acid rain

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Precipitation that incorporates anthropogenic acids and acidic materials. The deposition of acidic materials on the Earth's surface occurs in both wet and dry forms as rain, snow, fog, dry particles, and gases. Although 30% or more of the total deposition may be dry, very little information that is specific to this dry form is available. In contrast, there is a large and expanding body of information related to the wet form: acid rain or acid precipitation. Acid precipitation, strictly defined, contains a greater concentration of hydrogen (H⁺) than of hydroxyl (OH⁻) ions, resulting in a solution pH less than 7. Under this definition, nearly all precipitation is acidic. The phenomenon of acid deposition, however, is generally regarded as being anthropogenic, that is, resulting from human activity.

Theoretically, the natural acidity of precipitation corresponds to a pH of 5.6, which represents the pH of pure water in equilibrium with atmospheric concentrations of carbon dioxide. Atmospheric moisture, however, is not pure, and its interaction with ammonia, oxides of nitrogen and sulfur, and windblown dust results in a pH between 4.9 and 6.5 for most “natural” precipitation. The distribution and magnitude of precipitation pH in the United States (illustration) suggest the impact of anthropogenic rather than natural causes. The areas of highest precipitation acidity (lowest pH) correspond to areas within and downwind of heavy industrialization and urbanization where emissions of sulfur and nitrogen oxides are high. It is with these emissions that the most acidic precipitation is thought to originate.

Distribution of rainfall pH in the eastern United States.

The transport of acidic substances and their precursors, chemical reactions, and deposition are controlled by atmospheric processes. In general, it is convenient to distinguish between physical and chemical processes, but it must be realized that both types may be operating simultaneously in complicated and interdependent ways. The physical processes of transport by atmospheric winds and the formation of clouds and precipitation strongly influence the patterns and rates of acidic deposition, while chemical reactions govern the forms of the compounds deposited.

There are a number of chemical pathways by which the primary pollutants, sulfur dioxide (SO₂) from industry, nitric oxide (NO) from both industry and automobiles, and reactive hydrocarbons mostly from trees, are transformed into acid-producing compounds. Some of these pathways exist solely in the gas phase, while others involve the aqueous phase afforded by the cloud and precipitation. As a general rule, the volatile primary pollutants must first be oxidized to more stable compounds before they are efficiently removed from the atmosphere. Ironically, the most effective oxidizing agents, hydrogen peroxide (H₂O₂) and ozone (O₃), arise from photochemical reactions involving the primary pollutants themselves. See also Air pollution.

The effect of acid deposition on a particular ecosystem depends largely on its acid sensitivity, its acid neutralization capability, the concentration and composition of acid reaction products, and the amount of acid added to the system. As an example, the major factors influencing the impact of acidic deposition on lakes and streams are (1) the amount of acid deposited; (2) the pathway and travel time from the point of deposition to the lake or stream; (3) the buffering characteristics of the soil through which the acidic solution moves; (4) the nature and amount of acid reaction products in soil drainage and from sediments; and (5) the buffering capacity of the lake or stream.

Acid precipitation may injure trees directly or indirectly through the soil. Foliar effects have been studied extensively, and it is generally accepted that visible damage occurs only after prolonged exposure to precipitation of pH 3 or less (for example, acid fog or clouds). Measurable effects on forest ecosystems will then more likely result indirectly through soil processes than directly through exposure of the forest canopy. Many important declines in the condition of forest trees have been reported in Europe and North America during the period of increasing precipitation acidity. These cases include injury to white pine in the eastern United States, red spruce in the Appalachian Mountains of eastern North America, and many economically important species in central Europe. Since forest trees are continuously stressed by competition for light, water, and nutrients; by disease organisms; by extremes in climate; and by atmospheric pollutants, establishing acid deposition as the cause of these declines is made more difficult. Each of these sources of stress, singly or in combination, produces similar injury. However, a large body of information indicates that accelerated soil acidification resulting from acid deposition is an important predisposing stress that in combination with other stresses has resulted in increased decline and mortality of sensitive tree species and widespread reduction in tree growth. See also Forest ecosystem.

