and loss of fish populations in the Adirondack Mountains of New York. The increased dissolved inorganic aluminum leaching from watersheds due to increased acidity proved toxic to fish in this region. In addition to dissolved aluminum toxicity, increased acidification leads to a large-scale disruption of lake food webs. For example, the experimental acidification of an entire Canadian lake (pH 6.8 to pH 5.09 from 1976 to 1983) led progressively to a loss of freshwater shrimp, all minnow species, and crayfish. These were important food sources for the lake trout population. By the end of the experiment all fish reproduction had ceased. There were also large changes in the species compositon of smaller organisms (insects and crustaceans) lower in the food chain.

Another aquatic impact of acid deposition is episodic acidification. For example, one form of episodic acidification occurs during spring snow melt. When a winter snowpack first melts, acids and other soluble material are concentrated and released, causing an initial "acid pulse" of meltwater, with acidity levels that may be higher than any of the original snowfall. These highly acid episodes, which are also often associated with high dissolved aluminum concentration in runoff, can be especially damaging in streams where fish and other organisms cannot seek refuge in less acid waters. Large storms, which produce high amounts of runoff during other seasons, can also produce episodic acidification.

Historical evidence has linked acidic deposition to the acidification of surface waters in many regions of eastern North America and Europe. Thousands of lakes and streams in these areas are significantly more acid than they were a few decades ago. Large regions of eastern Canada lying on the resistant bedrock of the Precambrian Shield are sensitive to ongoing acidification. In the eastern United States, surface water acidification has occurred in the Adirondack Mountains, the Pocono/Catskill region, the mid-Appalachians, the eastern portion of the Upper Midwest, the New Jersey Pine Barrens, and to a lesser extent, the Florida panhandle. Even with reduced emissions, acidification continues today in many regions. One reason improvements have been smaller than expected is that declines in sulfur emissions have also been accompanied by declines in emissions and deposition of acid-neutralizing agents found in atmospheric dust, both in North America and Europe. The most likely causes for the declines in atmospheric dust are cleaner combustion processes, controls on particulate emissions from smokestacks, changing agricultural practices (no-till), and fewer unpaved roads.

Controlling the effects of acid deposition by the use of lime or other acid-neutralizing compounds has been tried, but mainly on an experimental basis. Adding lime to lakes usually has only a short-term effect in terms of neutralizing lake acidity. The longevity of the effect is directly related to lake's water residence time, or how long it takes for the lake volume to be replaced with new water. Another experimental control is the liming of a lake or stream watershed. While such an approach can improve forest health, as well as reduce lake acidity for a more extended time, it is prohibitively expensive as a widespread solution to acid deposition.

Terrestial Effects

The documentation of regional level terrestrial consequences of acid deposition is complicated. For example, forested ecosystems in eastern North America can be influenced by other factors such as high atmospheric ozone concentrations, drought, insect outbreaks and disease, sometimes from nonnative sources. However there is a general consensus on some impacts of acidic depositon on both soils and forests in sensitive regions.

In the eastern United States, high elevation red spruce and fir forests found in the Northeast have suffered significant injury and mortality. Significant but lesser amounts of damage have also been found in high elevation spruce-fir forests in the southern Appalachians. Damage can occur directly to trees. For example, foliar leaching of plant nutrients such as calcium, and susceptability to winter cold damage thought to be the result of exposure to highly acid cloudwater can be direct impacts. Besides direct effects, acid deposition on poorly buffered, nutrient deficient soils has caused the leaching of valuable plant nutrients such as calcium, magnesium and other base cations and the release of aluminum, which can be injurious to plants and, as mentioned earlier, toxic to aquatic life. The loss of soil base cations can have long-term deleterious effects and may delay recovery of stressed ecosystems for decades or even longer. Such long-term soil nutrient losses also occur in sensitive low elevation forested ecosystems.

The role of nitrogen in the form of nitrate (NO 3-) from both wet and dry nitric acid deposition can have both positive and negative effects on ecosystems. NO 3- is an important plant nutrient and in nitrogen-poor soils can lead to increased plant growth and vigor. However in many acid-sensitive soils receiving high acidic deposition, nitrogen in soils is at or near saturation and can lead to the leaching of other important plant nutrients such as base cations or the release of aluminum into solution. The relative importance of nitric acid deposition will continue to grow as substantial reductions in SO 2 emissions occur and emissions of NO x do not appreciably decline.

CONCLUSIONS

Acid deposition is a regional problem wherever large amounts of fossil fuels are consumed. There have been significant efforts in both Europe and North America to reduce acid deposition because of its many deleterious effects. This effort has focused mainly on the reduction of SO 2 emissions. In the future acid deposition will have to be addressed in eastern Asia, where rapid industrialization and increased use of fossil fuels is likely.

In the United States the passage of the CAAAs of 1990, and their implementation starting in 1995, was an attempt to "solve" the acid rain problem mainly by reducing SO 2 emissions from electric power plants. While significant reductions in SO 2 emissions have occurred, and there already has been a reduction in deposition of sulfur and acidity, there have not been significant improvements in some sensitive regions such as the Adirondack Mountains of New York, the Green and White Mountains of New England, and the southern Appalachians. However, further reductions in SO 2 (and less so for NO x) are expected until the year 2005. However, this too may not be enough. It is likely that further reductions in emissions of both SO 2 and NO x, beyond those required by the CAAA, will be necessary if the goal is to protect sensitive ecosystems and public health. One reason recovery has been very limited is that acid deposition over decades has removed base cations from watersheds that are crucial to maintaining proper soil chemistry for plant growth and acid-neutralizing capacity for aquatic ecosystems. Recovery may be a slow process. Another concern is the very limited reduction in NO x emissions over the last decade. Nitric acid is becoming a major component of acid deposition and significant reductions in NO x emissions will probably be necessary to "solve" the acid rain problem.

Tom Butler

See also: Air Pollution; Atmosphere.

BIBLIOGRAPHY

Cowling, E. B. (1982). "Acid Precipitation in Historical Perspective." Environmental Science and Technology 16: 110A–123A.

Crowther, C., and Ruston, H. G. (1911). Journal of Agricultural Science 4:25–55.

Dannevig, A. (1959). Jager og Fisker 3:116–118.

Gorham, E. (1957). Limnology and Oceanography 2:22

Gorham, E (1958). Phil. Trans. Royal Soc. London, Ser. B 247:147–178.

Likens, G. E., and Bormann, F. H. (1974). "Acid Rain: A Serious Environmental Problem." Science 184:1176–1179

Likens, G. E.; Bormann, F. H.; and Johnson, N. M. (1972). "Acid rain." Environment 14:33–40.

Oden, S. (1968). "The Acidification of Air and Precipitation and Its Consequences in the Natural Environment." Ecology Committee Bulletin No. 1, Swedish National Science Research Council, Stockholm. Arlington, VA, Translation Consultants Ltd.

Schindler, D. W.; Mills, K. H.; Malley, D. F.; Findlay, D. L.; Shearer, J.A.; Davies, I. J.; Turner, M. A.; Linsey, G. A.; Cruikshank, D. R. (1985). "Long Term Ecosystem Stress: The Effects of Years of Experimental Acidification on a Small Lake." Science 228:1395–1401.

Smith, R. A. (1872). Air and Rain: The Beginnings of a Chemical Climatology.London: Longmans Greene.

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Acid Rain

Acid rain

"Acid rain" is a popularly used phrase that refers to the deposition of acidifying substances from the atmosphere and the environmental damage that this causes. Acid rain became a prominent issue around 1970, and since then research has demonstrated that the deposition of atmospheric chemicals is causing widespread acidification of lakes and streams, and possibly soil . The resulting biological effects include the extirpation (or local extinction ) of many populations of fish . Scientific understanding of the causes and consequences of acid rain, in conjunction with lobbying of government by environmental organizations, has resulted in large reductions in the atmospheric emissions of pollutants in North America and parts of Europe . If these reductions prove to be large enough, acid rain will be less of an environmental problem in those regions.


Atmospheric deposition

Strictly speaking, the term "acid rain" should only refer to rainfall, or so-called wet precipitation . However, the proper meaning of acid rain is "the deposition of acidifying substances from the atmosphere." This is because acidification is not just caused by acidic rain, but also by chemicals in snow and fog , and by inputs of gases and particulates when precipitation is not occurring.

Of the many chemicals that are deposited from the atmosphere, the most important in terms of causing acidity in soil and surface waters (such as lakes and streams) are: (1) dilute solutions of sulfuric and nitric acids (H2SO4 and HNO3, respectively) deposited as acidic rain or snow, (2) the gases sulfur dioxide (SO2) and oxides of nitrogen (NO and NO2, together called NOx), and (3) tiny particulates, such as ammonium sulfate ([NH4]2SO4) and ammonium nitrate (NH4NO3).

The depositions of these gases and particulates primarily occur when it is not raining or snowing. This type of atmospheric input is known as "dry deposition." Large regions of Europe and North America are exposed to these acidifying depositions. However, only certain types of ecosystems are vulnerable to becoming acidified by these atmospheric inputs. These usually have a thin cover of soil that contains little calcium , and sits upon a bedrock of hard minerals such as granite or quartz. There is convincing evidence that atmospheric depositions have caused an acidification of freshwater ecosystems in such areas. Many lakes, streams, and rivers have become acidic, resulting in declining or locally extirpated populations of some plants and animals. However, there is not yet conclusive evidence that terrestrial ecosystems have been degraded by acidic deposition (except for cases of severe pollution by toxic SO2).

Chemistry of precipitation

The acidity of an aqueous solution is measured as its concentration of hydrogen ions (H+). The pH scale expresses this concentration in logarithmic units to the base 10, ranging from very acidic solutions of pH 0, through the neutral value of pH 7, to very alkaline (or basic) solutions of pH 14. It is important to recognize that a one-unit difference in pH (for example, from pH 3 to pH 4) implies a 10-fold difference in the concentration of hydrogen ions. The pHs of some common solutions include: lemon juice, pH 2; table vinegar, pH 3; milk, pH 6.6; milk of magnesia, pH 10.5.

As just noted, an acidic solution, strictly speaking, has a pH less than 7.0. However, in environmental science the operational definition of acidic precipitation is a pH less than 5.65. This is the pH associated with the weak solution of carbonic acid (H2CO3) that forms when water droplets in clouds are in chemical equilibrium with carbon dioxide (CO2), an atmospheric gas with a concentration of about 360 ppm (parts per million; this is a unit of concentration).

Water in precipitation contains a mixture of positively charged ions (or cations) and negatively charged ions (or anions). The most abundant cations are usually hydrogen (H+), ammonium (NH4+), calcium (Ca2+), magnesium (Mg2+), and sodium (Na+), while the major anions are sulfate (SO42-), chloride (Cl-), and nitrate (NO3-). The principle of conservation of electrochemical neutrality of aqueous solutions states that the total number of cation charges must equal that of anions, so the net electrical charge is zero . Following from this principle, the quantity of H+ in an aqueous solution is related to the difference in concentration of the sum of all anions, and the sum of all cations other than H+.

Data for the chemistry of precipitation in a region experiencing severe acid rain are available from Hubbard Brook, New Hampshire, where one of the world's best long-term studies of this phenomenon has been undertaken. The average pH of precipitation at Hubbard Brook is 4.2, and H+ accounts for 71% of the total amount of cations, and SO42- and NO3- for 87% of the anions. Therefore, most of the acidity of precipitation at Hubbard Brook occurs as dilute sulfuric and nitric acids. The SO42- is believed to originate from SO2 emitted from power plants and industries, and oxidized by photochemical reactions in the atmosphere to SO42-. The NO3- originates with emissions of NOx (i.e., NO and NO2) gases from these sources and automobiles. Not surprisingly, air masses that pass over the large emission sources of Boston and New York produce storms with the highest concentrations of H+, SO42-, and NO3- at Hubbard Brook.

Regions differ greatly in their precipitation chemistry. This can be demonstrated using data for precipitation chemistry monitored during a study in eastern Canada. The village of Dorset in southern Ontario is close to large sources of emission of SO2 and NOx. On average, the precipitation at Dorset is highly acidic at pH 4.1, and the large concentrations of SO42- and NO3- suggest that the acidity is caused by dilute sulfuric and nitric acids. In comparison, the Experimental Lakes Area (ELA) is in a remote landscape in northwestern Ontario that is infrequently affected by polluted air masses. The ELA site has a less acidic precipitation (average pH 4.7) and smaller concentrations of SO42- and NO3- than at Dorset. Another site near the Atlantic Ocean in Nova Scotia receives air masses that pass over large sources of emissions in New England and southeastern Canada. However, by the time Nova Scotia is reached much of the acidic SO42- and NO3- have been removed by prior rain-out, and the precipitation is only moderately acidic (pH 4.6). Also, because Nova Scotia is influenced by the ocean, its precipitation chemistry is characterized by high concentrations of Na+ and Cl-. Finally, Lethbridge in southern Alberta is in a prairie landscape, and its precipitation is not acidic (average pH 6.0) because of the influence of calcium-rich, acid-neutralizing dusts blown into the atmosphere from agricultural fields.


In some places, fog moisture can be especially acidic. For example, fogwater at coastal locations in New England can be as acidic as pH 3.0-3.5. At high-elevation locations where fog is frequent there can be large depositions of cloudwater and acidity. At a site in New Hampshire where fog occurs 40% of the time, cloudwater deposition to a conifer forest is equivalent to 47% of the water input by rain and snow, and because of its large concentrations of some chemicals, fog deposition accounted for 62% of the total inputs of H+, and 81% of those of SO42- and NO3-.



Spatial patterns of acidic precipitation

Large regions are affected by acidic precipitation in North America, Europe, and elsewhere. A relatively small region of eastern North America is known to have experienced acidic precipitation before 1955, but this has since expanded so that most of the eastern United States and southeastern Canada is now affected.

Interestingly, the acidity of precipitation is not usually greater close to large point-sources of emission of important gaseous precursors of acidity, such as smelters or power plants that emit SO2 and NOx. This observation emphasizes the fact that acid rain is a regional phenomenon, and not a local one. For instance, the acidity of precipitation is not appreciably influenced by distance from the world's largest point-source of SO2 emissions, a smelter in Sudbury, Ontario. Furthermore, when that smelter was temporarily shut down by a labor dispute, the precipitation averaged pH 4.49, not significantly different from the pH 4.52 when there were large emissions of SO2.

Dry deposition of acidifying substances

Dry deposition occurs in the intervals of time between precipitation events. Dry deposition includes inputs of tiny particulates from the atmosphere, as well as the uptake of gaseous SO2 and NOx by plants, soil, and water. Unlike wet deposition, the rates of dry deposition can be much larger close to point-sources of emission, compared with further away.

Once they are dry deposited, certain chemicals can generate important quantities of acidity when they are chemically transformed in the receiving ecosystem . For example, SO2 gas can dissolve into the water of lakes or streams, or it can be absorbed by the foliage of plants. This dry-deposited SO2 is then oxidized to SO42-, which is electrochemically balanced by H+, so that acidity results. Dry-deposited NOx gas can similarly be oxidized to NO3- and also balanced by H+.

In relatively polluted environments close to emissions sources, the total input of acidifying substances (i.e., wet + dry depositions) is dominated by the dry deposition of acidic substances and their acid-forming precursors. The dry deposition is mostly associated with gaseous SO2 and NOx, because wet deposition is little influenced by distance from sources of emission.

For example, within a 25 mi (40 km) radius of the large smelter at Sudbury, about 55% of the total input of sulfur from the atmosphere is due to dry deposition, especially SO2. However, less than 1% of the SO2emission from the smelter is deposited in that area, because the tall smokestack is so effective at widely dispersing the emissions.

Because they have such a large surface area of foliage and bark , forests are especially effective at absorbing atmospheric gases and particles. Consequently, dry inputs accounted for about 33% of the total sulfur deposition to a hardwood forest in New Hampshire, 56-63% of the inputs of S and N to a hardwood forest in Tennessee, and 55% of their inputs to a conifer forest in Sweden.


Chemical changes in the forest canopy

In any forest, leaves and bark are usually the first surfaces encountered by precipitation. Most rainwater penetrates the foliar canopy and then reaches the forest floor as so-called throughfall, while a smaller amount runs down tree trunks as stemflow. Throughfall and stemflow have a different chemistry than the original precipitation. Because potassium is easily leached out of leaves, its concentration is especially changed. In a study of several types of forest in Nova Scotia, the concentration of potassium (K+) was about 10 times larger in throughfall and stemflow than in rain, while calcium (Ca2+) and magnesium (Mg2+) were three to four times more concentrated. There was less of a change in the concentration of H+; the rainwater pH was 4.4, but in throughfall and stemflow of hardwood stands pH averaged 4.7, and it was 4.4-4.5 in conifer stands. The decreases in acidity were associated with ion-exchange reactions occurring on foliage and bark surfaces, in which H+ is removed from solution in exchange for Ca2+, Mg2+, and K+. Overall, the "consumption" of hydrogen ions accounted for 42-66% of the input of H+ by precipitation to these forests. Similarly, H+ consumption by the tree canopy was 91% in a hardwood forest at Hubbard Brook, New Hampshire, 21-80% among seven stands in New Brunswick, and 14-43% in stands in upstate New York.

In areas polluted by SO2 there can be large increases in the sulfate concentration of throughfall and stemflow, compared with ambient precipitation. This is caused by the washoff of SO2 and SO4 that had been previously dry-deposited to the canopy. At Hubbard Brook this SO4 enhancement is about four times larger than ambient precipitation, while in central Germany it is about two to three times greater. These are both regions with relatively large concentrations of particulate SO4 and gaseous SO2 in the atmosphere.


Chemical changes in soil

Once precipitation reaches the forest floor, it percolates into the soil. Important chemical changes take place as: (1) microbes and plants selectively absorb, release, and metabolize chemicals; (2) ions are exchanged at the surfaces of particles of clay and organic matter ; (3) minerals are made soluble by so-called acid-weathering reactions; and (4) secondary minerals such as certain clays and metal oxides are formed through chemical precipitation of soluble ions of aluminum , iron , and other metals. These various chemical changes can contribute to: soil acidification, the leaching of important chemicals such as calcium and magnesium, and the mobilization of toxic ions of aluminum, especially Al3+. These are all natural, closely linked processes, occurring wherever there is well-established vegetation, and where water inputs by precipitation are greater than evapotranspiration (i.e., evaporation from vegetation and non-living surfaces). A potential influence of acid rain is to increase the rates of some of these processes, such as the leaching of toxic H+ and Al3+ to lakes and other surface waters.

