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 (NO_x ) 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 NO_x 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 NO_x 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 NO_x 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 (NO_x )—the main air pollutants that cause the problems—from 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 NO_x emission reduction program aims to reduce annual NO_x emissions from coal-fired electric utility boilers by 2 million tons below what they would have been without Title IV. The NO_x component of the program does not include a cap on NO_x 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 dramatic—and 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 NO_x have been reduced by 1.5 million tons from 1990 levels (about 3 million tons lower than projected growth). Because the NO_x component of the program includes no cap, there is no guarantee that NO_x 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):789–800.

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):180–198.

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):185–198.

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):244–246.

Lovett, Gary. "Atmospheric Deposition of Nutrients and Pollutants in North America: An Ecological Perspective." Ecological Applications 4, no. 4 (1994):629–650.

Stoddard, John L. et al. "Regional Trends in Aquatic Recovery from Acidification inNorth America and Europe." Nature 401 (7 Oct. 1999):575–578.

U.S. Environmental Protection Agency, Clean Air Markets Division. EPA Acid Rain Program—2001 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.

acid rain Acid rain became one of the most emotive environmental issues of the 1970s and 1980s, yet the presence of high concentrations of acids in the urban rainfall of Manchester, in comparison with surrounding rural areas, was identified in a systematic way over a hundred years ago by Angus Smith, an industrial chemist who was employed as the first Inspector of Factories. As a contemporary environmental issue, the acid rain problem came to the forefront of scientific and public awareness in Europe after the first United Nations Environment Conference in Stockholm in 1972 when ‘evidence’ for a number of detrimental environmental impacts of acid rain was presented. The impacts identified were a rapid increase in the acidity of European rainfall, a parallel increase in the acidity of Swedish rivers and lakes, a decline in fish populations in these rivers and lakes, and a decline in forest growth. In the decades since this conference, the scientific community has endeavoured to evaluate in a systematic way whether there is a sound body of evidence to substantiate these claims. Providing substantive evidence for the effects listed above has proved to be a challenging task for environmental scientists, not least because of the complexity in linking essentially gaseous sources to the environmental effects of acid rain. In consequence, an understanding of the acid rain problem needs to focus upon some of the fundamental principles and concepts involved, of which the first is ‘What do we mean by the term “acid rain”?’

Natural rainfall contains many impurities, including dissolved gases, dusts, and salts which it picks up during its passage through the atmosphere to the ground. These constituents can come from a variety of natural sources, which include volcanic and biologically produced gases, sea spray and wind-blown dusts. Impurities in rainfall also come from a variety of polluting sources, such as fossil-fuel combustion and motor vehicles; these impurities include both gaseous and particulate emissions. In order to understand the effect that both natural and polluting substances have on the acidity of rainfall it is essential to start with an examination of the way in which acidity is measured.

Acidity is measured on what is called the pH scale. The scale was developed by chemists at the beginning of the twentieth century to provide a simple way of expressing the concentration of free hydrogen ions (H⁺) in a solution. (Free hydrogen is the hydrogen in a solution which is not part of the water molecule, H₂O.) The pH scale is potentially confusing to the non-chemist, since it is negative and is based upon logarithmic units. This means that the concentration of H⁺ increases as the pH value falls. Starting at a neutral pH of 7, where there are no free hydrogen ions, each single unit decrease in pH means that there is ten times more hydrogen in the solution. Between pH 6 and pH 2, therefore, there is a 10 000-fold increase in concentration. The relationship between hydrogen ion concentration and pH is shown in Fig. 1.

One of the most common natural atmospheric gases is carbon dioxide, CO₂, which dissolves in rainwater to form carbonic acid, giving it a pH of around 5.6. The presence of other natural acids can reduce the pH of unpolluted rain to around 5. Dust in rain can change the pH by making it either more or less acidic according to the chemical properties of the dust. While natural rainfall is acidic, the presence of pollutant gases, particularly those containing oxides of nitrogen, sulphur, and chlorine, serves to increase the acidity. In the industrial regions of North America and Europe the presence of these dissolved gases generally increases the acidity by a factor of 10 (i.e. it produces rainfall with a pH of around 4).

A second fundamental issue in understanding the acid rain problem is to identify the types of pollutant gaseous emissions which produce acid rain and to identify how, and at what rate, acid rain is subsequently produced. As indicated above, the principal pollutant emissions leading to the formation of acid rain are oxides of nitrogen and sulphur. Sulphur is present in fossil fuels and is liberated by their combustion, especially in power generating stations and by the smelting of ores. Nitrogen oxides are generated during the combustion process in the internal combustion engine because of the presence of nitrogen in the air. The degree to which this process occurs depends on the temperature of the reaction. Some nitrogen gases, especially ammonia, are alkaline when dissolved in rain. As ammonia is oxidized, however, it will eventually contribute to acidity. Some 80 per cent of ammonia in the atmosphere is estimated to have come from livestock wastes. Chlorine is a third acid-forming gas, of which 75 per cent in Europe is estimated to come from the burning of fossil fuels. The rate at which sulphur, nitrogen, and chlorine are oxidized in the atmosphere to produce acid rain depends in part on the presence of volatile organic carbons that are also liberated by industrial processes. Furthermore, industry produces considerable quantities of carbon dioxide which influence the amount of carbonic acid present in rainfall. Emissions from point sources such as chimney stacks can be dispersed downwind for hundreds or even thousands of kilometres and are usually retained within an atmospheric layer less than a kilometre thick. The rate at which acids are produced from gases through the oxidation process is fairly slow, and it has been estimated that the conversion rate is only between 1 and 3 per cent per hour. These gases and oxides are also diluted by as much as 10 000 times in the atmosphere as they disperse.