Acidic deposition impacts aquatic ecosystems by harming individual organisms and by disrupting flows of energy and materials through the ecosystem. The effect of acid deposition is commonly assessed by studying aquatic invertebrates and fish. Aquatic invertebrates live in the sediments of lakes and streams and are vitally important to the cycling of energy and material in aquatic ecosystems. These small organisms break down large particulate organic matter for further degradation by microorganisms, and they are an important food source for fish, aquatic birds, and predatory invertebrates.

Currently, there are concerns that acid deposition is causing the loss of fish species, through physiological damage and by reproductive impairment. While fish die from acidification, their numbers and diversity are more likely to decline from a failure to reproduce.The effects of acid deposition on individuals in turn elicit changes in the composition and abundance of communities of aquatic organisms. The degree of change depends on the severity of acidification, and the interaction of other factors, such as metal concentrations and the buffering capacity of the water. The pattern most characteristic of aquatic communities in acidified waters is a loss of species diversity, and an increase in the abundance of a few, acid-tolerant taxa.

Community-level effects may occur indirectly, as a result of changes in the food supply and in predator-prey relations. Reduction in the quality and amount of periphyton may decrease the number of herbivorous invertebrates, which may in turn reduce the number of organisms (predatory invertebrates and fish) that feed upon herbivorous invertebrates. The disappearance of fish may result in profound changes in plant and invertebrate communities. Dominant fish species function as keystone predators, controlling the size distribution, diversity, and numbers of invertebrates. Their reduction alters the interaction within and among different levels of the food web and the stability of the ecosystem as a whole.

The impact of acid deposition on terrestrial and aquatic ecosystems is not uniform. While increases in acid deposition may stress some ecosystems and reduce their stability and productivity, others may be unaffected. The degree and nature of the impact depend on the acid input load, organismal susceptibility, and buffering capacity of the particular ecosystem. See also Biogeochemistry.

"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.

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

Bibliography

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

— MORTON LIPPMANN

When fossil fuels are burned, dioxides of sulphur and nitrogen are released into the air. When inhaled, these dry deposits can lead to breathing problems. The pollutants dissolve in atmospheric water particles to form acid rain.

Industrial development, particularly in the mid-latitudes of the Northern Hemisphere, has been responsible for the emission of increasing quantities of such atmospheric pollutants, which can travel large distances, generally being carried eastwards by the prevailing westerly winds; the OECD estimated in the early 1990s that 85% of the sulphur dioxide deposition on the Nordic countries was ‘imported’. The primary sources were the former USSR, Germany, and Poland.

Any form of atmospheric water, such as rain, dew, or snow, with a pH of less than 5.6 is properly termed acid precipitation. (Note that the pH scale is logarithmic, so that an increase of one point represents a tenfold increase in acidity.) When the concentration of sulphur dioxide reaches 0.2 p.p.m., acid precipitation is toxic to vegetation. Extensive damage has been reported, for example, in the Black Forest, Germany. Humans are at risk when the concentration rises above 1 p.p.m.

Within soils, acidification seems to limit bacterial activity, displace nutrient ions by hydrogen ions, and liberate toxic heavy metals such as aluminium and lead, which may contaminate drinking water. High levels of aluminium in lake water in Scandinavia have been linked to acid emissions from the UK, and have caused the destruction of aquatic flora and fauna. Aquatic ecosystems seem to react more rapidly than terrestrial systems to acidification. Acid precipitation may also attack building-stone containing calcium and magnesium carbonates.