Some of these effects have been examined by experiments in which simulated "rainwater" of various pHs was added to soil contained in plastic tubes. These experiments have shown that very acidic solutions can cause: (1) an acidification of the soil; (2) increased leaching of the so-called "basic cations" Ca, Mg, and K, resulting in nutrient loss, decreased base saturation of cation exchange capacity, and increased vulnerability of soil to acidification; (3) increased solubilization of toxic ions of metals such as aluminum, iron, manganese, lead , and zinc; and (4) saturation of the ability of soil to absorb sulfate, after which sulfate leaches at about the rate of input. The leaching of sulfate has a secondary influence on soil acidification if it is accompanied by the loss of base cations, and it can cause acidifying and toxic effects in surface waters if accompanied by Al3+ and H+.

Soil acidification can occur naturally. This fact can be illustrated by studies of ecological succession on newly exposed parent materials of soil. At Glacier Bay, Alaska, the melting of glaciers exposes a mineral substrate with a pH of about 8.0, with up to 7-10% carbonate minerals. As this material is colonized and modified by vegetation and climate, its acidity increases, reaching about pH 4.8 after 70 years when a conifer forest has established. Accompanying this acidification is a reduction of carbonates to less than 1%, caused by leaching and uptake by plants.

Several studies have attempted to determine whether naturally occurring soil acidification has been intensified as a result of acid rain and associated atmospheric depositions. So far, there is no conclusive evidence that this has occurred on a wide scale. It appears that soil acidification is a potential, longer-term risk associated with acid rain.


Chemistry of surface waters

Compared with the water of precipitation, that of lakes, ponds, streams, and rivers is relatively concentrated in ions, especially in calcium, magnesium, potassium, sodium, sulfate, and chloride. These chemicals have been mobilized from the terrestrial part of the watersheds of the surface waters. In addition, some surface waters are brown-colored because of their high concentrations of dissolved organic compounds, usually leached out of nearby bogs. Brown-water lakes are often naturally acidic, with a pH of about 4 to 5.

Seasonal variations in the chemistry of surface waters are important. Where a snowpack accumulates, meltwater in the springtime can be quite acidic. This happens because soils are frozen and/or saturated during snowmelt, so there is little possibility to neutralize the acidity of meltwater. So-called "acid shock" events in streams have been linked to the first meltwaters of the snowpack, which are generally more acidic than later fractions.

A widespread acidification of weakly-buffered waters has affected the northeastern United States, eastern Canada, Scandinavia, and elsewhere. In 1941, for example, the average pH of 21 lakes in central Norway was 7.5, but only 5.4-6.3 in the 1970s. Before 1950 the average pH of 14 Swedish water bodies was 6.6, but 5.5 in 1971. In New York's Adirondack Mountains, 4% of 320 lakes had pH less than 5 in the 1930s, compared with 51% of 217 lakes in that area in 1975 (90% were also devoid of fish). The Environmental Protection Agency sampled a large number of lakes and streams in the United States in the early 1990s. Out of 10,400 lakes, 11% were acidic, mostly in the eastern United States. Atmospheric deposition was attributed as the cause of acidification of 75% of the lakes, while 3% had been affected by acidic drainage from coal mines, and 22% by organic acids from bogs. Of the 4,670 streams considered acidic, 47% had been acidified by atmospheric deposition, 26% by acid-mine drainage, and 27% by bogs.

Surface waters that are vulnerable to acidification generally have a small acid-neutralizing capacity. Usually, H+ is absorbed until a buffering threshold is exceeded, and there is then a rapid decrease in pH until another buffering system comes into play. Within the pH range of 6 to 8, bicarbonate alkalinity is the natural buffering system that can be depleted by acidic deposition. The amount of bicarbonate in water is determined by geochemical factors, especially the presence of mineral carbonates such as calcite (CaCO3) or dolomite (Ca,MgCO3) in the soil, bedrock, or aquatic sediment of the watershed . Small pockets of these minerals are sufficient to supply enough acid-neutralizing capacity to prevent acidification, even in regions where acid rain is severe. In contrast, where bedrock, soil, and sediment are composed of hard minerals such as granite and quartz, the acid-neutralizing capacity is small and acidification can occur readily. Vulnerable watersheds have little alkalinity and are subject to large depositions of acidifying substances; these are especially common in glaciated regions of eastern North America and Scandinavia, and at high altitude in more southern mountains (such as the Appalachians) where crustal granite has been exposed by erosion .

High-altitude, headwater lakes and streams are often at risk because they usually have a small watershed. Because there is little opportunity for rainwater to interact with the thin soil and bedrock typical of headwater systems, little of the acidity of precipitation is neutralized before it reaches surface water.

In overview, the acidification of freshwaters can be described as a titration of a dilute bicarbonate solution with sulfuric and nitric acids derived from atmospheric deposition. In waters with little alkalinity, and where the watershed provides large fluxes of sulfate accompanied by hydrogen and aluminum ions, the waterbody is vulnerable to acidification.


Effects of acidification on terrestrial plants

Few studies have demonstrated injury to terrestrial plants caused by an exposure to ambient acid rain. Although many experiments have demonstrated injury to plants after treatment with artificial "acid rain" solutions, the toxic thresholds are usually at substantially more acidic pHs than normally occur in nature.

For example, some Norwegian experiments involved the treating of young forests with simulated acid rain. Lodgepole pine watered for three years grew 15-20% more quickly at pHs 4 and 3, compared with a "control" treatment of pH 5.6-6.1. The height growth of spruce was not affected over the pH range 5.6 to 2.5, while Scotch pine was stimulated by up to 15% at pHs of 2.5 to 3.0, compared with pH 5.6-6.1. Birch trees were also stimulated by the acid treatments. However, the feather mosses that dominated the ground vegetation were negatively affected by acid treatments.

Because laboratory experiments can be well controlled, they are useful for the determination of dose-response effects of acidic solutions on plants. In general, growth reductions are not observed unless treatment pHs are more acidic than about 3.0, and some species are stimulated by more acidic pHs than this. In one experiment, the growth of white pine seedlings was greater after treatment at pHs of 2.3 to 4.0 than at pH 5.6. In another experiment, seedlings of 11 tree species were treated over the pH range 2.6 to 5.6. Injuries to foliage occurred at pH 2.6, but only after a week of treatment with this very acidic pH.

Overall, it appears that trees and other vascular plants are rather tolerant of acidic rain, and they may not be at risk of suffering direct, short-term injury from ambient acidic precipitation. It remains possible, however, that even in the absence of obvious injuries, stresses associated with acid rain could decrease plant growth. Because acid rain is regional in character, these yield decreases could occur over large areas, and this would have important economic implications. This potential problem is most relevant to forests and other natural vegetation. This is because agricultural land is regularly treated with liming agents to reduce soil acidity, and because acid production by cropping and fertilization is much larger than that caused by atmospheric depositions.

Studies in western Europe and eastern North America have examined the possible effects of acid rain on forest productivity. Recent decreases in productivity have been shown for various tree species and in various areas. However, progressive decreases in productivity are natural as the canopy closes and competition intensifies in developing forests. So far, research has not separated clear effects of regional acid rain from those caused by ecological succession, insect defoliation, or climate change.


Effects of acidification on freshwater organisms

The community of microscopic algae (or phytoplankton ) of lakes is quite diverse in species. Non-acidic, oligotrophic (i.e., unproductive) lakes in a temperate climate are usually dominated by golden-brown algae and diatoms , while acidic lakes are typically dominated by dinoflagellates, cryptomonads, and green algae.

An important experiment was performed in a remote lake in Ontario, in which sulfuric acid was added to slowly acidify the entire lake, ultimately to about pH 5.0 from the original pH of 6.5. During this whole-lake acidification, the phytoplankton community changed from an initial domination by golden-brown algae to dominance by green algae. There was no change in the total number of species, but there was a small increase in algal biomass after acidification because of an increased clarity of the water.

In some acidified lakes the abundance of larger plants (called macrophytes) has decreased, sometimes accompanied by increased abundance of a moss known as Sphagnum. In itself, proliferation of Sphagnum can cause acidification, because these plants efficiently remove cations from the water in exchange for H+, and their mats interfere with acid neutralizing processes in the sediment.

Zooplankton are small crustaceans living in the water column of lakes. These animals can be affected by acidification through: (1) the toxicity of H+ and associated metals ions, especially Al3+; (2) changes in their phytoplankton food; and (3) changes in predation, especially if plankton-eating fish become extirpated by acidification. Surveys have demonstrated that some zooplankton species are sensitive to acidity, while others are more tolerant. In general, higher-pH lakes are richer in zooplankton species. For example, a survey of lakes in Ontario found 9-16 species with three to four dominants at pH greater than pH 5, but only 1-7 species with one to two dominants at more acidic pHs.

In the whole-lake experiment mentioned previously, the abundance of zooplankton increased by 66-93% after acidification, a change attributed to an increase in algal biomass. Although there was little change in dominant species, some less common species were extirpated.

Fish are the best-known victims of acidification. Loss of populations of trout, salmon , and other species have occurred in many acidified freshwaters. A survey of 700 Norwegian lakes, for example, found that brown trout were absent from 40% of the water bodies and sparse in another 40%, even though almost all of the lakes had supported healthy fish populations prior to the 1950s. Surveys during the 1930s in the Adirondack Mountains of New York found brook trout in 82% of the lakes. However, in the 1970s fish did not occur in 43% of 215 lakes in the same area, including 26 definite extirpations of brook trout in re-surveyed lakes. This dramatic change paralleled the known acidification of these lakes. Other studies documented the loss of fish populations from lakes in the Killarney region of Ontario, where there are known extirpations of lake trout in 17 lakes, while smallmouth bass have disappeared from 12 lakes, largemouth bass and walleye from four, and yellow perch and rock bass from two.

Many studies have been made of the physiological effects of acidification on fish. Younger life-history stages are generally more sensitive than adults, and most losses of fish populations can be attributed to reproductive failure, rather than mortality of adults (although adults have sometimes been killed by acid-shock episodes in the springtime).

There are large increases in concentration of certain toxic metals in acidic waters, most notably ions of aluminum. In many acidic waters aluminium ions can be sufficient to kill fish, regardless of any direct effect of H+. In general, survival and growth of larvae and older stages of fish are reduced if dissolved aluminium concentrations are larger than 0.1 ppm, an exposure regularly exceeded in acidic waters. The most toxic ions of aluminium are Al3+ and AlOH2+.

Although direct effects of acidification on aquatic birds have not been demonstrated, changes in their habitat could indirectly affect their populations. Losses of fish populations would be detrimental to fish-eating waterbirds such as loons , mergansers, and osprey. In contrast, an increased abundance of aquatic insects and zooplankton, resulting from decreased predation by fish, could be beneficial to diving ducks such as common goldeneye and hooded merganser, and to dabbling ducks such as the mallard and black duck.



Reclamation of acidified water bodies

Fishery biologists especially are interested in liming acidic lakes to create habitat for sportfish. Usually, acidic waters are treated by adding limestone (CaCO3) or lime (Ca[OH]2), a process analogous to a whole-lake titration to raise pH. In some parts of Scandinavia liming has been used extensively to mitigate the biological damages of acidification. By 1988 about 5,000 water bodies had been limed in Sweden, mostly with limestone, along with another several hundred lakes in southern Norway. In the early 1980s there was a program to lime 800 acidic lakes in the Adirondack region of New York.

Although liming rapidly decreases the acidity of a lake, the water later re-acidifies at a rate determined by size of the drainage basin , the rate of flushing of the lake, and continued atmospheric inputs. Therefore, small headwater lakes have to be re-limed more frequently. In addition, liming initially stresses the acid-adapted biota of the lake, causing changes in species dominance until a new, steady-state ecosystem is achieved. It is important to recognize that liming is a temporary management strategy, and not a long-term solution to acidification.


Avoiding acid rain

Neutralization of acidic ecosystems treats the symptoms, but not the sources of acidification. Clearly, large reductions in emissions of the acid-forming gases SO2 and NOx are the ultimate solution to this widespread environmental problem. However, there is controversy over the amount that the emissions must be reduced in order to alleviate acidic deposition, and about how to pursue the reduction of emissions. For example, should large point sources such as power plants and smelters be targeted, with less attention paid to smaller sources such as automobiles and residential furnaces? Not surprisingly, industries and regions that are copious emitters of these gases lobby against emission controls, for which they argue the scientific justification is not yet adequate.

In spite of many uncertainties about the causes and magnitudes of the damage associated with acid rain and related atmospheric depositions, it is intuitively clear that what goes up (that is, the acid-precursor gases) must come down (as acidifying depositions). This common-sense notion is supported by a great deal of scientific evidence, and because of public awareness and concerns about acid rain in many countries, politicians have began to act effectively. Emissions of sulfur dioxide and oxides of nitrogen are being reduced, especially in western Europe and North America. For example, in 1992 the governments of the United States and Canada signed an air-quality agreement aimed at reducing acidifying depositions in both countries. This agreement calls for large expenditures by government and industry to achieve substantial reductions in the emissions of air pollutants during the 1990s. Eventually, these actions should improve environmental conditions related to damage caused by acid rain.

However, so far the actions to reduce emissions of the precursor gases of acidifying deposition have only been vigorous in western Europe and North America. Actions are also needed in other, less wealthy regions where the political focus is on industrial growth, and not on control of air pollution and other environmental damages that are used to subsidize that growth. In the coming years, much more attention will have to be paid to acid rain and other pollution problems in eastern Europe and the former USSR, China, India, southeast Asia , Mexico, and other so-called "developing" nations. Emissions of important air pollutants are rampant in these places, and are increasing rapidly.

See also Sulfur dioxide.


Resources

books

Edmonds, A. Acid Rain. Sussex, England: Copper Beech Books, Ltd., 1997.

Ellerman, Danny. Markets for Clean Air: The U. S. Acid RainProgram. Cambridge: Cambridge University Press, 2000.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Hancock P. L. and Skinner B. J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.


periodicals

Anonymous. National Acid Precipitation Assessment Program Integrated Assessment Report. Washington, DC: Superintendent of Documents, U.S. Government Printing Office, 1989.

Brimblecombe, P. "Acid Rain 2000." Water, Air, and Soil Pollution 130, 1-4 (2001): 25-30.

Galloway, James N. "Acidification of the World: Natural and Anthropogenic." Water, Air, and Soil Pollution 130, no. 1-4 (2001): 17-24.

Krajick, K. "Acid Rain: Long-term Data Show Lingering Effects from Acid Rain." Science 292, no. 5515 (2001): 195-196.

Milius, S. "Red Snow, Green Snow." Science News no. 157 (May 2000): 328-333.


other

The United Nations. "The Conference and Kyoto Protocol," homepage [cited March 2003]. <http://unfccc.int/resource/convkp.html>.

United Stated Geological Survey. "What is Acid Rain?" [cited March 2003]. <http://pubs.usgs.gov/gip/acidrain/2.html>.


Bill Freedman

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acid mine drainage

—Surface water or groundwater that has been acidified by the oxidation of pyrite and other reduced-sulfur minerals that occur in coal and metal mines and their wastes.

Acid shock

—A short-term event of great acidity. This phenomenon regularly occurs in freshwater systems that receive intense pulses of acidic water when an accumulated snowpack melts rapidly in the spring.

Acidic rain (acidic precipitation)

—(1) Rain, snow, sleet or fog water having a pH less than 5.65. (2) The deposition of acidifying substances from the atmosphere during a precipitation event.

Acidification

—An increase over time in the content of acidity in a system, accompanied by a decrease in acid-neutralizing capacity.

Acidifying substance

—Any substance that causes acidification. The substance may have an acidic character and therefore act directly, or it may initially be non-acidic but generate acidity as a result of its chemical transformation, as happens when ammonium is nitrified to nitrate, and when sulfides are oxidized to sulfate.

Acidity

—The ability of a solution to neutralize an input of hydroxide ion (OH—). Acidity is usually measured as the concentration of hydrogen ion (H+), in logarithmic pH units (see also pH). Strictly speaking, an acidic solution has a pH less than 7.0.

Acidophilous

—Refers to organisms that only occur in acidic habitats, and are tolerant of the chemical stresses of acidity.

Conservation of electrochemical neutrality

—Refers to an aqueous solution, in which the number of cation equivalents equals the number of anion equivalents, so that the solution does not have a net electrical charge.

Equivalent

—Abbreviation for mole-equivalent, and calculated as the molecular or atomic weight multiplied times the number of charges of the ion. Equivalent units are necessary for a charge-balance calculation, related to the conservation of electrochemical neutrality (above).

Leaching

—The movement of dissolved chemicals with water percolating through soil.

pH

—The negative logarithm to the base 10 of the aqueous concentration of hydrogen ions in units of moles per liter. An acidic solution has pH less than 7, while an alkaline solution has pH greater than 7. Note that a one-unit difference in pH implies a 10-fold difference in the concentration of hydrogen ions.

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Acid Rain

Acid rain


Acid rain is rain that has been made strongly acidic by pollutants in the atmosphere. It is caused by the burning of fossil fuels and results in declining fish populations and damaged and dead trees. It also damages buildings and statues and aggravates respiratory conditions.

Precipitation or rainwater has always been slightly acidic since water naturally dissolves atmospheric carbon dioxide, but the precipitation described as acid rain often shows an alarmingly high concentration of dangerous acids like sulfuric acid and nitric acid. These powerful acids are in the atmosphere because of human activities, despite the fact that no one deliberately or even accidentally put them there. Rather, acid rain is the indirect result of human technological and industrial progress. Automobiles, factories, and power plants usually burn fossil fuels such as oil, gas, natural gas, or coal. Ever since the Industrial Revolution began in the eighteenth century, scientists have noticed a connection between air pollution, acid rain, and downwind damage to animals and plants. The term acid rain was first used in England around 1870.

HOW ACID RAIN IS FORMED

Acid rain is formed in Earth's atmosphere by a very simple and natural chemical process. First the atmosphere receives a steady dose of gases, such as sulfur and nitrogen oxides, that result from burning huge quantities of fossil fuels. Gas-burning cars and coal-burning electric power plants are two examples of items that give off sulfur dioxide and nitrogen oxides. Sulfur dioxide is an especially smelly toxic gas given off when coal is burned. These gases rise into the sky where they usually react with water, oxygen, and sunlight. It is mainly the moisture in the clouds, after reacting with these gases, that creates nitric acid and sulfuric acid. Sulfuric acid is an especially strong acid and is responsible for well over half of the extra acidity found in acid rain. Nitric acid makes up most of the rest of acid rain, and it too is a very caustic acid.

HOW ACIDIC IS ACID RAIN?

