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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.


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,


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,, 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" ( 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,


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


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.


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.


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


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, (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.)


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.


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. 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.


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.


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, 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.


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.


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,, 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,, 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,, 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,


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.


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



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 (, 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.


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


Regional changes in air quality and deposition of sulfur and nitrogen, 198991 and 200305
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, (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, 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, 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.


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.


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, (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


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 <>.

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 <>.

Internet Resources

Clean Air Markets. U.S. Environmental Protection Agency. <>.

National Acid Precipitation Assessment Program. <>.

National Atmospheric Deposition Program. <>.

Nilles, Mark A. Atmospheric Deposition Program of the U.S. Geological Survey. U.S. Geological Survey. <>.


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" 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] )


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, 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.


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

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." The Columbia Encyclopedia, 6th ed.. . 18 Dec. 2017 <>.

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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." A Dictionary of Biology. . 18 Dec. 2017 <>.

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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


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." Plant Sciences. . 18 Dec. 2017 <>.

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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." World Encyclopedia. . 18 Dec. 2017 <>.

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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." A Dictionary of Earth Sciences. . 18 Dec. 2017 <>.

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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." The Oxford Pocket Dictionary of Current English. . 18 Dec. 2017 <>.

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acid rain

acid rain See acid precipitation.

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"acid rain." A Dictionary of Ecology. . 18 Dec. 2017 <>.

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