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

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

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

Efforts to abate acid rain have focused on two pollutants, sulfur dioxide, a by-product of burning coal or fuel oil, and nitrogen oxides generated largely by automobiles and power plants. In the United States the federal Clean Air Act (CAA) of 1970 restricted both pollutants. However, acid rain was not the motivating factor behind the CAA, and studies later suggested the law may have worsened the problem. When monitoring devices were placed near factories, many firms simply built taller smokestacks to disperse pollutants higher into the atmosphere, away from the monitors. Consequently acid rain spread even wider. In 1977 amendments to the CAA required that utilities install scrubbers in each new coal-fired power plant. Implementation of these and additional amendments in the 1980s are credited for decreasing annual sulfur dioxide emissions in the United States from 26 to 21 million metric tons by 1989. Similarly nitrogen oxide emissions, which peaked at 22 million metric tons in 1981, fell to 19 million tons by 1990.

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

Bibliography

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

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

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

—Hugh Gorman/W. P.

Rainwater that is acidic because it contains sulfur dioxide and other pollutants emitted from some industrial facilities. In parts of the United States and Canada, acid rain has damaged and even caused the death of forest trees many hundreds of miles from the source of the emissions.

A type of precipitation made up of dilute acids, primarily a by-product of heavy industry.

Rain which contains materials, sulfates and oxidation products of nitrogen and sulfur particularly, produced by combustion of coal and oil in industrial processes and by motor vehicles; lowers the pH of water bodies; see acid waters; credited with local decline of fish culture.

• Environment, Ecology, and Animal Behavior - acid rain: rain with abnormally high acid level, esp. due to combustion of fossil fuels
• Disasters and Phenomena - acid rain: precipitation with excess acidity due to industrial pollutants

Processes involved in acid deposition (note that only SO₂ and NO_x play a significant role in acid rain).

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen oxides which react with the water molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the release of sulfur dioxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes and sulfur dioxide is produced by volcanic eruptions. The chemicals in acid rain can cause paint to peel, corrosion of steel structures such as bridges, and erosion of stone statues.

Contents 1 Definition 2 History 2.1 History of acid rain in the United States 3 Emissions of chemicals leading to acidification 3.1 Natural phenomena 3.2 Human activity 4 Chemical processes 4.1 Gas phase chemistry 4.2 Chemistry in cloud droplets 5 Acid deposition 5.1 Wet deposition 5.2 Dry deposition 6 Adverse effects 6.1 Surface waters and aquatic animals 6.2 Soils 6.3 Forests and other vegetation 6.4 Human health effects 6.5 Other adverse effects 7 Affected areas 8 Prevention methods 8.1 Technical solutions 8.2 International treaties 8.3 Emissions trading 9 See also 10 References 11 External links

Definition

"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloudwater, and dew) and dry (acidifying particles and gases) acidic components. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline. “Clean” or unpolluted rain has a slightly acidic pH of over 5.7, because carbon dioxide and water in the air react together to form carbonic acid, but unpolluted rain also contains other chemicals. A common example is nitric acid produced by electric discharge in the atmosphere such as lightning.^[1] Carbonic acid is formed by the reaction

H₂O (l) + CO₂ (g) H₂CO₃ (aq)

Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:

H₂O (l) + H₂CO₃ (aq) HCO₃⁻ (aq) + H₃O⁺ (aq)

Acid deposition as an environmental issue would include additional acids to H₂CO₃.

History

Trees killed by acid rain

The corrosive effect of polluted, acidic city air on limestone and marble was noted in the 17th century by John Evelyn, who remarked upon the poor condition of the Arundel marbles.^[2] Since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased.^[3][4] In 1852, Robert Angus Smith was the first to show the relationship between acid rain and atmospheric pollution in Manchester, England.^[5] Though acidic rain was discovered in 1852, it was not until the late 1960s that scientists began widely observing and studying the phenomenon.^[6] The term "acid rain" was coined in 1872 by Robert Angus Smith.^[7] Canadian Harold Harvey was among the first to research a "dead" lake. Public awareness of acid rain in the U.S increased in the 1970s after The New York Times promulgated reports from the Hubbard Brook Experimental Forest in New Hampshire of the myriad deleterious environmental effects demonstrated to result from it.^[8][9]