Normal rain has a slightly acidic pH of 5.6. This is on a typical pH acid/base scale of 0 to 14 where 7 indicates neutrality. Numbers less than 7 indicate increasing acidity and numbers greater than 7 represent increasing alkalinity. Acid rain has been found to have a pH ranging between 3.4 and 4.5, which is very acidic. This acid-bearing moisture reaches Earth's surface when it rains, snows, or when fog covers the ground. It is known to be particularly harmful to organisms that live in water. Acid rain can also fall to Earth as particles or gases in a process called dry deposition.

EFFECTS OF ACID RAIN

Some regions are more sensitive to acid rain than others, and an area that has highly alkaline soil tends to neutralize much of the acidity, making it less harmful. However, areas that have thin soil on top of granite do not have the same advantages. What happens to the aquatic organisms in these areas is that the ponds, lakes, and streams they live in become acidic to the point where water conditions actually start to kill the inhabitants. The young and small fish die first and are followed by the amphibians (frogs and salamanders). This alters the ecological balance and usually results in a boom in the mosquito population since their natural predators are gone. If the situation continues, the lake will eventually die from a lack of oxygen after becoming full of dead organisms.

For plants, too much acid in the soil eventually reduces soil fertility. More important however, is the effect that acid soil has on a plant's leaves. Since the acid destroys a leaf's waxy outer coating, it takes away both its protection and its ability to photosynthesize (the process by which the plant makes its own food using sunlight). It is easy to notice evergreen forests damaged by acid rain since the trees usually die from the top down. High altitude trees suffer the worst effects since they are exposed to the highest concentration of acid rain. Plants and trees not killed directly by acid rain are put under stress, making them susceptible to other diseases and enemies.

Besides acidifying water bodies and damaging forests, acid rain is linked to corrosion of statues, monuments, and some buildings. Such wellknown monuments like the Lincoln Memorial and the Roman Colosseum were built out of limestone and marble and have shown signs of corroding. Both stone types are composed of calcium carbonate which reacts with the acids and slowly "melts" away the surface detail.

Today, acid rain is recognized as an international problem since airborne pollutants do not stop at national boundaries. For example, much of the acid rain that harms the forests of Norway, Sweden, and Finland is blown there by prevailing winds from western and eastern Europe where the rain originated. Beginning with the passage of the 1970 Clean Air Act in the United States, national legislation and international agreements have been passed to reduce the amount of acid rain that is produced in developed countries. New technologies like "scrubbers," whose purpose it is to control the release of sulfur, are now being used in factories and power plants. Automotive emissions also have been controlled somewhat by the use of catalytic converters. Corrective actions like these have improved the situation in certain parts of the world, yet not all nations are making their best efforts, especially when pollution reduction is so expensive. However, it is important that all countries attempt to control their pollution since the future of the world's ecology depends on it.

[See alsoAcid and base; Forests; Pollution ]

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Acid Rain

Acid Rain

Acid rain is a general term describing the pollution that occurs when acids fall out of the atmosphere (mass of air surrounding Earth). The principal pollutants that produce acids in the atmosphere are sulfur dioxide (SO2) and nitrogen oxides, like nitrogen oxide (NO) and nitrogen dioxide (NO2). These compounds combine with water in the atmosphere to form sulfuric acid (H2SO4), and nitric acid (HNO3). Acid rain has significantly affected the waters that flow into lakes and rivers, as well as the lakes and rivers themselves. In turn, the plants and animals that depend on lakes, rivers and oceans are harmed by acid rain.

When describing acid rain, scientists use the more precise term acid deposition. Scientists distinguish between two types of acid deposition: dry and wet. Dry deposition includes acidic gases and solid particles containing sulfuric and nitric acid that settle out of the air and land on the ground or other surfaces. Dry deposition usually occurs very close to the point where the pollutants are released. Wet deposition occurs when precipitation, such as rain, sleet, fog, and snow, becomes acidic and falls to the ground. Wet deposition can occur hundreds of miles (kilometers) from the place where the air pollution originates.

Acid rain and the pH scale

The scale that is used to measure the acidity of a substance is called the pH scale. The pH scale runs from 0 to 14. If a material has a pH of 7 it is neutral, meaning that it is neither acidic nor alkaline (basic). Substances with pH values less than 7 are acidic and substances with pH values greater than 7 are alkaline. Distilled water is neutral, with a pH of 7. Lemon juice and vinegar are both acidic; they have pH values of 2.3 and 3.3, respectively. Baking soda, with a pH of 8.2, and milk of magnesia, with a pH of 10.5, are both alkaline. Combining an alkaline substance with an acidic substance results in a substance with a pH value that is closer to 7 than either of the original substances. This is called neutralization.

A substance that has a pH of 3 is ten times more acidic than a substance that has a pH of 4; a substance that has a pH of 3 is one hundred times more acidic than a substance with a pH of 5, and so on. Given their respective pH values of 2.3 and 3.3, lemon juice is a ten times stronger acid than vinegar.

Natural rain water is slightly acidic. Chemical reactions between pure water and carbon dioxide in the atmosphere result in a weak acid. The pH of natural rainwater is between 5 and 6. This acidity is useful because when the rain falls to the ground, it can dissolve minerals in the soil that plants use to grow. Acid rain is anywhere from ten to ten thousand times more acid than natural rain, with a pH between 4.5 and 1.5.

The major sources of acid deposition

Acid deposition forms from the burning of fossil fuels, which are used in cars, factories, electricity generation, and other industries. Fossil fuels were formed over thousands of years by dead plants and animals. After these plants and animals died they were buried under sediments (particles of sand, silt, and clay). The intense pressure and increases in temperature under these sediments chemically changed the dead plants and animals into the fuels that are used to drive cars and generate electricity today. When fuel is burned it not only releases the energy that is used to power electrical devices, but it also releases chemicals, such as sulfur dioxide and nitrogen oxides that form acid rain.

Art and Acid Rain

Acid deposition is extremely corrosive, especially to soft stones. Many famous buildings throughout the world show signs of acid damage. For example, the Parthenon in Athens, the Coliseum in Rome, and the Taj Mahal in India have all been damaged by acid deposition. Monuments in Poland and stained glass windows in Sweden have also suffered from corrosion. Several famous cathedrals in England including St. Paul's, York Minster and Westminster Abbey have shown the effects of acid deposition. Most of this damage is the result of dry deposition.

All of the acid damage on famous structures results in very high restoration costs. In 1984 the Statue of Liberty in New York harbor had to be dismantled at substantial cost because of damage to its metal frame and copper covering by acid deposition. A study in England showed that if sulfur emissions were reduced by 30%, the savings in repair to these famous buildings could be as high as $20 billion.

Car exhaust is a major source of the nitrogen oxides in the air. A second major source of nitrogen oxides in the air come in smelting plants (factories that process metal), electrical facilities, and factories. Factories and power plants are also the major source of sulfur compounds that cause acid rain. The U.S. Environmental Protection Agency (EPA) reports that about two-thirds of all sulfur dioxide and one-quarter of all nitrogen oxides in the atmosphere originate from coal burning electric power plants.

Acid deposition in lakes and rivers

Under natural conditions, rainwater, which is slightly acidic, runs through the soils near a lake. These soils often contain limestone or calcium, which is alkaline and neutralizes the acid. The water in a healthy lake usually has a pH around 6.5, which allows for the growth of a variety of plants, invertebrates (animals without a backbone), and fish.

When acid rain falls on the ground and runs into lakes, initially it is neutralized by the alkaline substances in the soils. Eventually however, these substances are used up and the water that runs into lakes and rivers is extremely acidic. This causes lakes to become acidic as well. This acidity is highly damaging to the plants and animals that live in lakes. For example, at pH values lower than about 6, crustaceans, mollusks, snails, salmon, rainbow trout, many insects, and plankton cannot survive. At pH values lower than about 5.5, small fish such as whitefish and grayling will die. At pH values lower than about 4.5, all but the hardiest life dies.

In addition, as more acidic water passes through the soils, chemical reactions occur in the soils that cause harmful minerals such as aluminum to be released. These minerals run into the lake where they are taken up by plants and invertebrates. The plants and invertebrates are then eaten by fish, which are consumed by birds that live nearby. Because the birds must eat so many fish in order to survive, the aluminum is concentrated in their bodies. High levels of aluminum cause the birds to lay eggs with very fragile shells. Often the eggs break or become dry inside. Other times, baby birds are born with physical deformities.

The EPA completed a survey of one thousand lakes in the United States in areas where acid deposition is suspected to be a problem. They found that 75% of the lakes surveyed did suffer from acidity. In addition, nearly half the streams sampled showed evidence of acidity. The major places where acid deposition was found to be a problem in the United States were Adirondacks and Catskill Mountains in New York State, the Appalachian mountains along the east coast, the northern Midwest, and mountainous areas of the Western United States. The report also mentioned that air pollution in the United States contributed to acidification of lakes and streams in Canada.

Acid deposition in oceans

Because of its chemical composition, the nitrogen and sulfur-based acids that cause acid deposition in fresh water lakes and rivers do not have a strong effect on the acidity of the ocean. However, carbon compounds in the atmosphere are responsible for increased acidity. Burning of fossil fuels releases carbon dioxide (CO2) into the atmosphere. Carbon dioxide levels in the atmosphere are currently the highest they have been in 55 million years. When it combines with seawater, this carbon dioxide produces carbonic acid, which makes seawater more acidic. This acidity will have a very negative effect on all marine organisms that make shells out of calcium carbonate, such as corals and mollusks, because it reduces the availability of calcium ions (the building blocks of shells) in seawater.

Acid deposition in forests

The ways that acid rain harms forests are complicated and interconnected. Acid rain harms both the soils that trees use to grow and the trees themselves. As acid rain falls on the soil in a forest, it washes away nutrients such as calcium and magnesium that are needed by trees to grow. In addition, acid rain releases from the soil toxic (poisonous) minerals such as aluminum that are then absorbed by the plants' roots. This causes severe damage to the trees' roots and weakens the trees. As acid rain falls on the trees themselves, it burns the needles at the top and at the tips of branches, which are then shed. This reduces the ability of the trees to make food from photosynthesis (process of converting the energy of sunlight into food) and to grow. Trees are then more vulnerable to environmental stresses like disease, drought (prolonged periods of dry weather), and insects. A tree that is exposed to acid rain will absorb extra alkaline substances from the soil, making the soil acidic. This means that the acid rain falling on the soil makes the soil even more acidic, compounding the problems of acid rain.

Decline in forests due to acid rain has been a serious problem throughout the Northern Hemisphere. In the 1990s surveys of the Black Forest in Germany showed that half of the trees were dead or dying as a result of acid deposition. Between 1970 and 1998 nearly half the red spruce trees in the northeastern United States died. Many sugar maples in Canada and the United States are also dying. Throughout Scandinavia, forests are dying because of acid rain. Most of the acid rain that affects these countries travels hundreds of miles (kilometers) from its sources in other parts of Europe.

Corrosion due to acid deposition

Acid deposition damages most surfaces on which it falls. In particular, dry deposition etches the paint on cars, corrodes metals, and deteriorates stone. In particular, buildings made of limestone and marble contain a lot of calcium carbonate. The acid in dry deposition, reacts with the calcium carbonate to form a powder. This powder is easily washed away when it rains. A variety of famous buildings and sculptures, especially in Europe, have been damaged by acid deposition.

The acid rain program

In 1990 the EPA established the Acid Rain Program as part of the Clean Air Act. The goal of the program is to reduce the emissions of sulfur dioxide and nitrogen oxides. Much of the work in this program involves creating the correct economic incentives for factories and electrical plants to improve the quality of the materials they release into the air. Companies decide how they want to achieve emissions reductions. Some may choose to install special devices on their smokestacks that cleanse the pollutants out of the emissions. Others may use fuel that is less polluting or may use renewable energy sources. Finally, companies can trade for emissions allowances (the amount of pollutants that can be legally released) from companies that have already reduced their emissions below the standard levels.

Black Forest

Beginning in the 1960s scientists noticed that many of the trees of Central Europe were dying. In particular, in the Black Forest, which is located in Southwestern Germany, a large number of trees showed signs of weakening and dying. The term daldsterben, or tree death, was coined to describe the problem. The first trees to be struck with the affliction were the pines, followed by deciduous trees (trees that lose their leaves each year). By 1990 at least half the trees in the Black Forest were harmed. Many trees dried out and died, while others dropped leaves or became discolored. The problem was eventually attributed to acid deposition in the forest. Although several types of remediation techniques, such as replanting trees, were tried, none have yet been successful. Scientists assume that the damage to the soil has made it so acidic that new trees can no longer grow in these ancient forests under current conditions.

The Acid Rain Program has been more successful at controlling sulfur pollutants than nitrogen pollutants. Since 1980 sulfur emissions from large factories have fallen by nearly one-half, from 9.4 to 4.7 million tons of sulfur dioxide a year. Much of these improvements in emissions have occurred in the parts of the country where pollution is the biggest problem, such as in Ohio and Indiana. As a result the concentration of sulfuric acid in the Northeast and the Mid-Atlantic states has fallen by about 25% and lakes in these regions are showing signs of recovery. The emissions of nitrogen oxides have remained fairly constant over the last decade. As a result, the deposition of nitric acid into the environment has remained essentially unchanged.

Juli Berwald, Ph.D.

For More Information

Books

Morgan, Sally. Acid Rain (Earth Watch). London: Franklin Watts, 1999.

Raven, Peter H., Linda R. Berg, and George B. Johnson. Environment. 2nd ed. Fort Worth, TX: Saunders College Publishing, 1998.

Sylvester, Doug. The Environment: Global Warming, Ozone, Acid Rain And Pollution. San Diego: Rainbow Horizons, 1998.

Periodicals

"Acid Oceans Spell Doom for Coral." The Daily Star (August 31, 2004): 1A. This article can also be found online at http://www.thedailystar.net/2004/08/31/d40831011715.htm (accessed on September 3, 2004).

Websites

"Acid Rain." Encyclopedia of the Atmospheric Environment.http://www.ace.mmu.ac.uk/eae (accessed on September 3, 2004).

"Acid Rain." U.S. Environmental Protection Agency.http://www.epa.gov/airmarkets/acidrain (accessed on September 3, 2004).

United States Geological Survey. "Acid Rain: Do You Need to Start Wearing a Rainhat?" Water Science for Schools.http://ga.water.usgs.gov/edu/acidrain.html (accessed on September 3, 2004).

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Acid Rain

CHAPTER 7
ACID RAIN

WHAT IS ACID RAIN?

Acid rain is the common name for acidic deposits that fall to Earth from the atmosphere. The term was coined in 1872 by English chemist Robert Angus Smith to describe the acidic precipitation in Manchester, England. Today scientists study both wet and dry acidic deposits. Although there are natural sources of acid in the atmosphere, acid rain is primarily caused by emissions of sulfur dioxide (SO2) and nitrous oxide (N2O) from electric utilities burning fossil fuels, especially coal. These chemicals are converted to sulfuric acid and nitric acid in the atmosphere and can be carried by the winds for many miles from where the original emissions took place. (See Figure 7.1.)

Wet deposition occurs when the acid falls in rain, snow, or ice. Dry deposition is caused by very tiny particles (or particulates) in combustion emissions. They may stay dry as they fall or pollute cloud water and precipitation. Moist deposition occurs when the acid is trapped in cloud or fog droplets. This is most common at high altitudes and in coastal areas. Whatever its form, acid rain can create dangerously high levels of acidic impurities in water, soil, and plants.

Measuring Acid Rain

The acidity of any solution is measured on a potential hydrogen (pH) scale numbered from zero to 14, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic (the pH of baking soda is eight); values lower than seven are considered acidic (the pH of lemon juice is two). The pH scale is a logarithmic measure. This means that every pH change of one is a ten-fold change in acid content. Therefore, a decrease from pH seven to pH six is a ten-fold increase in acidity; a drop from pH seven to pH five is a 100-fold increase in acidity; and a drop from pH seven to pH four is a 1,000-fold increase. (See Figure 7.2.)

Pure, distilled water has a neutral pH of seven. Normal rainfall has a pH value of about 5.6. It is slightly acidic because it accumulates naturally occurring sulfur oxides (SO5) and nitrogen oxides (NO5) as it passes through the atmosphere. Acid rain has a pH of less than 5.6.

Figure 7.3 shows the average rainfall pH measured during 2002 at various field laboratories around the country by the National Atmospheric Deposition Program, a cooperative project between many state and federal government agencies and private entities. Rainfall was most acidic in the Mid-Atlantic states, particularly New York, Pennsylvania, Maryland, Ohio, West Virginia, and in portions of western Virginia, North Carolina, eastern Tennessee, and Kentucky. Unfortunately, the areas with lowest rainfall pH contain some of the country's most sensitive natural resources—the Appalachian Mountains, Adirondack Mountains, Chesapeake Bay, and Great Smoky Mountains National Park.

SOURCES OF SULFATE AND NITRATE IN THE ATMOSPHERE

Natural Sources

Natural sources of sulfate in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide released from the decomposition of organic matter found in the Earth. Natural sources of nitrogen or nitrates include NO5 produced by microorganisms in soils, by lightning during thunderstorms, and by forest fires. Scientists generally speculate that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources (this is a rough estimate as there is no way to measure natural emissions as opposed to those that are manmade.)

Sources Caused by Human Activity

The primary anthropogenic (human-caused) contributors to acid rain are SO2 and NO5, resulting from the burning of fossil fuels, such as coal, oil, and natural gas.

Figure 5.15 in Chapter 5 shows the breakdown of U.S. SO2 emissions by source from 1983 to 2002. Fuel combustion by fossil-fueled electric utilities historically has been by far the greatest source of these emissions, accounting for 85 percent of them in 2002. Lesser sources included transportation vehicles and industrial processes.

NO5 emission sources are shown in Figure 5.6 in Chapter 5. Transportation vehicles are the primary source, accounting for 56 percent of the total in 2002. Fuel combustion in power plants is another major source, accounting for 37 percent of the total. Emissions by industry and miscellaneous sources (for example, agriculture) accounted for only 7 percent of the total. Agricultural emissions of nitrogen compounds are due to windblown fertilizers.

Nitrogen pollution of waters has historically been blamed on surface runoff from fertilizer, animal waste, sewage, and industrial waste. Although these are still significant causes, scientists have come to believe that airborne nitrates account for one-fourth of all nitrogen, the second most prevalent cause after fertilizers. Scientists also blame ammonia emissions, which come largely from agricultural activities such as manure handling and fertilizing, for contributing to acid rain. According to the U.S. Geological Survey (USGS), ammonium levels in precipitation increased throughout the 1990s across most of the country. The average increase was 24 percent.

NATURAL FACTORS THAT AFFECT ACID RAIN DEPOSITION

Air Movement

Several factors contribute to the impact of acid rain on an area. Transport systems—primarily the movement of air—distribute acid emissions in definite patterns around the planet. The movement of air masses transports emitted pollutants many miles, during which the pollutants are transformed into sulfuric and nitric acid by mixing with clouds of water.

