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

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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.