Occasional pH readings in rain and fog water of well below 2.4 have been reported in industrialized areas.^[3] Industrial acid rain is a substantial problem in China and Russia^[10][11] and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity.^[12] The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation.^[13][14] Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the greatest deposition (simply because of their higher rainfall). An example of this effect is the low pH of rain which falls in Scandinavia.^[15]

History of acid rain in the United States

Since 1998, Harvard University wraps some of the bronze and marble statues on its campus, such as this "Chinese stele", with waterproof covers every winter, in order to protect them from erosion caused by acid rain (or, actually, acid snow)^[16]

In 1980, the U.S. Congress passed an Acid Deposition Act.^[17] This Act established a 18-year assessment and research program under the direction of the National Acidic Precipitation Assessment Program (NAPAP). NAPAP looked at the entire problem from a scientific perspective. It enlarged a network of monitoring sites to determine how acidic the precipitation actually was, and to determine long term trends, and established a network for dry deposition. It looked at the effects of acid rain and funded research on the effects of acid precipitation on freshwater and terrestrial ecosystems, historical buildings, monuments, and building materials. It also funded extensive studies on atmospheric processes and potential control programs. From the start, policy advocates from all sides attempted to influence NAPAP activities to support their particular policy advocacy efforts, or to disparage those of their opponents.^[17] For the U.S. Government's scientific enterprise, a significant impact of NAPAP were lessons learned in the assessment process and in environmental research management to a relatively large group of scientists, program managers and the public.^[18]

In 1991, DENR provided its first assessment of acid rain in the United States. It reported that 5% of New England Lakes were acidic, with sulfates being the most common problem. They noted that 2% of the lakes could no longer support Brook Trout, and 6% of the lakes were unsuitable for the survival of many species of minnow. Subsequent Reports to Congress have documented chemical changes in soil and freshwater ecosystems, nitrogen saturation, decreases in amounts of nutrients in soil, episodic acidification, regional haze, and damage to historical monuments.

Meanwhile, in 1989, the US Congress passed a series of amendments to the Clean Air Act. Title IV of these amendments established the Acid Rain Program, a cap and trade system designed to control emissions of sulfur dioxide and nitrogen oxides. Title IV called for a total reduction of about 10 million tons of SO₂ emissions from power plants. It was implemented in two phases. Phase I began in 1995, and limited sulfur dioxide emissions from 110 of the largest power plants to a combined total of 8.7 million tons of sulfur dioxide. One power plant in New England (Merrimack) was in Phase I. Four other plants (Newington, Mount Tom, Brayton Point, and Salem Harbor) were added under other provisions of the program. Phase II began in 2000, and affects most of the power plants in the country.

During the 1990s, research continued. On March 10, 2005, EPA issued the Clean Air Interstate Rule (CAIR). This rule provides states with a solution to the problem of power plant pollution that drifts from one state to another. CAIR will permanently cap emissions of SO₂ and NOx in the eastern United States. When fully implemented, CAIR will reduce SO₂ emissions in 28 eastern states and the District of Columbia by over 70 percent and NOx emissions by over 60 percent from 2003 levels.^[19]

Overall, the Program's cap and trade program has been successful in achieving its goals. Since the 1990s, SO₂ emissions have dropped 40%, and according to the Pacific Research Institute, acid rain levels have dropped 65% since 1976.^[20][21] However, this was significantly less successful than conventional regulation in the European Union, which saw a decrease of over 70% in SO₂ emissions during the same time period.^[22]

In 2007, total SO₂ emissions were 8.9 million tons, achieving the program's long term goal ahead of the 2010 statutory deadline.^[23]

The EPA estimates that by 2010, the overall costs of complying with the program for businesses and consumers will be $1 billion to $2 billion a year, only one fourth of what was originally predicted.^[20]