In the United States a typical transport pattern occurs from the Ohio River Valley to the northeastern United States and southeastern Canada, as prevailing winds tend to move from west to east and from south to north. About one-third of the total sulfur compounds deposited over the eastern United States originates from sources in the Midwest more than 300 miles away.

Climate

In drier climates, such as those of the western United States, windblown alkaline dust moves more freely through the air and tends to neutralize atmospheric acidity. The effects of acid rain can be greatly reduced by the presence of basic (also called alkali) substances. Sodium, potassium, and calcium are examples of basic chemicals. When a basic and an acid chemical come into contact, they react chemically and neutralize each other. On the other hand, in more humid climates where there is less dust, such as along the eastern seaboard, precipitation is more acidic.

Topography/Geology

Areas most sensitive to acid rain contain hard, crystalline bedrock and very thin surface soils. When no alkaline-buffering particles are in the soil, runoff from rainfall directly affects surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity, such as flat land, neutralizes acid rain better. Lakes tend to be most susceptible to acid rain because of low alkaline content in lake beds. A lake's depth, its watershed (the area draining into the lake), and the amount of time the water has been in the lake are also factors.

The lakes and forests in and near the Adirondack Mountains in upstate New York are an example of what occurs in areas that do not have carbonate rock to quickly neutralize acid. Approximately half the lakes above the altitude of 2,000 feet have a pH of less than 5.0. Ninety percent of these lakes contain no aquatic life.

The states bordering and east of the Mississippi River contain approximately 17,000 lakes and 112,000 miles of streams. An estimated 25 percent of the land contains soil and bedrock that allow acidity to travel through underground water to these lakes and streams. Approximately half of these bodies of water have such a limited ability to neutralize acid that acid-laden pollutants will eventually cause acidification.

EFFECTS OF ACID RAIN ON OUR ENVIRONMENT

In nature, the combination of rain and oxides is part of a natural balance that nourishes plants and aquatic life. However, when the balance is upset, the results to the environment can be harmful and destructive. (See Table 7.1.)

Human health and ecosystem Effects Recovery benefits
Human healthIn the atmosphere, sulfur dioxide and nitrogen oxides become sulfate and nitrate aerosols, which increase morbidity and mortality from lung disorders, such as asthma and bronchitis, and impacts to the cardiovascular system.Decrease emergency room visits, hospital admissions, and deaths.
Surface watersAcidic surface waters decrease the survivability of animal life in lakes and streams and in the more severe instances eliminate some or all types of fish and other organisms.Reduce the acidic levels of surface waters and restore animal life to the more severely damaged lakes and streams.
ForestsAcid deposition contributes to forest degradation by impairing trees' growth and increasing their susceptibility to winter injury, insect infestation, and drought. It also causes leaching and depletion of natural nutrients in forest soil.Reduce stress on trees, thereby reducing the effects of winter injury, insect infestation, and drought, and reduce the leaching of soil nutrients, thereby improving overall forest health.
MaterialsAcid deposition contributes to the corrosion and deterioration of buildings, cultural objects, and cars, which decreases their value and increases costs of correcting and repairing damage.Reduce the damage to buildings, cultural objects, and cars, and reduce the costs of correcting and repairing future damage.
VisibilityIn the atmosphere, sulfur dioxide and nitrogen oxides form sulfate and nitrate particles, which impair visibility and affect the enjoyment of national parks and other scenic views.Extend the distance and increase the clarity at which scenery can be viewed, thus reducing limited and hazy scenes and increasing the enjoyment of national parks and other vistas.
source: "Appendix I: Effect of Acid Rain on Human Health and Selected Ecosystems and Anticipated Recovery Benefits," in Acid Rain: Emissions Trends and Effects in the Eastern United States, U.S. General Accounting Office, Washington, DC, March 2000

Aquatic Systems

Although pH levels vary considerably from one body of water to another, a typical pH range for the lakes and rivers in the United States is six to eight.

Low pH levels kill fish eggs, frog eggs, and fish food organisms. The degree of damage depends on several factors, one of which is the buffering capacity of the watershed soil—the higher the alkalinity, the more slowly the lakes and streams acidify. The exposure of fish to acidified freshwater lakes and streams has been intensely studied since the 1970s. Scientists distinguish between sudden shocks and chronic (long-term) exposure to low pH levels.

Sudden, short-term shifts in pH levels result from snowmelts, which release acidic materials accumulated during the winter, or sudden rainstorms that can wash residual acid into streams and lakes. The resulting acid shock can be devastating to fish and their ecosystems. At pH levels below 4.9, damage occurs to fish eggs. At acid levels below 4.5, some species of fish die. Below pH 3.5, most fish die within hours. (See Table 7.2.)

Mountainous streams in New York, North Carolina, Pennsylvania, Tennessee, and Arkansas have shown an acidity during rainstorms and snowmelts of three to 20 times that experienced during the rest of the year. Because many species of fish hatch in the spring, even mild increases in acidity can harm or kill the new life. Temporary increases in acidity also affect insects and other invertebrates, such as snails and crayfish, on which the fish feed.

Gradual decreases of pH levels over time affect fish reproduction and spawning. Moderate levels of acidity in water can confuse a salmon's sense of smell, which it uses to find the stream from which it came. Atlantic salmon are unable to find their home streams and rivers because of acid rain. In addition, excessive acid levels in female fish cause low amounts of calcium, thereby preventing the production of eggs. Even if eggs are produced, their development is often abnormal. Over time the fish population decreases while the remaining fish population becomes older and larger.

Increased acidity can also cause the release of aluminum and manganese particles stored in a lake or river bottom. High concentrations of these metals are toxic to fish.

In 1988 the Environmental Defense Fund (EDF), an environmental watch group, sounded one of the first alarms that the coastal waters of the eastern United States were receiving large inputs of nitrogen. The nitrogen led to an excessive growth of algae on the surface of the water. This in turn resulted in the loss of oxygen and light to the water and the long-term decline of marine life. The EDF concluded that the major sources of the nitrogen were human activities—the runoff of fertilizer, animal waste from farms, and discharge from sewage treatment plants and industrial facilities. Researchers noted that the significant decline of the Chesapeake Bay and other estuaries could also be attributed to the increase in NO5 from automobiles and electric power plants, along with toxic chemicals, pesticides, and wetland destruction.

pH range Effect
6.5–9No effect
6.0–6.4Unlikely to be harmful except when carbon dioxide levels are very high (1000 mg I 1)
5.0–5.9Not especially harmful except when carbon dioxide levels are high (20 mg I 1) or ferric ions are present
4.5–4.9Harmful to the eggs of salmon and trout species (salmonids) and to adult fish when levels of Ca2, Na and Cl are low
4.0–4.4Harmful to adult fish of many types which have not been progressively acclimated to low pH
3.5–3.9Lethal to salmonids, although acclimated roach can survive for longer
3.0–3.4Most fish are killed within hours at these levels
source: "Generalized Short-Term Effects of Acidity on Fish," in National Water Quality Inventory: 1998 Report to Congress, U.S. Environmental Protection Agency, Washington, DC, June 2000

During the 1990s acidic and polluted waters caused the disappearance of many aquatic species, leaving gaping holes in the food chain and diminishing the biological balance and diversity that keeps Earth genetically healthy. According to the American Fisheries Society and the Environmental Protection Agency (EPA), many species of freshwater fish have become extinct since the late 1970s, and additional species have become endangered, threatened, or listed as "of special concern" for their ultimate survival.

Soil and Vegetation

Acid rain is believed to harm vegetation by changing soil chemistry. Soils exposed to acid rain can gradually lose valuable nutrients, such as calcium, magnesium, and potassium, and become too concentrated with dissolved inorganic aluminum, which is toxic to vegetation. Long-term changes in soil chemistry may have already affected sensitive soils, particularly in forests. Forest soils saturated in nitrogen cannot retain other nutrients required for healthy vegetation. Subsequently, these nutrients are washed away. The EPA reports that nitrogen saturation has already been found in a number of regions, including northeastern forests, the Colorado Front Range, and mountain ranges near Los Angeles, California. The same effects have been reported in Canada and Europe. Nutrient-poor trees are more vulnerable to climatic extremes, pest invasion, and the effects of other air pollutants, such as ozone.

Some researchers believe that acid rain disrupts soil regeneration, which is the recycling of chemical and mineral nutrients through plants and animals back to the Earth. They also believe acids suppress decay of organic matter, a natural process needed to enrich the soils. Valuable nutrients like calcium and magnesium are normally bound to soil particles and are, therefore, protected from being rapidly washed into groundwater. Acid rain, however, may accelerate the process of breaking these bonds to rob the soil of these nutrients. This, in turn, decreases plant uptake of vital nutrients. (See Figure 7.4.)

Acid deposition can cause leafy plants such as lettuce to hold increased amounts of potentially toxic substances like the mineral cadmium. Research has also found a decrease in carbohydrate production in the photosynthesis process of some plants exposed to acid conditions. Research is underway to determine whether acid rain could ultimately lead to a permanent reduction in tree growth, food crop production, and soil quality. Effects on soils, forests, and crops are difficult to measure because of the numerous species of plants and animals, the slow rate at which ecological changes occur, and the complex interrelationships between plants and their environment.

TREES.

The effect of acid rain on trees is influenced by many factors. Some trees adapt to environmental stress better than others; the type of tree, its height, and its leaf structure (deciduous or evergreen) influence how well it will adapt to acid rain. Acid rain may affect trees in at least two ways: in areas with high evaporation rates, acids will concentrate on leaf surfaces; in regions where a dense leaf canopy does not exist, more acid may seep into the Earth to affect the soil around the tree's roots.

Scientists believe that acid rain directly harms trees by leaching calcium from their foliage and indirectly harms them by lowering their tolerance to other stresses. Trees are exposed to many natural threats, including drought, ice storms, invasive species, and forest fires. These stresses, combined with increased air and water pollution, can prove too much for sensitive tree species.

A 1994 joint report of the European Commission and the UNECE surveyed 102,300 trees at 26,000 sampling plots in 35 European countries and found that almost one-quarter of the trees in Europe were defoliated by more than 25 percent. The report showed that forest damage is a problem in virtually all European countries. The most severely affected country was the Czech Republic, where 53 percent of all trees had suffered moderate or severe defoliation or died. The least affected was Portugal, where 7.3 percent of trees were damaged.

In 1998 the National Acid Precipitation Assessment Program (NAPAP) identified forest ecosystems in the United States that are most at risk to acid rain damage due to natural sensitivity and high acid deposition rates. (See Figure 7.5.) The EPA blames acid deposition, along with other pollutants and natural stress factors, for increased death and decline of northeastern red spruce at high elevations (for example, in the Adirondacks) and decreased growth of red spruce in the southern Appalachians. Acid rain is also closely linked to the decline of sugar maple trees in Pennsylvania.

In Soil Calcium Depletion Linked to Acid Rain and Forest Growth in the Eastern United States, the USGS reported in March 1999 that calcium levels in forest soils had declined at locations in ten states in the eastern United States. Calcium is necessary to neutralize acid rain and is an essential nutrient for tree growth. Sugar maple and red spruce trees, in particular, showed reduced resistance to stresses such as insect defoliation and low winter temperatures. Although the specific relationships among calcium availability, acid rain, and forest growth are uncertain, Gregory Lawrence, a scientist and coauthor of the report, speculated: "Acid rain releases aluminum from the underlying mineral soil layer.… The result is that aluminum replaces calcium, and the trees have a harder time trying to get the needed calcium from the soil layer."

In 2001 the Hubbard Brook Research Foundation reported that more than half of large-canopy red spruce trees in the Adirondack Mountains and the Green Mountains had died since the 1960s. Acid rain was considered the primary cause.

According to the EPA, acid rain has also been implicated in impairing the winter hardening process of some trees, making them more susceptible to cold-weather damage. In some trees, the roots are prone to damage because the movement of acidic rain through the soil releases aluminum ions, which are toxic to plants.

One area in which acid rain has been linked to direct effects on trees is from moist deposition via acidic fogs and clouds. The concentrations of acid and SO5 in fog droplets are much greater than in rainfall. In areas of frequent fog, such as London, significant damage has occurred to trees and other vegetation because the fog condenses directly on the leaves.

The Forest Health Monitoring Program is a joint effort supported by the U.S. Department of Agriculture (USDA) and private and academic entities. The program monitored precipitation pH and the deposition of SO5, nitrate, ammonium, and total nitrogen in U.S. forests from 1979 to 1995. It estimates that nearly half of forest area in the North and just over 20 percent of forest area in the South are covered by relatively high SO5 deposition. Northern forests were much more exposed to nitrate deposition (40 percent) than were southern forests (less than 1 percent). High ammonium deposition was a problem for more than 62 percent of forests in the North, but less than 20 percent of forests in the South.

Birds

Increased freshwater acidity harms some species of migratory birds. Experts believe the dramatic decline of the North American black duck population since the 1950s is due to decreased food supplies in the acidified wetlands. The U.S. Fish and Wildlife Service reports that ducklings in wetlands created by humans in Maryland are three times more likely to die before adulthood if raised in acidic waters.

Acid rain leaches calcium out of the soil and robs snails of the calcium they need to form shells. Because tit-mice and other species of songbirds get most of their calcium from the shells of snails, the birds are also perishing. The eggs they lay are defective—thin and fragile. The chicks either do not hatch or have bone malformations and die.

In 2002 researchers at Cornell University released the results of a large-scale study showing a clear link between acid rain and widespread population declines in a song-bird called the wood thrush. The scientists believe that calcium depletion has had a negative impact on the birds' food source, mainly snails, earthworms, and centipedes. The birds may also be ingesting high levels of metals that are more likely to leach out of overly acidic soils. Declining wood thrush populations were most pronounced in the higher elevations of the Adirondack, Great Smoky, and Appalachian mountains. The researchers warned that acid rain may also be contributing to population declines in other songbird species.

Materials

Investigations into the effects of acid rain on objects such as stone buildings, marble statues, metals, and paints only began in the 1990s. A joint study conducted by the EPA, the Brookhaven National Laboratory, and the Army Corps of Engineers in 1993 found that acid rain was causing $5 billion worth of damage annually in a 17-state region. Two-thirds of the damage was created by pollution whose source was less than 30 miles away. Many of the country's historical monuments and buildings are located in eastern states that have been most hard-hit by acid rain.

Acid rain is suspected, in part, of damaging the Statue of Liberty and the Egyptian pyramids. Examination of the 700-year-old, 37-foot-tall bronze Great Buddha of Kamakura, an important symbol of Japanese culture, shows pock marks and rust stains, the result of acid rain.

New kinds of protective chemicals that adhere to limestone and marble are helping to save some of the world's decomposing monuments from acid rain and other pollutants. These chemicals, called consolidants, were developed in the 1960s in response to widespread water damage to stone buildings in Venice. Among the monuments getting close attention are the Taj Mahal in India; the Parthenon in Athens, Greece; the Lincoln Memorial in Washington, D.C.; and the Alamo in Texas. Experts report, however, that these chemicals have many limitations. They are toxic and difficult to apply, and their effects are only temporary, yet they permanently alter the nature of the stone. Most important is that their long-term effects are uncertain. For those reasons their use was banned on the Acropolis in Athens, Greece.

AUTOMOTIVE COATINGS.

Reports of damage to automotive coverings have been increasing. The general consensus within the automobile industry is that the damage is caused by some form of "environmental fallout"—the term used in the automobile industry. Automakers suspect acid rain damage to automobile paint, especially to many newer models that have clear protective overcoats. Chemical analyses of the damaged areas of some car finishes have showed elevated levels of SO5, implicating acid rain.

The auto industry began using clear-coat finishes in the mid-1980s. Although the new high-gloss paints look better, complaints are mounting over marred surfaces, especially on dark-or metallic-colored cars in the northeastern and southeastern United States. Automakers believe that when acid rain falls on autos the moisture evaporates, leaving a permanent blemish caused by sulfuric acid and nitric acid—the composition of acid rain. Some car dealers now offer optional protective sealants at added expense to buyers. Higher-priced cars often include protective sealants in the purchase price.

Human Health

Acid rain has several direct and indirect effects on human health. Particulates are extremely small pollutant particles that can threaten human health. Particulates related to acid rain include fine particles of SO5 and nitrates. These particles can travel long distances and, when inhaled, penetrate deep into the lungs. Studies of death rates across the United States, such as that reported in "Lung Cancer, Cardiopulmonary Mortality, and Long-Term Exposure to Fine Particulate Air Pollution" (Journal of the American Medical Association, March 6, 2002), have found some correlation between elevated mortality levels and high SO5 levels. Acid rain and the pollutants that cause it can lead to the development of bronchitis and asthma in children. Acid rain is also believed to be responsible for increasing health risks to those over the age of 65; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

THE POLITICS OF ACID RAIN

Scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation in North America was not identified until 1972, when scientists found that precipitation was acidic in eastern North America, especially in northeastern and eastern Canada. In 1975 the First International Symposium on Acid Precipitation and the Forest Ecosystem convened in Columbus, Ohio, to define the acid rain problem. Scientists used the meeting to propose a precipitation-monitoring network in the United States that would cooperate with the European and Scandinavian networks and to set up protocols for collecting and testing precipitation.

In 1977 the Council on Environmental Quality was asked to develop a national acid rain research program. Several scientists drafted a report that eventually became the basis for NAPAP. This initiative eventually translated into legislative action with the Energy Security Act (PL 96-264) in June 1980. Title VII of the Energy Security Act (the Acid Precipitation Act of 1980) produced a formal proposal that created NAPAP and authorized federally financed support.

The first international treaty aimed at limiting air pollution was the UNECE Convention on Long-Range Transboundary Air Pollution, which went into effect in 1983. It was ratified by 38 of the 54 UNECE members, which included not only European countries but also Canada and the United States. The treaty targeted sulfur emissions, requiring that countries reduce emissions 30 percent from 1980 levels—the so-called "30 percent club."

The early acid rain debate centered almost exclusively on the eastern United States and Canada. The controversy was often defined as a problem of property rights. The highly valued production of electricity in coal-fired utilities in the Ohio River Valley caused acid rain to fall on land in the Northeast and Canada. An important part of the acid rain controversy in the 1980s was the adversarial relationship between U.S. and Canadian government officials over emission controls of SO2 and NO2. More of these pollutants crossed the border into Canada than the reverse. Canadian officials very quickly came to a consensus over the need for more stringent controls, while this consensus was lacking in the United States.

Throughout the 1980s the major lawsuits involving acid rain all came from eastern states, and the states that passed their own acid rain legislation were those in the eastern part of the United States.