Environmental scientists have identified a number of ways by which the oxides produced by atmospheric conversion can reach the ground surface without necessarily involving rainfall. The oxides can exist in dusts which reach vegetation surfaces or the ground surface as dry particulate fallout, or as dissolved constituents which may reach the ground surface as wet fallout, which includes not only rain, but snow, mist, and low cloud. In the case of low cloud, acid droplets are deposited on vegetated surfaces by a process called ‘occult deposition’. A generalized scheme showing the production, dispersal, chemical transformation, and deposition of acids is shown in Fig. 2.

It has been estimated that the emission of sulphur through the combustion of fossil fuels increased from 1850, when emissions were around 0.5 Mtonne (0.5 × 10⁶ metric tonnes), to 1965, when emissions reached a peak of some 3.5 Mtonne. After 1965, global emissions declined sharply as a result of changing practices in industry and the use of cleaner fuels such as gas and nuclear power. While sulphur emissions have declined, nitrogen emissions have increased, particularly as a result of the increase in road traffic. Although there have been substantial reductions in sulphur emissions, there is little evidence to suggest that this is having a dramatic impact upon the acidity of rainfall.

In trying to link the acidity of rainfall to the acidity of soils and water it is essential to understand the role that soils play in releasing chemicals into solution as a result of natural weathering. Soils are made up of two major components: organic matter, which results from the decomposition of vegetation, and the physically and chemically altered parent material. In soils with a high content of organic matter, and in areas of high rainfall, water draining through the soil becomes more acidic with the release of organic acids. Some parent materials, especially those containing carbonate rocks, can buffer this acidity and raise the pH of drainage waters. The pathways that water takes through the soil to the rivers are also important because they will determine whether or not the drainage water comes into contact with that part of the soil which can buffer the acidity. The sensitivity of different areas of the UK to acid inputs is highly variable. In regions with high rainfall, acid soils, and hard crystalline rocks, there is a potentially high susceptibility to the input of acid rain with little opportunity to buffer the input (Fig. 3).

The presence of hydrogen ions in rainfall, soil solutions, rivers, and lakes has a secondary impact on the environment through the release of potentially toxic metals, particularly aluminium, which is present in trace amounts in almost all soils. Aluminium is most toxic in the pH range 5–6. It has a number of known effects, such as impairing respiration in fish and possibly reducing the growth rate of phytoplankton communities in fresh water.

Vegetation is important for two reasons. First, it produces leaf litter, which decomposes in the soil to produce organic matter which in turn liberates organic acids. Some tree species especially conifers, are known to produce more organic acids than others. Scientific evidence obtained from studies of lake sediments in North Wales has, however, indicated that acidification began well before the planting of upland forests. It seems, nevertheless, that the rate of acidification increased after forest plantation. Vegetation also plays an important role in trapping dusts and acid aerosols by occult deposition. The combination of these two processes, in addition to the release of organic acids from leaf surfaces, also serves to increase the acidity of water dripping through the forest canopy (as throughfall: see hydrological cycle). Some studies have shown that the pH of throughfall can be at least one unit lower than the acidity of rain as it reaches the canopy surface, that is, the concentration of hydrogen ions is ten times greater. The effect of acidity upon forest growth rates is unproved. Forest decline is not a specific disease with a single cause; many factors, such as acidity, climate change, and the preferential removal of essential elements like magnesium from soils, may all play a part in the decline in forest growth.

The body of scientific evidence accumulated since the 1972 United Nations Stockholm conference has improved our understanding of the acid rain problem. There are, however, many unresolved issues and only recently have scientists begun to turn their attention to the next major phase in acid rain research, which is to find out whether or not the observed chemical and biological changes to the environment are reversible and sustainable.

Ian D. L. Foster

Bibliography

Battarbee, R. W. (1988) Lake acidification in the United Kingdom. HMSO, London.
Foster, I. D. L. (1991) Environmental pollution. Oxford University Press.
Howells, G. (1995) Acid rain and acid waters (2nd edn). Ellis Horwood, London.
Regens, J. L. and and R. W. Rycroft (1988) The acid rain controversy. University of Pittsburgh Press, Pittsburgh.
Steinberg, C. E. W. and R. F. Wright (eds) (1994) Acidificationof freshwater ecosystems; implications for the future. John Wiley and Sons, Chichester.
The Quality of Urban Air Group (1993) Urban air quality in the United Kingdom. Department of the Environment, Bradford.

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 hundreds—even thousands—of 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 alkaline—or basic—a 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 ]

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 (1817–1884), 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 filters—devices 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

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.

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 .

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; NO_x (Nitrogen Oxides); Petroleum; Sulfur Dioxide; Vehicular Pollution.

Internet Resource

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

William E. Sharpe

acid rain or acid deposition, form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids ( p H 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 .

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).

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.

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.

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.

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

acid rain See acid precipitation.