Emissions of chemicals leading to acidification

The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form nitric acid are of increasing importance due to stricter controls on emissions of sulfur containing compounds. 70 Tg(S) per year in the form of SO₂ comes from fossil fuel combustion and industry, 2.8 Tg(S) from wildfires and 7-8 Tg(S) per year from volcanoes.^[24]

Natural phenomena

The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes. Thus, for example, fumaroles from Laguna Caliente crater of Poás Volcano create extremely high amounts of acid rain and fog with acidity 2 of pH, clearing an area of any vegetation and frequently causing irritation to the eyes and lungs of inhabitants in nearby settlements.^[25] Acid-producing gasses are created also by biological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur containing compounds is dimethyl sulfide.

Nitric acid in rainwater is an important source of fixed nitrogen for plant life, and is also produced by electrical activity in the atmosphere such as lightning.

Acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.^[13]

Soils of Coniferous forests are naturally very acidic due to the shedding of needles and this phenomenon should not be confused with acid rain.

Human activity

The coal-fired Gavin Power Plant in Cheshire, Ohio

The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, factories, and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometers in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke but this caused many problems locally; thus, factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage.

Chemical processes

Combustion of fuels produces sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid.^[26]

Gas phase chemistry

In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:^[5]

SO₂ + OH· → HOSO₂·

which is followed by:

HOSO₂· + O₂ → HO₂· + SO₃

In the presence of water, sulfur trioxide (SO₃) is converted rapidly to sulfuric acid:

SO₃ (g) + H₂O (l) → H₂SO₄ (l)

Nitrogen dioxide reacts with OH to form nitric acid:

NO₂ + OH· → HNO₃

Chemistry in cloud droplets

When clouds are present, the loss rate of SO₂ is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets.

Hydrolysis

Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:

SO₂ (g) + H₂O SO₂·H₂O

SO₂·H₂O H⁺ + HSO₃⁻

HSO₃⁻ H⁺ + SO₃²⁻

Oxidation

There are a large number of aqueous reactions that oxidize sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).^[5]

Acid deposition

Wet deposition

Wet deposition of acids occurs when any form of precipitation (rain, snow, and so on.) removes acids from the atmosphere and delivers it to the Earth's surface. This can result from the deposition of acids produced in the raindrops (see aqueous phase chemistry above) or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosols are both of importance for wet deposition.

Dry deposition

Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition.^[27] This occurs when particles and gases stick to the ground, plants or other surfaces.

Adverse effects

This chart shows that not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid; for example, frogs can tolerate water that is more acidic (i.e., has a lower pH) than trout.

Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms as well as causing damage to buildings and having impacts on human health.

Surface waters and aquatic animals

Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals. At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish. As lakes and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States.^[28] However, the extent to which acid rain contributes directly or indirectly via runoff from the catchment to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable. The United States Environmental Protection Agency's (EPA) website states: "Of the lakes and streams surveyed, acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams".^[28]

Soils

Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pHs and are killed.^[29] The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins such as aluminium, and leach away essential nutrients and minerals such as magnesium.^[30]

2 H⁺ (aq) + Mg²⁺ (clay) 2 H⁺ (clay) + Mg²⁺ (aq)

Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain thereby affecting sensitive species, such as sugar maple (Acer saccharum).^[31][32]

Forests and other vegetation

Effect of acid rain on a forest, Jizera Mountains, Czech Republic

Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.

Other plants can also be damaged by acid rain, but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death.^[33][34]

Human health effects

Acid rain does not directly affect human health. The acid in the rainwater is too dilute to have direct adverse effects. However, the particulates responsible for acid rain (sulfur dioxide and nitrogen oxides) do have an adverse effect. Increased amounts of fine particulate matter in the air do contribute to heart and lung problems including asthma and bronchitis.^[35]

Other adverse effects

Effect of acid rain on statues

Acid rain can also damage buildings and historic monuments and statues, especially those made of rocks, such as limestone and marble, that contain large amounts of calcium carbonate. Acids in the rain react with the calcium compounds in the stones to create gypsum, which then flakes off.