Legislative attempts to restrict emissions of pollutants were often defeated after strong lobbying by the coal industry and utility companies. Those industries advocated further research for pollution-control technology rather than placing restrictions on utility company emissions.

THE ACID RAIN PROGRAM—CLEAN AIR ACT AMENDMENTS, TITLE IV

In 1980 Congress established NAPAP to study the causes and effects of acid deposition. About 2,000 scientists worked with an elaborate multimillion-dollar computer model in an eight-year, $570 million undertaking. In 1988 NAPAP produced an overwhelming 6,000-page report on its findings, including:

  • Acid rain had adversely affected aquatic life in about 10 percent of eastern lakes and streams.
  • Acid rain had contributed to the decline of red spruce at high elevations by reducing that species' cold tolerance.
  • Acid rain had contributed to erosion and corrosion of buildings and materials.
  • Acid rain and related pollutants had reduced visibility throughout the Northeast and in parts of the West.

The report concluded, however, that the incidence of serious acidification was more limited than originally feared. The Adirondacks area of New York was the only region showing widespread, significant damage from acid at that time.

Results indicated that electricity-generating power plants were responsible for two-thirds of SO2 emissions and one-third of NO5 emissions. In response, Congress created the Acid Rain Program under Title IV (Acid Deposition Control) of the 1990 Clean Air Act Amendments (PL 101-549).

The goal of the Acid Rain Program is to reduce annual emissions of SO2 and NO5 from electric power plants nationwide. The program set a permanent cap on the total amount of SO2 that could be emitted by these power plants. That cap was set at 8.95 million tons (approximately half the number of tons of SO2 emitted by these plants during 1980). The program also established NO5 emissions limitations for certain coal-fired electric utility plants. The objective of the NO5 program was to achieve and maintain a two-million-ton reduction in NO5 emission levels by the year 2000 compared to the emissions that would have occurred in 2000 if the program had not been implemented.

The reduction was implemented in two phases. Phase 1 began in 1995 and covered 263 units at 110 utility plants in 21 states with the highest levels of emissions. Most of these units were at coal-burning plants in eastern and midwestern states. They were mandated to reduce their annual SO2 emissions by 3.5 million tons. An additional 182 units joined Phase 1 voluntarily, bringing the total of Phase 1 units to 445.

Phase 2 began in 2000. It tightened annual emission limits on the Phase 1 group and set new limits for more than 2,000 cleaner and smaller units in all 48 contiguous states and the District of Columbia.

A New Flexibility in Meeting Regulations

Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amount, and what technology to use. Title IV, however, gave utilities flexibility in choosing how to achieve these reductions. For example, utilities could reduce emissions by switching to low-sulfur coal, installing pollution-control devices called scrubbers, or shutting down plants.

Utilities took advantage of their flexibility under Title IV to choose less costly ways to reduce emissions, such as switching from high-to low-sulfur coal, and they have been achieving sizable reductions in their SO2 emissions. Fifty-five percent of Phase 1 plants opted to switch to low-sulfur coal, 16 percent chose to install scrubbers, and only 3 percent initially planned to purchase allowances (which allow plants to emit extra SO2). Not surprisingly, the market for low-sulfur coal is growing as a result of Title IV, and the market for high-sulfur coal is decreasing.

Allowance Trading

Title IV also allows electric utilities to trade allowances to emit SO2. Utilities that reduce their emissions below the required levels can sell their extra allowances to other utilities to help them meet their requirements.

Title IV allows companies to buy, sell, trade, and bank pollution rights. Utility units are allocated allowances based on their historic fuel consumption and a specific emissions rate. Each allowance permits a unit to emit one ton of SO2 during or after a specific year. For each ton of SO2 discharged in a given year, one allowance is retired and can no longer be used. Companies that pollute less than the set standards will have allowances left over. They can then sell the difference to companies that pollute more than they are allowed, bringing them into compliance with overall standards. Companies that clean up their pollution would recover some of their costs by selling their pollution rights to other companies.

The EPA holds an allowance auction each year. The sale offers allowances at a fixed price. This use of market-based incentives under Title IV is regarded by many as a major new method for controlling pollution.

From 1995 to 1998 there was considerable buying and selling of allowances among utilities. Because the utilities that participated in Phase 1 reduced their sulfur emissions more than the minimum required, they did not use as many allowances as they were allocated for the first four years of the program. Those unused allowances could be used to offset SO2 emissions in future years. From 1995 to 1998 a total of 30.2 million allowances were allocated to utilities nationwide; almost 8.7 million, or 29 percent, of the allowances were not used, but were carried over (banked) for subsequent years.

Figure 7.6 shows the status of the allowance bank from 1995 through 2002. In 2002 a total of 9.54 million allowances were allocated. Another 9.30 million banked allowances were carried over from previous years. The allowance bank reached a maximum during 2000 and began to decline after that. The EPA expects that the allowance bank will gradually be depleted.

EMISSIONS AND DEPOSITION

Each year the EPA publishes a report detailing the progress achieved by the Acid Rain Program. The latest report is titled Acid Rain Program, 2002 Progress Report and was published in November 2003.

The report notes that in 2002 there were 3,208 electric generating units subject to the SO2 provisions of the Acid Rain Program. They emitted 10.2 million tons of SO2 into the air as shown in Figure 7.7. The EPA expects that the 8.95-million-ton annual cap will be achieved by the year 2010. SO2 emissions from sources covered by the program decreased by 41 percent between 1980 and 2002.

The downward trend in SO2 emissions was accompanied by a decrease in SO2 concentrations measured in the air and in sulfate deposition recorded at monitoring sites operated by the National Atmospheric Deposition Program. Between 1990 and 2002 average SO2 concentrations in the atmosphere decreased by 54 percent. Wet sulfate deposition across the Northeast and Midwest declined by approximately 50 percent.

Between 1990 and 2002 NO5 emissions from power plants subject to the Acid Rain Program decreased from 6.7 million tons per year to 4.5 million tons per year. In 2000 the program achieved its goal of reducing emissions by at least two million tons; 8.1 million tons were originally predicted in 1990 to be emitted in the year 2000 without the program in place.

Decreased NO5 emissions have not resulted in uniformly lower levels of NO5 in the atmosphere or in deposits measured at recording stations. The EPA reports that concentrations of wet nitrates in the atmosphere generally remained constant between 1989 and 2002 across much of the country. In a few areas, concentrations actually increased. Progress for wet nitrate deposition was a little more promising. Large decreases in deposition were reported across the Northeast and the state of Michigan. Unfortunately, most of the Midwest and the mid-Atlantic regions showed little to no significant improvement.

ARE ECOSYSTEMS RECOVERING?

Recovery Times

The EPA reports that ecosystems harmed by acid rain deposition can take a long time to fully recover even after harmful emissions cease. The most chronic aquatic problems can take years to be resolved. Forest health is even slower to improve following decreases in emissions, taking decades to recover from damage by acid deposition. Finally, soil nutrient reserves (such as calcium) can take centuries to replenish.

Recent Studies Show Mixed Results

According to the USGS in Trends in Precipitation Chemistry in the United States, 19831994: An Analysis of the Effects in 1995 of Phase 1 of the Clean Air Act Amendments of 1990, Title IV, rainwater tested at 109 test sites across the United States was less acidic in 1995 than in 1983, particularly along the Ohio River Valley and in the Mid-Atlantic region. SO5 had declined at 92 percent of the sites. The USGS attributed the improvement to the standards put in place by the Clean Air Act Amendments Title IV program. For nitrates, approximately as many sites showed decreased levels as reported increased levels. Overall, nitrate levels rose slightly, with the largest increases occurring in the western states.

In April 1999 NAPAP released findings from the study National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment. The study warned that, despite important strides in reducing air pollution, acid rain remained a serious problem in sensitive areas. The report provided additional evidence that acid rain is more "complex and intractable than was believed 10 years ago." Among the findings were the following:

  • New York's Adirondack Mountain waterways suffer from serious levels of acid. Even though sulfur levels are declining, nitrogen levels there are climbing. The agency predicted that by 2040 about half the region's 2,800 lakes and ponds will be too acidic to sustain life.
  • The Chesapeake Bay is suffering from excess nitrogen, which is causing algae blooms that suffocate other life forms.
  • High elevation forests in Colorado, West Virginia, Tennessee, and southern California are nearly saturated with nitrogen, a key ingredient in acid rain.
  • High elevation lakes and streams in the Sierra Nevadas, the Cascades, and the Rocky Mountains may be on the verge of "chronically high acidity."

The report concluded that further reductions in sulfur and nitrogen would be needed. The report also found, however, that the 1990 Clean Air Act Amendments have reduced sulfur emissions and acid rain in much of the United States. Some scientists believe that the problems associated with acid rain are theoretically reversible. That is, recovery is possible if a threshold of damage is not passed.

In March 2000 the EPA and the U.S. General Accounting Office (GAO) concluded in Acid Rain: Emissions Trends and Effects in the Eastern United States that some surface waters in New England harmed by acid rain were showing signs of recovery. However, ecosystems considered most severely affected—such as the Adirondacks—were not yet showing improvement. The GAO reported that acidified lakes in the Adirondack Mountains were taking longer to recover than lakes elsewhere and might not recover fully or at all without further reductions in acid deposition. Recovery was considered dependent on improving the nearby soil condition.

In early 2001 the EPA released a report on progress made by the United States and Canada on cross-border air pollution. The study, U.S.-Canada 2000 Air Quality Agreement Progress Report, is the fifth biennial report related to the 1991 agreement between the two countries. The report says that SO5 deposition was reduced by up to 25 percent between 1995 and 1998 over a large area of the eastern United States. Most of the reduction was in the Northeast, where many sensitive ecosystems are located. SO5 concentrations in lakes and streams decreased all over North America. Declines in nitrate concentrations were much smaller and rarer. Only one region, Vermont/Quebec, showed recovery as evidenced by decreasing acidity or increasing alkalinity.

In 2001 the Hubbard Brook Research Foundation released Acid Rain Revisited, which examined progress since 1990. The report concluded that acid rain was still a significant problem in the Northeast, despite declines in sulfur emissions. Researchers urged for tighter controls on emissions of NO5 and ammonia, two problems that have not been well addressed by the Acid Rain Program.

The report noted that many ecosystems in the northeast have reached or passed their tolerance for acid input, making recovery unlikely under the existing emissions reductions scheme. The researchers called for an additional 40 to 80 percent reduction in sulfur emission from electric utilities in addition to what is mandated now. Reductions at the 80 percent level are predicted to allow recovery of acidic streams to nonacidic status in approximately 20 to 25 years.

A GLOBAL PROBLEM

Because of the transport properties of acid rain, it is not a localized problem. Emissions can originate hundreds of miles from where acid deposition occurs. Canadian authorities estimate that more than 30 percent of the acid rain that falls in Canada is due to U.S. emission sources.

In Europe pollutants are carried from the smokestacks of the United Kingdom over Sweden. In southwestern Germany many trees of the famed Black Forest are dying from the effects of acid rain transported to the region by wind. Germans have coined a word for the phenomenon, waldsterben (forest death).

Acid rain is a growing problem in Asia. According to the International Institute for Applied Systems Analysis, SO2 emissions in Asia are surpassing those in Europe and North America. In China acid rain is implicated in large die-offs in southwestern forests. A study by China's National Environment Protection Agency found that farmland is also affected by acid rain, so crops are at risk as well. According to Todd Johnson et al., in Clear Water, Blue Skies: China's Environment in the New Century, (Washington, DC: World Bank, 1997), the World Bank estimated annual forest and crop losses at $5 billion. Researchers believe it will be difficult to control pollution from China, the world's biggest consumer of coal, as that nation goes through an accelerated economic expansion that involves increased coal consumption.

Scientists estimate that about one-third of Japan's sulfur deposition comes from China. Atmospheric acidity levels are highest in the winter and early spring. During this time, huge air masses from continental Asia move to Japan, propelled by the prevailing monsoons. As in other countries,

Great deal % Fair amount % Only a little % Not at all % No opinion %
2004 Mar 8–11202627261
2003 Mar 3–5242627212
2002 Mar 4–7252331192
2001 Mar 5–7282826162
2000 Apr 3–9343119151
1999 Apr 13–14293523112
1991 Apr 11–14343020143
1990 Apr 5–8343018144
1989 May 4–7412719113
source: "Please tell me if you personally worry about this problem a great deal, a fair amount, only a little, or not at all. Acid rain?," in Poll Topics and Trends: Environment, The Gallup Organization, Princeton, NJ, March 17, 2004 [Online] www.gallup.com [accessed March 30, 2004]

forests in Japan have experienced abnormally high death rates, particularly in stands of red pine and Japanese cedar. Japanese laws governing the emission of gases that acidify rain are among the strictest in the world. Nonetheless, Japan's rain is increasingly acidic. In Kawasaki, where NO5 levels are posted every day outside City Hall, the rain is sometimes as acidic as grapefruit juice.

In 2001 atmospheric scientists at Princeton University said that acid rain in Asia could triple over the next 30 years due to large expected increases in industrial emissions of NO5. Already, nearly 25 percent of China's NO5 emissions return to Earth in acid rain. Chinese emissions are blamed for more than 27 percent of NO5 acid rain in Japan and more than half in North Korea.

PUBLIC OPINION ABOUT ACID RAIN

Every year the Gallup Organization polls Americans about their attitudes regarding environmental issues. The most recent poll was conducted in March 2004. As shown in Figure 1.8 in Chapter 1, acid rain ranked last among the environmental problems considered during the poll. Only 20 percent of respondents expressed a great deal of worry about acid rain. Table 7.3 shows a dramatic decline in concern about acid rain since the late 1980s. In 1989 Gallup found that 41 percent of respondents felt a great deal of concern about acid rain and 11 percent felt none at all. By 2004 only 20 percent of people polled were concerned a great deal about acid rain and more than a quarter of those asked expressed no concern about the acid rain issue.

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Acid Rain

CHAPTER 9
ACID RAIN

WHAT IS ACID RAIN?

Sulfur dioxide and nitrogen oxides are gases that occur naturally in the Earth's atmosphere. These gases react with water, oxygen, and other chemicals in the atmosphere to form various acidic compounds, including mild sulfuric acid and nitric acid. In nature, the combination of rain and these oxides is part of a natural balance that nourishes plants and aquatic life. However, when human activity increases the amount of acid-forming chemicals in the air, the results can be harmful to humans and the environment.

Acid rain is the common name applied to any form of precipitation that contains a greater than normal amount of acid. It would be more accurate to call it "acid deposition." It occurs in two ways: wet and dry. Acidic fog, hail, rain, sleet, and dew are wet deposition. Dry deposition consists of acidic aerosols, particles, and gases. About half of the acid in the atmosphere falls to earth as dry deposition.

Dry deposition introduces acidic particles and gases into water in two ways: direct deposition onto the water body surface and indirect deposition. One example of indirect deposition is snowpack melt. In cold parts of the country, pollutants are concentrated in the upper layers of the snowpack because of wet deposition during snowfall, and dry deposition during periods of clear weather. During the spring snowmelt, runoff containing large amounts of acidic particles accumulated over the winter can flow into a lake or river, causing acid shock to aquatic inhabitants. (See Figure 9.1.)

Origin of the Term "Acid Rain"

The concept of acid rain originated in 1872 when the term was first used by Robert Angus Smith, an English chemist, to describe acidic precipitation in and around the city of Manchester, England. Subsequent scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation was not identified in North America until 1972, when scientists found that precipitation was acidic in eastern North America, especially in the northeastern United States and eastern Canada. The 1975 First International Symposium on Acid Precipitation and the Forest Ecosystem (in Columbus, Ohio) helped scientists define the acid rain problem and initiated further research.

Formation of Acid Rain

One of the main components of acid rain is sulfur dioxide. When sulfur dioxide reaches the atmosphere, it oxidizes to first form a sulfate ion. It becomes sulfuric acid when it joins with hydrogen atoms in the air and falls back to earth. The most oxidation occurs in clouds, especially in heavily polluted air where other compounds such as ammonia and ozone help catalyze (accelerate) the reaction, converting more sulfur dioxide to sulfuric acid. Not all of the sulfur dioxide, however, is converted. In fact, a substantial amount of sulfur dioxide can float up into the atmosphere, be transported to another location, and return to earth unconverted.

Nitric oxide and nitric dioxide are the other major components of acid rain. Like sulfur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also catalyzed in heavily polluted clouds where iron, manganese, ammonia, and hydrogen peroxide are present.

Figure 9.2 illustrates how sulfur and nitrogen oxides, as well as hydrocarbons, are carried into the air to become acid deposition. Gases and particulate matter are carried into the atmosphere where they mix with moisture and other pollutants to form dry and wet acid deposition. Wet deposition returns to earth as precipitation, which enters the water body directly, percolates through the soil, or becomes runoff to nearby water bodies. Dry deposition builds up over time on all dry surfaces to be transported to water bodies in runoff during periods of precipitation, or falls directly onto a water surface.

Measuring Acid Rain

The acidity of any solution is measured on a pH (potential hydrogen) scale, numbered from zero to fourteen, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic (the pH of baking soda, a mild alkali, is eight); values that are lower than seven are considered acidic (the pH of lemon juice, an acid, is two). Pure, distilled water has a pH level of seven. The pH scale is a logarithmic measure. This means that every pH drop of one is a tenfold increase in acid content. Therefore, a decrease from pH six to pH five is a tenfold increase in acidity; a drop from pH six to pH four is a hundredfold increase in acidity; and a drop from pH six to pH three is a thousandfold increase. (See Figure 9.3.)

Normal rainfall has a pH value of 5.65. It is not pure because it accumulates naturally occurring sulfur oxides and nitrogen oxides as it passes through the atmosphere. In comparison, acid rain has a pH of about four. "Normal" pH for freshwater streams and lakes in the United States is about six. The introduction over time of large volumes of acid deposition into unbuffered water bodies (mostly freshwater systems) can increase natural acidity as much as a hundredfold. Buffers are substances in the soils or water that offer resistance to changes in pH. When levels of alkaline chemicals that neutralize the acid rain are low in the soil or water, acid deposition directly affects the surface water pH.

FACTORS AFFECTING ACID DEPOSITION

The interplay of soil, water, climate, and winds can have a profound impact on the effects of acid deposition. The effects of acid rain can be greatly reduced by the presence of alkali (basic) substances. Sodium, potassium, magnesium, calcium, and bicarbonate are examples of chemicals with buffering (neutralizing) capacity. In areas where soils contain limestone (calcium carbonate) or other minerals with high buffering capacities, acidity is reduced as runoff travels over the soil, mixes with dust, and percolates through the soil. Brackish and salt water are more resistant to pH change from acid deposition than freshwater because they contain many substances with good buffering capacity.

Areas most sensitive to acid deposition have hard, crystalline bedrock and very thin surface soils. When no buffering particles are in the soil, acid rainfall and runoff directly affect surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity neutralizes acid rain better. Generally, lakes tend to be most susceptible to acid rain because of low alkaline content in lakebeds, the water, and the watershed soils, and the longer residence time of water in lakes.

Like lakes, freshwater streams flowing over stream-beds and draining watersheds with low buffering capacity can also be susceptible to acid deposition. For example, according to the Environmental Protection Agency (EPA) in "Effects of Acid Rain: Lakes & Streams" (http://www.epa.gov/acidrain/effects/surfacewater.html, November 12, 2003), 580 freshwater streams in the Mid-Atlantic Coastal Plain have been identified as acidic because of acid deposition.

In drier climates, such as the western United States, windblown alkaline dust blows freely and tends to help neutralize atmospheric acidity. On the other hand, the more acidic dust on the eastern seaboard contributes to atmospheric acidity.

Some acid deposition events are more severe than others. Episodic acidification refers to brief periods during which pH levels drop because of runoff from the influx of large amounts of water, such as heavy downpours and snowmelt. An example would be heavy rainfall following a long dry period. The runoff would be very acidic because of the combination of the acidic rain and the dry acid deposition washed from all surfaces.

Freshwater lakes and streams in many areas of the United States are susceptible to episodic acidification, that is, they become temporarily acidic during and immediately after storms and snowmelt. Effects may last for several hours or days, depending on the water flow, as opposed to waters that are acidic year-round. For example, during rainstorms and snowmelts, mountain streams in New York, North Carolina, Pennsylvania, Tennessee, and Arkansas have shown acidity from three to twenty times the level occurring the rest of the year. If episodic acidification occurs during periods when fish are spawning or seed is germinating, the results can be devastating. In severe cases, it has caused fish kills.

The prevailing winds in an area are determinants in the transport systems that distribute acid pollutants in definite patterns across the planet. The movement of air masses transports air pollutants many miles, during which time these pollutants are transformed into sulfuric and nitric acid by mixing with other pollutants, clouds, and water. This process is known as "transport and transformation." For example, a typical European transport pattern carries pollutants from the smokestacks of the United Kingdom over Sweden. In southwestern Germany, many trees of the famed Black Forest (mostly coniferous) are dying from the effects of acid rain transported from industrial sites to the region by wind.

Northeastern United States Hit Hardest

The areas of greatest acidity (lowest pH values) in the nation occur in the northeastern United States. This high acidity is caused by the large number of cities, dense populations, and the concentration of power and industrial plants in the Northeast. Because the area is characterized by generally acidic soils and copious freshwater lakes and streams with low buffering capacity, it is very vulnerable to the effects of acid deposition.

The prevailing wind direction in the Northeast also brings storms and air pollutants from the Ohio River Valley, an area rich in coal-fired utilities. A typical transport pattern brings pollutants from the Ohio River Valley to the northeastern United States on prevailing winds that tend to move from west to east and from south to north. As the Attorney General for the State of Maine, Steven Rowe, said in a November 2002 press release, the Clean Air Act benefits people in Maine most because the state is at the end of the country's "air pollution tailpipe"("AG Rowe to Sue Bush Administration for Gutting Clean Air Act," http://www.maine.gov/ag/press_release_pop_up.php?press_id=111, November 22, 2002). About one-third of the total sulfur compounds deposited over the eastern United States originate from sources in the Midwest, more than 300 miles away. In addition to the problems in the northeastern United States, eastern areas of Canada have also been affected by pollutants from the Ohio River Valley.

SOURCES OF SULFATE AND NITRATE IN
THE ATMOSPHERE

Natural Causes

Natural causes of sulfate (sulfur oxides) in the atmosphere include ocean spray, volcanic emissions, and readily oxidized hydrogen sulfide released from the decomposition of organic matter found on land and in water. Natural sources of nitrogen or nitrates include nitrogen oxides produced by microorganisms in soils, by lightning during thunderstorms, and by forest fires. Scientists believe that one-third of the sulfur and nitrogen emissions in the United States comes from these natural sources.

The island of Hawaii provides a good example of the natural occurrence of acid rain. Sulfur dioxide gas and other pollutants emitted from the Kilauea volcano on the island of Hawaii combine and interact chemically in the atmosphere with water, oxygen, dust, and sunlight to produce "vog" (volcanic smog) and acid rain. Vog is a visible haze consisting of gas and aerosols (a suspended mix of very tiny liquid and solid particles) that can be a health hazard because it aggravates preexisting respiratory ailments. When rain falls in areas that have vog, the crops and local water supplies can be damaged by the resulting acid rain.

Many residents and visitors on the island of Hawaii report physical symptoms associated with vog. These include headache, breathing difficulties, greater susceptibility to respiratory ailments, general lack of energy, sore throat, watery eyes, and other flu-like symptoms. Although the amount of particulate material in the air does not routinely exceed the federal standards, sulfur dioxide concentrations do. Sulfur dioxide emission rates from Kilauea were first measured in 1975 and have been measured on a regular basis since 1979. Periodic reporting of these sulfuric dioxide emission rates is done by the U.S. Geological Survey.

The tiny sulfuric acid droplets in vog have the corrosive properties of diluted battery acid. When these droplets combine with moisture in the air to form acid rain, plant damage and acceleration of the rusting of metal objects such as vehicles and machinery occurs. Crop damage is another frequent occurrence, even in greenhouses, because the vog enters through vents and mixes with the moisture on plant leaves.

The combination of vog and acid rain created an unusual water supply problem on Hawaii. Many homes relied on rooftop rainwater catchment basins for drinking water. In 1988 the drinking water in more than 40% of the homes was found to contain elevated lead levels. Upon further study it was determined that the process of acid-induced leaching from lead roofing and plumbing materials was the cause of the elevated lead levels. Tests confirmed elevated lead levels in the blood of residents. This finding led to a major island-wide effort to remove lead materials from rainwater catchment systems.

Human Causes

Most human-made emissions of sulfur dioxide and nitrogen oxides are the result of burning fossil fuels (coal, oil, and gas) for energy. This includes fossil-fueled electric utilities and industrial plants, motor vehicles using gasoline or diesel fuel, and commercial or residential heating. Nonenergy sources of emission include metal smelters that emit sulfur compounds and nitrogen compounds from agricultural fertilizers that are carried by the wind to other areas.

Levels of pollutants are measured in two ways: emissions and concentrations. Emissions are those pollutants expelled into the air by a source, whereas concentrations are the total saturation of a contaminant over time. Table 9.1 shows sulfur dioxide emission sources from 1970 to 2002, while Table 9.2 shows nitrogen oxide emissions for that same period. Fuel combustion from fossil-fueled utilities accounts for most of the man-made sulfur dioxide emissions but less than half of the nitrogen oxides. The primary source of human-generated nitrogen oxides is transportation (car, truck, bus, and other vehicle emissions), which accounts for a little more than half of man-made nitrogen oxides emissions.

There has been great progress in reducing sulfur dioxide concentration. Between 1980 and 1999 the average annual mean concentration of sulfur dioxide dropped by half. According to EPA data, annual mean nitrogen oxides concentrations declined in the early 1980s, were relatively stable during the mid-to-late 1980s, and resumed their decline in the 1990s. Concentrations of nitrogen oxides declined 25% between 1980 and 1999.

EFFECTS OF ACID DEPOSITION ON
LIVING ORGANISMS

An ecosystem is a particular environment and the biological organisms that live there. Ecosystems can be global or tiny. The plants and animals living within an ecosystem are interdependent. For example, frogs eat water insects. If the insects disappear because of acid deposition effects, the frogs may not thrive because part of their food supply has disappeared. Because of the many and varied interconnections among the plants, animals, and microorganisms living in an ecosystem, changes in pH may change the ecosystem's biodiversity or overall health.

The duration of the effects of acid rain on living organisms can vary from a few hours to many years. For example, soils that are depleted of essential nutrients may take decades or even centuries to recover. Once acid rain is reduced to normal levels, the slow process of nutrient buildup in the soil is dependent on the gradual succession over time of plant life. Plants that are tolerant of depleted soils will restore nutrients over time as they grow, die, and decompose, putting essential nutrients back in the soils. In the natural order, these plants will be followed by other plants that require more nutrient-rich soil, and as they grow, die, and decompose they will return more nutrients to the soil. Animals attracted to each stage of plant succession will also add their wastes to the process, bringing in additional nutrients. This process will continue through time until a healthy, balanced population appropriate to the ecosystem is restored.

Aquatic Systems

The effects of acid rain on aquatic systems are varied and many. They include great harm or death to fish, diminished fish populations, loss of a species in a particular water body, and reduction in biodiversity. As acid rain moves through soils in a watershed, aluminum is released from soils into the lakes and streams. As the pH lowers in a water body, the aluminum level climbs. Both low pH and elevated aluminum levels are toxic to fish (aluminum burns the gills of fish and accumulates in organs, causing organ damage). They can also cause chronic stress, which does not immediately kill an individual fish, but impairs its ability to take in the oxygen, salts, and nutrients needed to stay alive.

Freshwater fish need to maintain their osmoregulation to stay alive. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules stimulate the formation of mucus in the gills, which interferes with their ability to absorb oxygen. If mucus buildup continues, the fish suffocate. In addition, a low pH disrupts the balance of salts in fish and other aquatic life, interfering with reproduction and maintenance of bones or exoskeleton.

Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
  Total all sources 31,218 28,043 25,925 23,307 23,076 18,619 18,385 18,840 18,944 17,545 16,347 15,932 15,353
Fuel combustion total (tier 0–01) 23,456 22,661 21,391 20,021 20,290 16,230 16,252 16,649 16,743 15,338 14,163 13,735 13,168
Fuel combustion electric utility (tier 1–01) 17,398 18,268 17,469 16,272 15,909 12,080 12,767 13,195 13,416 12,583 11,396 10,850 10,293
Coal15,79916,75616,07315,63015,22011,60312,24112,61412,46911,74610,62310,0049,732
  Bituminous9,57410,161NA14,02913,3718,6099,0339,5169,3569,3138,4347,8667,317
  Subbituminous4,7165,005NA1,2921,4152,3452,6322,4902,4861,6691,5511,5311,949
  Anthracite & lignite1,5091,590NA309434649576608627763638607465
Oil1,5981,5111,395612639413461514762594482529343
  Residual1,5781,462NA604629408454509756559446492330
  Distillate2049NA810565635373713
Gas1111197661772322628
OtherNANANANANANA55122544542197
Internal combustionNANANA30495553565712141313
Fuel combustion industrial (tier 1–02) 4,568 3,310 2,951 3,169 3,550 3,357 2,849 2,805 2,740 2,135 2,139 2,243 2,299
Coal3,1291,8701,5271,8181,9141,7281,3111,3061,2731,0541,0241,0961,143
  Bituminous2,1711,2971,0581,3471,0501,003875876857646628661634
  Subbituminous66939932628508163636146476090
  Anthracite & lignite289174144906768616057605864117
  OtherNANANA353746576312306298301291311302
Oil1,2291,1391,065862927912805764738526554579534
  Residual956825851671687701623578559366395403368
  Distillate9814485111198191158161156142141157148
  Other17517112980422023252318191818
Gas140263299397543548574582578407415414472
Other70386086158147139133132132129137139
Internal combustionNANANA792320191916171810
Fuel combustion other (tier 1–03) 1,490 1,082 971 579 831 793 636 648 586 620 628 642 575
Commercial/institutional coal109147110158212200178184196146148152148
Commercial/institutional oil883638637239425397307314250256261267258
Commercial/institutional gas11127810101012121514
Miscellaneous fuel combustion (except residential)NANANA1655557775
Residential wood671313777556554
Residential other492290211167175176131130121195196197146
  Distillate oil21219615712813714410810697125125126132
  Bituminous/subbituminous coal260764329302417181846464611
  Other20181110986662525253
Industrial processes total (tier 0–02) 7,101 4,728 3,807 2,467 1,900 1,638 1,403 1,459 1,464 1,364 1,418 1,464 1,399
Chemical & allied product manufacturing (tier 1–04) 591 367 280 456 297 286 255 259 261 325 338 342 328
Organic chemical manufacturingNANANA161084445667
Inorganic chemical manufacturing591358271354214199173176178161165169167
  Sulfur compounds591358271346211195171174176141144148151
  OtherNANANA82422220212116
Polymer & resin manufacturingNANANA7101111111
Agricultural chemical manufacturingNANANA45511145514646
Paint, varnish, lacquer, enamel manufacturingNANANANANA00000000
Pharmaceutical manufacturingNANANA0000000000
Other chemical manufacturingNA810766774767677112115119106
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
Metals processing (tier 1–05) 4,775 2,849 1,842 1,042 726 530 389 407 405 304 313 332 271
Non-ferrous metals processing4,0602,1651,279853517361266276274193199211155
  Copper3,5071,9461,08065532317793999848505333
  Lead77343412112912611111311479818763
  Zinc80729562605357595757586151
  Other39611371144655599108
Ferrous metals processing715684562172186151106114114939610299
Metals processing NECNANANA18221817171718181817
Petroleum & related industries (tier 1–06) 881 727 734 505 430 369 335 344 342 312 316 319 348
Oil & gas production1111731572041228990909098100102102
  Natural gas1111731572021208889908995979996
  OtherNANANA2211113336
Petroleum refineries & related industries770554577300304271238246245205207208237
  Fluid catalytic cracking units480318330212183188157163162137137138148
  Other290236247881218381838368707089
Asphalt manufacturingNANANA1498889999
Other industrial processes (tier 1–07) 846 740 918 425 399 403 386 409 415 382 410 429 416
Agriculture, food, & kindred productsNANANA3334448889
Textiles, leather, & apparel productsNANANA0000000000
Wood, pulp & paper, & publishing products1691682231311161141011051079910310594
Rubber & miscellaneous plastic productsNANANA1001111120
Mineral products677571694286275282266285288250273289300
  Cement manufacturing618511630192181171167181183153162173177
  Other59606495941119910310597111116123
Machinery productsNANANA0010000000
Electronic equipmentNANANA0000000000
Transportation equipmentNANANANANA00000000
Miscellaneous industrial processesNANANA35413131423232414
Solvent utilization (tier 1–08) NA NA NA 1 0 1 1 1 1 1 1 1 2
DegreasingNANANA0000000001
Graphic artsNANANA0000000000
Dry cleaningNANANANANA00000000
Surface coatingNANANA1000000110
Other industrialNANANA0001111110
Storage & transport (tier 1–09) NA NA NA 4 7 2 5 5 5 6 6 7 5
Bulk terminals & plantsNANANANA001111110
Petroleum & petroleum product storageNANANA0500001111
Petroleum & petroleum product transportNANANA1001122220
Service stations: stage IINANANANANA00000000
Organic chemical storageNANANA1000000002
Organic chemical transportNANANANA000000000
Inorganic chemical storageNANANA0000000000
Inorganic chemical transportNANANA0000000000
Bulk materials storage storageNANANA1112222222
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
Waste disposal & recycling (tier 1–10) 8 46 33 34 42 47 32 33 34 34 34 35 28
Incineration4292125323527272827262720
  IndustrialNANANA10586677773
  Other4292115262720212120202018
Open burning41712911115555554
  IndustrialNANANA0000000000
  Land clearing debrisNANANANANANANANANANANANANA
  Other41712810115555554
POTWNANANANA000000000
Industrial waste waterNANANANA000000001
TSDFNANANANA000000000
LandfillsNANANA0001111122
  IndustrialNANANA0000000000
  OtherNANANA0000001111
OtherNANANA0000000011
Transportation total (tier 0–03) 551 635 717 808 874 742 715 725 732 776 697 688 696
Highway vehicles (tier 1–11) 273 334 394 455 503 335 302 304 300 300 260 248 275
Light-duty gas vehicles & motorcycles1291251201161111121121051061101039693
  Light-duty gas vehicles1281241201161111111121051061091029693
  Motorcycles1111100000000
Light-duty gas trucks24303642526972707072707065
  Light-duty gas trucks 113172125314548474748474845
  Light-duty gas trucks 211131517212324242424232220
Heavy-duty gas vehicles17171616161717151515141212
Diesel1031622212803241381011131091047370105
Off-highway (tier 1–12) 278 301 323 354 371 406 413 422 432 475 437 440 420
Non-road gasoline67899111110101111118
Non-road diesel315579105131168176181187222198204198
Aircraft6677777888988
Marine vessels160162156173167160159165170176163161160
Railroads75717359565960585658565747
Other0000000000000
Miscellaneous (tier 0–04) 110 20 11 11 12 10 15 7 6 67 70 44 91
Miscellaneous (tier 1–13) 110 20 11 11 12 10 15 7 6 67 70 44 91
Agriculture & forestryNANANANANANANANANA0NANANA
Other combustion102011111210156667704491
Fugitive dustNANANANA000000000
OtherNANANANANANA0000000
Note:
Subtotals are provided at selected tier levels.
Total all sources = sum of the 4 tier 0 categories which are bolded and separated by blank lines.
The tier 0 categories are further divided into the 13 tier 1 categories which are bolded with no line separation and are under their respective tier 0 categories.
The tier 1 categories are further divided into tier 2 categories which are not bolded and are under their respective tier 1 categories.
The tier 2 categories are further divided into tier 3 categories which are italicized and under their respective tier 2 categories.
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
  Total all sources 26,883 26,377 27,079 25,757 25,529 24,956 24,787 24,705 24,348 22,845 22,598 21,547 21,102
Fuel combustion total (tier 0–01) 10,061 10,486 11,320 10,048 10,894 10,827 10,513 10,554 10,383 9,198 8,819 8,454 8,294
Fuel combustion electric utility (tier 1–01) 4,900 5,694 7,024 6,127 6,663 6,384 6,164 6,276 6,232 5,721 5,330 4,917 4,699
Coal3,8884,8286,1235,2405,6425,5795,6015,6445,4364,9094,5634,2084,094
  Bituminous2,1122,5903,4394,3784,5323,8303,8023,8283,6353,5123,2182,9372,632
  Subbituminous1,0411,2761,6946688571,4751,5701,5911,5751,1371,0871,0351,296
  Anthracite & lignite344414542194254273229225226256255233163
  Other391548447NANANANANANA3333
Oil1,01286690119322196118145223201166170130
  Residual401013917820794116142220185152156121
  Distillate972765862151422231614138
  OtherNANANANA0NANANANANANANANA
GasNANANA646565562285319381434422359270
  NaturalNANANA646565562273306363426414352264
  ProcessNANANANANANA1213197876
OtherNANANANANANA782841404154
Internal combustionNANANA48235148153161164137140139152
Fuel combustion industrial (tier 1–02) 4,325 4,007 3,555 3,209 3,035 3,144 3,151 3,101 3,050 2,709 2,723 2,757 2,870
Coal771520444608584597540537524419408432447
  Bituminous532359306430399412366364357244237250232
  Subbituminous1641119414184646464435343662
  Anthracite & lignite75514433262619191822212335
  OtherNANANA131141112109108105118114122119
Oil332354286309265247224216209192201214175
  Residual22818617919118015614013012610411211586
  Distillate1041126389717373747281819180
  OtherNA56442914171112118888
Gas3,0602,9832,6191,5201,1811,3241,2041,1891,1751,0331,0481,0441,058
  Natural3,0532,8372,4691,2829671,102992970958835845848837
  Process855227211220210216215197202195219
  OtherNA14014511323331113
Other162149205118131123119113114142140148145
  Wood/bark waste10210813889898483798010099106100
  Liquid wasteNANANA128119887779
  Other60416717342827262536353636
Internal combustionNANANA6558738541,0641,0451,0289239269181,045
Fuel combustion other (tier 1–03) 836 785 741 712 1,196 1,298 1,197 1,177 1,101 768 766 779 725
Commercial/institutional coal23332537403833353736373838
Commercial/institutional oil2101761551069710395978081838575
Commercial/institutional gas120125131145200231247252243244249257229
Miscellaneous fuel combustion (except residential)NANANA11343022232335363724
Residential wood44397488464930303045333328
Residential other439412356326780847770740688326328330332
  Distillate oil118113857520921019318817253535356
  Natural gas242246238248449519470437400208209211235
  Other795433312111810811411765666642
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
Industrial processes total (tier 0–02) 1,215 697 666 891 892 873 950 994 1,010 940 943 977 1,000
Chemical & allied product manufacturing (tier 1–04) 271 221 213 262 168 158 125 127 129 102 105 107 105
Organic chemical manufacturing70535437182021212114151518
Inorganic chemical manufacturing201168159221276667788
Polymer & resin manufacturingNANANA22643332233
Agricultural chemical manufacturingNANANA143807450515248494948
Paint, varnish, lacquer, enamel manufacturingNANANA0000000000
Pharmaceutical manufacturingNANANA0000000000
Other chemical manufacturingNANANA38525445464731313229
Metals processing (tier 1–05) 77 73 65 87 97 98 83 89 89 86 89 94 84
Non-ferrous metals processingNANANA1614121112129999
Ferrous metals processing77736558788366717171737868
Metals processing NECNANANA13647767777
Petroleum & related industries (tier 1–06) 240 63 72 124 153 110 139 143 143 120 122 124 149
Oil & gas productionNANANA691045886888866676968
Petroleum refineries & related industries240637255474847484846464746
Asphalt manufacturingNANANA13577789935
Other industrial processes (tier 1–07) 187 182 205 327 378 399 433 460 467 451 479 501 487
Agriculture, food, & kindred productsNANANA5365558848
Textiles, leather, & apparel productsNANANA0001110000
Wood, pulp & paper, & publishing products18182473918986899193969983
Rubber & miscellaneous plastic productsNANANA0000000110
Mineral products169164181239270287327350355338361383385
  Cement manufacturing978998137151153196212214181190203214
  Glass manufacturing48536048596769747667717573
  Other2423235461666264659010010598
Machinery productsNANANA2372331111
Electronic equipmentNANANANA000000000
Transportation equipmentNANANA0000000000
Miscellaneous industrial processesNANANA810101212121111129
Solvent utilization (tier 1–08) NA NA NA 2 1 3 2 3 3 4 4 4 8
DegreasingNANANA0000000003
Graphic artsNANANA0011111111
Dry cleaningNANANANA000000000
Surface coatingNANANA2122223333
Other industrialNANANA0000000000
NonindustrialNANANANA000000000
Solvent utilization NECNANANANANA00000000
Storage & transport (tier 1–09) NA NA NA 2 3 6 15 16 16 14 15 16 16
Bulk terminals & plantsNANANANA012222220
Petroleum & petroleum product storageNANANA1207881111
Petroleum & petroleum product transportNANANA0000000000
Service stations: stage INANANANANANA0000000
Service stations: stage IINANANANANA00000000
Organic chemical storageNANANA1044443335
Organic chemical transportNANANANA000000000
Inorganic chemical storageNANANA0000000000
Bulk materials storageNANANA0012228999
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
Waste disposal & recycling (tier 1–10) 440 159 111 87 91 99 153 157 163 162 129 130 152
Incineration110563727495351535454565751
Open burning3301037459424498101106103686895
POTWNANANANA000000000
Industrial waste waterNANANANA000000000
TSDFNANANANA000000000
LandfillsNANANA0012224444
OtherNANANA0011110001
Transportation total (tier 0–03) 15,277 15,029 14,845 14,508 13,373 12,989 12,912 12,970 12,776 12,456 12,560 11,932 11,452
Highway vehicles (tier 1–11) 12,624 12,061 11,493 10,932 9,592 8,876 8,733 8,792 8,619 8,371 8,394 7,774 7,365
Light-duty gas vehicles & motorcycles8,5427,5876,6325,6814,2623,0492,8062,5222,3872,4302,3122,1812,166
  Light-duty gas vehicles8,5427,5836,6215,6634,2403,0332,7922,5072,3722,4152,2972,1682,152
  Motorcycles041118221514151515151414
Light-duty gas trucks1,5401,5591,5781,5981,5041,4611,4521,4591,4531,4501,4361,4691,401
  Light-duty gas trucks 18689159621,0099629971,0049739601,0149991,036974
  Light-duty gas trucks 2672644616589542464448486493436437434427
Heavy-duty gas vehicles723674624575567516506447417481453421404
Diesels,8332,2412,6593,0783,2593,8503,9684,3654,3624,0104,1923,7023,395
  Heavy-duty diesel vehicles1,7642,1752,5852,9973,1943,8163,9404,0214,0773,9864,1783,6873,378
  Light-duty diesel trucks7058473623141233127413699
  Light-duty diesel vehicles082644432116131112777
Off-highway (tier 1–12) 2,652 2,968 3,353 3,576 3,781 4,113 4,179 4,178 4,156 4,084 4,167 4,158 4,086
Non-road gasoline90102108114120141145163176204192190211
  Recreational4887789101011101011
  Construction44444556666606
  Industrial30272421181615161629141313
  Lawn & garden19253136425557687673868693
  Farm2233344445445
  Light commercial25811142021263032353539
  Logging0000000000001
  Airport service0000000002000
  Railway maintenance0000000000000
  Recreational marine vessels30303131323334343445353642
Non-road diesel3746539431,2461,4541,5851,6111,6131,6131,7341,6001,5881,600
  Recreational0111122222222
  Construction167312456600702764776774772858762754764
  Industrial356799131136139140139138126133132132
  Lawn & garden00011243538404238454649
  Farm112209307404478523532533533576530525521
  Light commercial052034486265676973727476
  Logging57545047393129262527222120
  Airport service00037899911999
  Railway maintenance0012233333333
  Recreational marine vessels361013161920202121222324
Aircraft50566369707374839196888181
Marine vessels9789869511,0431,0031,0491,0581,0089588571,0081,0131,011
  DieselNANA750822791835844801757672795799797
  Residual oilNANA201221212213213207200182214215214
  OtherNANANANANANANANANA2NANANA
Source category 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002
Railroads1,1361,1081,1859589451,0311,0481,0611,0739391,001999889
Other2463104145189234243251246255278286295
  Liquified petroleum gas54381120162204213221215215246254262
  Compressed natural gas19212325273030303139323233
Miscellaneous (tier 0–04) 330 165 248 310 369 267 412 187 179 251 276 184 356
Miscellaneous (tier 1–13) 330 165 248 310 369 267 412 187 179 251 276 184 356
Agriculture & forestryNANANANANANANANANA2NANA2
  Agricultural livestockNANANANANANANANANA2NANA2
Other combustion330165248310368265412187179249276184354
Health servicesNANANANANA00000000
Cooling towersNANANANANA0NANANANANANANA
Fugitive dustNANANANA110000000
OtherNANANANANANA000NANANANA
Subtotals are provided at selected tier levels.
Total all sources = sum of the 4 tier 0 categories which are bolded and separated by blank lines.
The tier 0 categories are further divided into the 13 tier 1 categories which are bolded with no line separation and are under their respective tier 0 categories.
The tier 1 categories are further divided into tier 2 categories which are not bolded and are under their respective tier 1 categories.
The tier 2 categories are further divided into tier 3 categories which are italicized and under their respective tier 2 categories.

Some aquatic plants and animals are able to tolerate more acidic waters. For example, frogs can tolerate lower pH than trout, crayfish, or clams. Acid-sensitive species, however, are lost as pH declines. It is usually the young of most species that are the most sensitive to environmental conditions. For example, at less than pH 5, trout and salmon eggs cannot hatch. At lower pH levels (pH 4 to 4.9), some adult fish die. Some extremely hardy fish such as the roach (a type of carp) can survive at a pH as low as 3.5 if the change is gradual and they have time to adjust.

Other effects include:

  • Sudden, short-term shifts in pH levels, resulting in acid shock to freshwater ecosystems
  • Gradual declines in fish populations and numbers of adult and juvenile fish over time as pH decreases
  • Unsuccessful reproduction by many aquatic species, including poor egg production, abnormal eggs, and poor juvenile survival
  • Physical impairment in juveniles of some species
  • Loss of the ability in salmon to find home streams because of impaired sense of smell
  • Loss of important components of the food web, leading to poor nutrition or starvation in species dependent on those components
  • Changes in the plant and animal species within an ecosystem

Nitrogen has been shown to play an important role in both episodic and long-term acidification. It is also an important nutrient, but excess nitrogen can cause water quality degradation. Of the nitrogen released into the atmosphere through human activities, 10% to 45% is transported to U.S. oceans and estuaries through air deposition. In Chesapeake Bay, for example, 30% of the nitrogen contributed from human-made sources is atmospheric deposition.

Another area in which excess nitrogen from acid deposition, among other sources, is being watched very carefully is along the coastline. Studies of a phenomenon nicknamed dead zones have been underway for many years. The term dead zone actually refers to a state of hypoxia. A water body that is suffering from hypoxia is one in which excess nitrogen has caused the dissolved oxygen in the water to deplete to the point where the water can no longer support life. The three sources of nitrogen known to cause large dead zones to appear every summer in the Gulf of Mexico are agricultural run-off, industrial waste, and acid deposition.

Forest Systems

Acid deposition can have serious impacts on trees and soils, causing slower growth, injury, or death of forests. Acid deposition has been implicated in forest and soil degradation in the eastern United States, particularly in the high elevations of the Appalachian Mountains from Georgia to Maine, an area including the Shenandoah and Great Smoky Mountain National Parks.

When rain falls to the forest floor, the buffering capacity of the soil may neutralize some or all of its acidity. Differences in soil-buffering capacity is the reason that some areas that receive a lot of acid rain show little damage while other areas that receive the same amount show a lot of damage. The ability of forest soil to resist becoming acidified depends on the thickness of the soil and the type of bedrock below the soil.

When the soils cannot buffer the acid rain, vital nutrients present in the soil, such as calcium and magnesium, are stripped away by the acid-driven reactions. Aluminum, a toxic element present in all soils, is made more available to the trees and taken up by their roots. The combination of toxic aluminum and poor nutrition retard growth, make the trees more vulnerable to infection, and can eventually kill the trees.

In March 1999 the U.S. Geological Survey (USGS), in Soil Calcium Depletion Linked to Acid Rain and Forest Growth in the Eastern United States, reported that calcium levels in forest soils had declined at locations in ten states in the eastern United States. Calcium is necessary to neutralize acid rain and is an essential nutrient for tree growth. Sugar maple and red spruce trees, in particular, showed reduced resistance to stresses such as insect defoliation and low winter temperatures. Although the specific relationships among calcium availability, acid rain, and forest growth are uncertain, Dr. Gregory Lawrence, scientist and coauthor of the report, speculated:

Acid rain releases aluminum from the underlying mineral soil layer, which is followed by the upward transport of the aluminum into the forest floor (the nutrient-rich organic soil layer where root activity is greatest) by root uptake and water movement. The result is that aluminum replaces calcium, and the trees have a harder time trying to get the needed calcium from the soil layer.

Acid deposition can affect trees in other ways. Sulfur dioxide that has not been converted to sulfuric acid has been shown to clog up the leaf stomata (tiny openings in leaves where gases diffuse in and out), impairing plant respiration and photosynthesis. Nitric acid and nitrogen oxide have been shown to stimulate tree growth outside the growing season, leaving trees vulnerable to winterkill. Forests in high mountain regions often are surrounded by acidic clouds and fog that are more acidic than rainfall. Scientists believe that when the tree leaves and needles are frequently wetted in this acid fog, essential nutrients are stripped away. Loss of nutrients in the foliage makes the trees more vulnerable to other environmental threats, particularly winterkill. Winterkill resulting in damage or death is the result of naturally occurring stress caused by cold, wind, ice, and dehydration on trees and other woody plants that have been weakened by insect damage, nutrient deficiency, or drought.

Plants that are found in locations that are susceptible to high acid deposition experience the same fate as trees. The processes causing growth retardation and ultimately death are believed to be the same.

Human Health

Acid rain feels, tastes, and looks just like clean rain. Sulfur dioxide and nitric oxides, the pollutants that cause acid rain, however, can damage human health. These gases interact with particulate in the atmosphere to form aerosols (a mixture of very tiny liquid and solid particles) that can travel long distances transported by winds. When inhaled, aerosols penetrate deep into the lungs and are readily retained. Because of their very fine size, they can also penetrate indoors through ventilation systems.

Air pollution studies have indicated that elevated levels of acidic particles can cause asthma attacks, particularly in adolescents, and can also impair the ability of the upper respiratory tract to remove other potentially harmful particles. Some scientific studies have also established a relationship between elevated levels of fine particles and increased deaths from heart and lung disorders, such as bronchitis and asthma. Other scientists believe that these pollutants may increase the health risks to those over age sixty-five; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

In "Effects of Acid Rain: Human Health" (Environmental Protection Agency, http://www.epa.gov/airmarkets/acidrain/effects/health.html, November 12, 2003), the EPA reported that sulfate aerosols make up about 25% of fine particles in the air in the eastern United States. Lowering emissions from power plants that contain fine sulfate and nitrate particles should eventually reduce the incidence and severity of the health problems believed related to these pollutants. The EPA estimated that when fully implemented in 2010, the public health benefits of the Acid Rain Program (created by Congress under Title IV of the 1990 Clean Air Act Amendments) would be about $50 billion annually in reduced health care costs because of decreases in emergency room visits, hospital admissions, and number of deaths.

Decreased nitric acid emissions are expected to lower the amount of ozone formed. Ozone is believed to increase the risk of illness or death from lung inflammation, including asthma and emphysema.

An indirect effect of acid deposition on human health is the increased reactivity in acid water of toxic metals and other chemicals. Increased reactivity means that the chemicals and toxic metals in the water are more likely to be taken up in fruits, vegetables, and animal tissue. Air deposition is believed to be the leading source of mercury bioaccumulation in fish. This sort of bioaccumulation has led to advisories against eating certain kinds of fish.

The principal pollutants generated by coal combustion that can cause health problems are particulate, sulfur and nitrogen oxides, trace elements (such as arsenic, fluorine, selenium, and the radionuclides, uranium, and thorium), and organic compounds as a result of incomplete coal combustion. Some of these trace elements have been shown to cause severe health effects in other countries, such as China, Romania, and Bulgaria.

The EPA conducted a detailed study of possible health effects that can come from the exposure to emissions of about twenty potentially toxic substances from coal-burning electric utilities. In this study, the EPA used USGS information on U.S. coal quality to assess the potential health impact of fourteen potentially toxic trace elements that may be mobilized by coal burning. The USGS fact sheet Health Impacts of Coal Combustion (July 2000) reported that, with the possible exception of mercury, there is no compelling evidence to indicate that emissions from U.S. coal-burning electric utilities cause human health problems. The absence of detectable health problems was credited in part to the use in the United States of coals that contain low to moderate amounts of sulfur and other potentially toxic trace elements. Another reason for the absence of detectable health problems was the common use of sophisticated pollution control systems by coal-burning utilities. These systems are specifically designed to reduce the emission of hazardous elements.

OTHER EFFECTS OF ACID RAIN

Reduction in Visibility

Sulfates and nitrates in the air contribute to reduced visibility, which means that people cannot see clearly or as far through the air. The air looks "hazy." Sulfate particles account for 50% to 70% of the visibility reduction in the eastern United States. Visibility in the East was expected to improve by 30% by 2010 because of acid rain program controls. The EPA has projected that this improvement will be worth more than $1 billion per year to the tourist industry in and around the eastern national parks.

In the western states, sulfates, nitrates, and carbon all play roles in reduced visibility. Sulfates have been shown to be important contributors to reduced visibility in many of the national parks found on the Colorado Plateau. These include Bryce Canyon, Grand Canyon, and Canyonlands. On the other hand, increased particulate in the air contributes to the spectacular sunrises and sunsets in the Red Rock country.

Human-Made Objects

Limestone and marble turn to gypsum, a crumbling substance, when exposed to acid. Many of the world's most beautiful buildings, monuments, and statuary are composed of these materials. Throughout the world, important art treasures and cultural and historic sites, such as the Taj Mahal in India; the Colosseum in Rome, Italy; and the Lincoln Memorial in Washington, D.C., are at risk. (See Figure 9.4.)

Investigations into the effects of acid rain on human-made objects in the United States, such as buildings, statues, metals, and paints, began only in the 1990s. A joint study conducted by the EPA, the Brookhaven National Laboratory, and the Army Corps of Engineers in 1993 found that acid rain caused $5 billion worth of damage annually in a seventeen-state region. Two-thirds of the damage was created by pollution whose source was less than thirty miles away.

New kinds of protective chemicals called consolidants, which adhere to limestone and marble, are being used to save some of the world's decomposing monuments from acid rain and other pollutants. Consolidants were developed in the 1960s in response to widespread water damage to stone buildings in Venice, Italy. Experts reported, however, that these chemicals have many limitations. They are toxic and difficult to apply. Their effects are only temporary, yet they permanently alter the nature of the stone. Most important, their long-term effects are uncertain. For those reasons, their use was banned on the Acropolis in Athens, Greece.

Automotive Coatings

During the 1990s, reports of damage to automotive textured roofs and paints increased. The damage generally occurs on flat, horizontal surfaces and appears as permanently etched, irregularly shaped areas. The damage is most easily observed on dark-hued vehicles, and can be detected with the aid of fluorescent lights on many vehicles that show no signs visible to the naked eye. Usually the damage is permanent.

The general consensus within the automobile industry is that the damage is caused by some form of environmental fallout. In the auto industry, the term "environmental fallout" refers to a wide variety of happenings, including bird droppings, decaying insects, acid rain, and tree sap. Chemical analysis of the damaged areas of some car finishes, however, has shown elevated levels of sulfate, implicating acid rain.

Quantifying the contribution of acid rain to paint finish damage relative to other forms of environmental fallout, deficient paint formulas, or improper paint application has been difficult. The best way to determine the exact cause of the damage is chemical testing, an expensive proposition.

The auto industry is actively pursuing the development of coatings that are more resistant to acid rain and other environmental fallout. Until acid rain is controlled or a universal protective technology is developed, the best protection for a vehicle is to keep it covered during precipitation events and wash it frequently, followed by hand drying.

POLITICS OF ACID RAIN

The early acid rain debate centered almost exclusively on the eastern portion of the United States and Canada. The controversy was often defined as a problem of property rights. The highly valued production of electricity in coal-fired utilities in the Ohio River Valley caused acid rain to fall on land in the Northeast and Canada. An important part of the acid rain controversy in the 1980s was the adversarial relationship between the U.S. and Canadian federal governments over emission controls of sulfur dioxide and nitrogen oxides. More of these pollutants crossed the border into Canada than the reverse. Canadian officials very quickly came to a consensus over the need for more stringent controls, while this consensus was lacking in the United States.

Throughout the 1980s, the major lawsuits involving acid rain all came from eastern states. States that passed their own acid rain legislation were also from the eastern part of the United States. There has been a clear difference in the intensity of interest between the eastern and western states regarding acid rain.

Legislative History

The U.S. Congress passed the first federal legislation aimed at reducing air pollution in 1967 (Air Quality Act of 1967). In 1970 the Environmental Protection Agency was founded and the Clean Air Act was passed. This law mandated the EPA to identify and set standards for pollutants identified as harmful to human health. The six pollutants identified and labeled "criteria" pollutants were:

  • Sulfur dioxide
  • Nitrogen dioxide
  • Carbon monoxide
  • Particulate matter less than or equal in size to ten micrometers
  • Lead
  • Ozone

Sulfur dioxide and nitrogen dioxide are the biggest contributors in the production of acid deposition.

In 1975 the First International Symposium on Acid Precipitation and the Forest Ecosystem convened in Columbus, Ohio, to define the acid rain problem. The scientists who took part in the symposium used the meeting to propose a precipitation monitoring network in the United States to cooperate with the European and Scandinavian networks and to set up protocols for collecting and testing precipitation.

In 1977 President Jimmy Carter's Council of Environmental Quality was asked to develop a national acid rain research program. Several scientists drafted a report that eventually became the basis for the National Acid Precipitation Assessment Program (NAPAP). Carter's initiative eventually translated into legislative action with the passage of the Energy Security Act (PL 96–264) in June 1980. Title VII of the Act (the Acid Precipitation Act of 1980) created the NAPAP and authorized federally financed support.

The Clean Air Act was amended in 1977. New legislation was added to address the problem of older fossil-fuel electric power producers that were not covered in the original law. The new program was called the New Source Review, under which these older plants would be required to undergo an EPA assessment if they chose to make changes to their operations. The EPA review would determine whether the planned changes would result in significantly higher emissions rates and, if so, these plants would be required to install pollution control technologies that brought them up to the new standards.

The first international treaty aimed at limiting air pollution was the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Trans-boundary Air Pollution, which went into effect in 1983. It was ratified by thirty-eight of the fifty-four UNECE member states, which included not only European countries but also Canada and the United States. The treaty targeted sulfur emissions, requiring that the parties reduce emissions 30% from 1980 levels, the so-called 30% club.

In 1990 the Clean Air Act was amended for a second time, and provisions designed specifically for reducing acid deposition were a significant part of the amendments passed.

Acid Rain Program—1990 Clean Air Act Amendments,
Title IV

Title IV of the 1990 Clean Air Act Amendments (PL 101–549) set as its objective achieving a ten-million-ton annual reduction in emissions from 1980 levels by the year 2010. Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amounts, and what technology to use. Title IV, however, gave utilities flexibility in choosing how to achieve these reductions. For example, utilities may reduce emissions by switching to low-sulfur coal, installing pollution control devices called scrubbers, or shutting down plants.

SULFUR DIOXIDE EMISSIONS.

Title IV introduced a new regulatory approach to reduce acid rain—allowing electric utilities to trade allowances to emit sulfur dioxide. Utilities that reduce their emissions below the required levels can sell their extra allowances to other utilities to help them meet their requirements. Because in 1990 electric utilities were the source of 70% of sulfur dioxide emissions and 30% of nitrogen oxide emissions, the Act targeted emissions from electric utilities. Of the desired ten-million-ton reduction in sulfur dioxide, 8.5 million tons is to come from the nation's major source, electric utilities.

The emissions reduction was implemented in two phases. In Phase I, the 263 units at 110 utility plants in twenty-one states with the highest levels of emissions were mandated to reduce their annual emissions by 3.5 million tons beginning January 1995. An additional 182 units joined Phase I voluntarily, bringing the total of Phase I units to 445. Phase II, which began January 1, 2000, affected 2,000 more units in all forty-eight contiguous states and the District of Columbia. Figure 9.5 shows the location of the highest-emitting plants in Phase I and the approximately 2,000 cleaner and smaller units throughout the nation that became involved in Phase II.

ALLOWANCE TRADING FOR SULFUR DIOXIDE.

Title IV allows companies to buy, sell, trade, and bank sulfur dioxide pollution rights. Utility units are allocated allowances based on their historic fuel consumption and a specific emissions rate. Each allowance permits a unit to emit one ton of sulfur dioxide during or after a specific year. For each ton of sulfur dioxide discharged in a given year, one allowance is retired and can no longer be used. Companies that pollute less than the set standards will have allowances left over (banked allowances). They can then sell the difference to companies that pollute more than they are allowed, bringing them into compliance with overall standards. Companies that can clean up their pollution less expensively by changing fuel or persuading their customers to conserve energy would recover some of their costs by selling their pollution rights to other companies. The EPA holds an allowance auction each year. The sale offers allowances at a fixed price. This use of market-based incentives by Title IV is regarded by many as a major new method for controlling pollution.

Utilities also took advantage of their flexibility under Title IV to choose less costly ways to reduce emissions, such as switching from high- to low-sulfur coal, and are achieving sizable reductions in their sulfur dioxide emissions. More than half of Phase I plants opted to switch to low-sulfur coal, and 16% chose to install scrubbers. Air scrubbers are treatment devices placed on the exhaust or smoke stack and used to reduce the particulate matter and other contaminants in plant emissions. Only 3% of plants initially planned to purchase allowances. Not surprisingly, the market for low-sulfur coal is growing as a result of Title IV, and the market for high-sulfur coal is decreasing.

From 1995 to 1998, however, there was considerable buying and selling of allowances among utilities. Because the utilities that participated in Phase I reduced their sulfur emissions more than the minimum required, they did not use as many allowances as they were allocated for the first four years of the program. Those unused allowances could be used to offset sulfur dioxide emissions in future years. According to figures published by the EPA in the Acid Rain Program, 2003 Progress Report (September 2004), a total of 9.5 million allowances were granted nationally for the year. To this was added the large stockpile of banked allowances carried over from prior years, making the total allowance stockpile available for 2003 19.93 million tons.

In 2003 utility sources emitted 10.6 million tons of sulfur dioxide, about a million tons more than the allowances granted in 2003 but far fewer than were in the pool of allowances available since that pool includes all the stockpiled allowances from prior years. Over time the bank of allowances is expected to be depleted further as plants use stockpiled allowances to comply with the more stringent emissions requirement of Phase II.

NITROGEN OXIDE EMISSIONS.

Title IV of the Clean Air Act Amendments (PL 101–549) maintained the traditional environmental "command and control by regulation" approach for nitrogen oxides. Under this approach, the EPA specifies how the pollution will be reduced, by what amounts, and what technology to use. The nitrogen oxides program establishes standard emissions limitations (the amount that can be discharged) for the affected units.

To encourage industry to reduce nitrogen oxides emissions before the required January 2000 date, the EPA adopted regulations in April 1995 that provided an incentive. The EPA allowed Phase II, Group I nitrogen oxides affected units, which would not have been subject to nitrogen oxide emission limits until January 2000, to use an "early election" compliance option. Under this provision, these Phase II, Group I units can demonstrate compliance with the higher Phase I limits for their boiler type from 1997 through 2007 and postpone having to meet the more stringent Phase II limits until 2008. There is one catch, however. If the utility fails to meet the annual Phase I limit for the boiler in any year, the unit is subject to the more stringent Phase II limit for Group I boilers beginning in 2000, or the year following the exceedance, whichever is later.

Clear Skies Act

On February 27, 2003, new legislation was introduced to the U.S. House of Representatives (HR 999) and the U.S. Senate (S 485), entitled the Clear Skies Act of 2003. This legislation would amend the Clean Air Act and create a mandatory program that would reduce power plant emissions of sulfur dioxide, nitrogen oxides, and mercury by setting national caps on each pollutant. The stated pollution caps are as follows:

  • Cut emissions of sulfur dioxide by 73%, reducing the emissions experienced in 2000 (eleven million tons) to a cap of three million tons in 2018.
  • Reduce nitrogen oxide emissions by 67%, reducing the 2000 emissions of five million tons to a cap of 1.7 million tons in 2018.
  • Cut mercury emissions by 69%, reducing the forty-eight tons emitted in 1999 to a cap of fifteen tons in 2018.

Environmental groups such as the Sierra Club, however, argued that the Clear Skies Act was essentially written by the energy industry, was riddled with loopholes for big polluters, and would actually weaken existing pollution rules. The Clear Skies Act of 2003 stalled in Congress. It was reintroduced in January 2005, and, as of that spring, appeared to have died in the Senate Environment and Public Works Committee.

ARE EFFORTS TO REDUCE
ACID RAIN WORKING?

The Good News

During the years since the enactment of the Acid Rain Program, much progress has been made in reducing power plant emissions across the nation, particularly in the Ohio Valley and the Northeast, where emissions have been historically highest. Figure 9.6 shows the average sulfur dioxide emissions from power plants by state for the year 1990 and for the two phases of the Program.

Figure 9.7 and Figure 9.8 present average power plant emissions of nitrogen oxide for the same time periods. Here one sees that the areas showing the most improvement are the Ohio Valley and the state of Texas.

According to the EPA's Acid Rain Program, 2003 Progress Report, great improvement has been made in reducing the pollutants that cause acid deposition. Sulfur dioxide emissions from power plants were reduced 38% between 1980 and 2003, though emissions were up slightly from 2002 to 2003. (See Figure 9.9.) Power plant emissions of nitrogen oxides over the period 1990–2003 were reduced 37%, from 6.7 million tons to 4.2 million tons. (See Figure 9.7.)

In 2003, the fourth year of the Acid Rain Program's Phase II implementation, all but one of the 3,497 Phase II affected utility units complied with the requirements to hold sufficient allowances to cover their sulfur dioxide emissions.

These reductions are even more encouraging in light of increased economic activity. The reduced sulfur and nitrogen oxide rates were achieved during a period of economic growth in which the amount of fuel burned to produceelectricityincreasedby28%.WithouttheAcid Rain Program's mandated reductions in emission rates, both sulfur dioxide and nitrogen oxide emissions from power plants would have been expected to rise in most parts of the nation as fossil fuel use rose. The fact that utility plant emissions of these pollutants have declined in the face of rising electrical production is very encouraging.

This news is offset somewhat by the contents of a report published by the U.S. Government Accountability Office, Air Pollution, Meeting the Future Electricity Demand Will Increase Emissions of Some Harmful Substances (October 2002). The report presents a forecast of energy requirements that was prepared by the Energy Information Administration. By the year 2020 electricity generation is expected to have risen by 42%. Emissions of pollutants by electric utilities are also expected to rise over this period. The good news in the report was that acid deposition producing pollutants were not expected to rise despite a significant increase in the production of electricity. Figure 9.10 presents pollution emissions by type that were projected to accompany the rise in electricity generation.

The Bad News

Important reductions in the emissions of acid rain producing pollutants have been documented. However, the ground-level data being collected on actual acid deposition has provided a somewhat less optimistic assessment. The Clean Air Status and Trends Network (CASTNET) is a scientific monitoring network established by the EPA under a mandate set forth in the 1990 Clean Air Act Amendments. The network is operated jointly by the EPA and the National Park Service.

In Clean Air Status and Trends Network 2002 Annual Report (November 2003), CASTNET presents data on the concentrations of sulfate and nitrogen particulates in the precipitation that has fallen on eastern states over the period 1990–2002. These data differ from the emissions data seen earlier. The power plant emissions of these two chemicals are down over the period in question. Their concentrations in precipitation are also down, but only slightly.

Figure 9.11, Figure 9.12, Figure 9.13, and Figure 9.14 present the sulfur particulate concentrations in precipitation collected by CASTNET in their eastern collection sites. Sulfate (SO42−) concentrations were down between 1990 and 2002, but by less than sulfate dioxide (SO2) emissions were over the same period.

Nitrate (NO3) concentrations presented for the same geographical area and time frame showed a more discouraging result. Figure 9.15 and Figure 9.16 show that nitrate concentrations in the eastern states showed no real decline over the period 1990–2001 despite declining utility plant emissions of nitrogen oxides over the period. Here again, the fact that nitrogen oxide emissions are down from sources participating in the Acid Rain Program does not mean that all human-made emissions are down.

Rising Nitrogen Oxide Emissions

A detailed analysis of the nitrogen oxide emissions data presented for all sources of emissions in the EPA's report National Air Quality and Emissions Trends Report 1999 (March 2001) shows clearly that total nitrogen oxide emissions were higher in 1999 than in 1970, 1980, and 1990.

Nitrogen oxide emissions from transportation sources were responsible for the greatest portion of the increase between 1970 and 1999, rising from 9.3 million short tons to 14.1, an increase of nearly 52%. The transportation category is divided into two sections, on-road vehicles and non-road engines and vehicles. The on-road portion rose but at a much slower pace than the non-road portion. On-road vehicle emissions, which include all automobiles and trucks, rose from 7.4 million short tons in 1970 to 8.6 million short tons in 1999, an increase of 16%. The non-road engine and vehicle category includes recreational vehicles, construction machinery, lawn and garden equipment, airplanes, railroad machinery, and marine vessels. This category rose from 1.9 million short tons in 1970 to 5.5 million in 1999, an increase of 189%. Nitrogen oxide emissions from fuel combustion sources, the second largest category in 1999, remained statistically unchanged over the period.

The Acid Rain Program had greater success in stemming sulfur dioxide emissions than it had, as of 1999, in stemming nitrogen oxide emissions.

A 1999 Assessment

In April 1999 the National Acid Precipitation Assessment Program released findings from the study National Acid Precipitation Assessment Program Biennial Report to Congress: An Integrated Assessment. The study warned that, despite important strides in reducing air pollution, acid deposition remains a serious problem in sensitive areas and provided more evidence that acid deposition is more "complex and intractable than was believed ten years ago." Among the findings were:

  • New York's Adirondack Mountain waterways suffer from serious levels of acid. Even though sulfur levels are declining, nitrogen levels are still climbing. The agency predicted that by 2040, about half the region's 2,800 lakes and ponds would be too acidic to sustain life.
  • Chesapeake Bay is suffering from excess nitrogen, which is causing algae blooms that can suffocate other life forms.
  • High elevation forests in Colorado, West Virginia, Tennessee, and Southern California are nearly saturated with nitrogen, a key ingredient in acid deposition. (Nitrogen saturation is a condition where the nitrogen levels in the soil exceed the plant needs with the result that excess nitrogen is flushed into streams where it can cause undesirable plant growth. As the nitrogen moves through the soil it strips away chemicals essential for forest fertility, increasing lake and stream acidity).
  • High elevation lakes and streams in the Sierra Nevada, the Cascades, and the Rocky Mountains may be on the verge of "chronically high acidity."

In conclusion, the report recognized the important strides made in reducing sulfur dioxide emissions since passage of the 1990 Clean Air Act Amendments. However, the slow recovery that was being recorded in lakes, streams, and forests indicated that further reductions in sulfur and nitrogen were necessary.

Anticipated Benefits of Further Abatement

There are many anticipated benefits of recovering from the effects of acid rain. Table 9.3 summarizes some of them. The benefits to human health are believed to be reduced illness and death from lung disorders and heart disease, resulting in decreased need for medical services and medical treatment. Aquatic systems and forests are expected to experience less stress, thereby preserving vital habitat and resources important to the nation's economy. In some locations, the return of badly damaged ecosystems to healthy, thriving ecosystems is another anticipated benefit.

Reduced destruction of human-made objects is also a potential benefit of further abatement of acid deposition. The reduced deterioration of vehicles, buildings, monuments, and other structures should reduce the costs that society has to pay to repair and correct destruction of this kind, and a reduction in damage and loss of cultural objects preserves our heritage for future generations.

Taken together, the overall benefits of further reducing acid deposition should be a healthier, more environmentally sound habitat for people and all living organisms.

Human health and ecosystem Effects Recovery benefits
Human healthIn the atmosphere, sulfur dioxide and nitrogen oxides become sulfate and nitrate aerosols, which increase morbidity and mortality from lung disorders, such as asthma and bronchitis, and impacts to the cardiovascular system.Decrease emergency room visits, hospital admissions, and deaths.
Surface watersAcidic surface waters decrease the survivability of animal life in lakes and streams and in the more severe instances eliminate some or all types of fish and other organisms.Reduce the acidic levels of surface waters and restore animal life to the more severely damaged lakes and streams.
ForestsAcid deposition contributes to forest degradation by impairing trees' growth and increasing their susceptibility to winter injury, insect infestation, and drought. It also causes leaching and depletion of natural nutrients in forest soil.Reduce stress on trees, thereby reducing the effects of winter injury, insect infestation, and drought, and reduce the leaching of soil nutrients, thereby improving overall forest health.
MaterialsAcid deposition contributes to the corrosion and deterioration of buildings, cultural objects, and cars, which decreases their value and increases costs of correcting and repairing damage.Reduce the damage to buildings, cultural objects, and cars, and reduce the costs of correcting and repairing future damage.
VisibilityIn the atmosphere, sulfur dioxide and nitrogen oxides form sulfate and nitrate particles, which impair visibility and affect the enjoyment of national parks and other scenic views.Extend the distance and increase the clarity at which scenery can be viewed, thus reducing limited and hazy scenes and increasing the enjoyment of national parks and other vistas.

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