CaCO₃ (s) + H₂SO₄ (aq) CaSO₄ (aq) + CO₂ (g) + H₂O (l)

The effects of this are commonly seen on old gravestones, where acid rain can cause the inscriptions to become completely illegible. Acid rain also increases the corrosion rate of metals, in particular iron, steel, copper and bronze.^[36][37]

Affected areas

Places significantly impacted by acid rain around the globe include most of eastern Europe from Poland northward into Scandinavia,^[38] the eastern third of the United States,^[39] and southeastern Canada. Other affected areas include the southeastern coast of China and Taiwan.^{[citation needed]}

Prevention methods

Technical solutions

Many coal-burning power plants use flue-gas desulfurization (FGD) to remove sulfur-containing gases from their stack gases. For a typical coal-fired power station, FGD will remove 95 percent or more of the SO2 in the flue gases. An example of FGD is the wet scrubber which is commonly used. A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoke stack gases from a power plant into the tower. Lime or limestone in slurry form is also injected into the tower to mix with the stack gases and combine with the sulfur dioxide present. The calcium carbonate of the limestone produces pH-neutral calcium sulfate that is physically removed from the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates.

In some areas the sulfates are sold to chemical companies as gypsum when the purity of calcium sulfate is high. In others, they are placed in landfill. However, the effects of acid rain can last for generations, as the effects of pH level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life.

Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles.

International treaties

A number of international treaties on the long range transport of atmospheric pollutants have been agreed for example, Sulphur Emissions Reduction Protocol under the Convention on Long-Range Transboundary Air Pollution. Canada and the US signed the Air Quality Agreement in 1991. Most European countries and Canada have signed the treaties.

Emissions trading

Main article: Emissions trading

See also: Acid Rain Program

In this regulatory scheme, every current polluting facility is given or may purchase on an open market an emissions allowance for each unit of a designated pollutant it emits. Operators can then install pollution control equipment, and sell portions of their emissions allowances they no longer need for their own operations, thereby recovering some of the capital cost of their investment in such equipment. The intention is to give operators economic incentives to install pollution controls.

The first emissions trading market was established in the United States by enactment of the Clean Air Act Amendments of 1990. The overall goal of the Acid Rain Program established by the Act^[40] is to achieve significant environmental and public health benefits through reductions in emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x), the primary causes of acid rain. To achieve this goal at the lowest cost to society, the program employs both regulatory and market based approaches for controlling air pollution.

See also

• Basic precipitation
• List of environmental issues
• Lists of environmental topics
• Rain dust (an alkaline rain)

References

1. ^ Likens, G. E., W. C. Keene, J. M. Miller and J. N. Galloway. 1987. Chemistry of precipitation from a remote, terrestrial site in Australia. J. Geophys. Res. 92(D11):13,299-13,314.
2. ^ E. S. de Beer, ed. The Diary of John Evelyn, III, 1955 (19 September 1667) p. 495.
3. ^ ^{a b} New Science Directorate Bio Mass Burning Redirect
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5. ^ ^{a b c} Seinfeld, John H.; Pandis, Spyros N (1998). Atmospheric Chemistry and Physics — From Air Pollution to Climate Change. John Wiley and Sons, Inc. ISBN 978-0-471-17816-3
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7. ^ EPA: Acid Rain in New England, A Brief History
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9. ^ "Search the HBES Publications". Hubbardbrook.org. doi:10.1029/2005JG000157. http://www.hubbardbrook.org/pubs/pub_search.php. Retrieved 2010-11-18. 
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20. ^ ^{a b} 'Cap-and-trade' model eyed for cutting greenhouse gases, San Francisco Chronicle, December 3, 2007.
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24. ^ Berresheim, H.; Wine, P.H. and Davies D.D., (1995). Sulfur in the Atmosphere. In Composition, Chemistry and Climate of the Atmophere, ed. H.B. Singh. Van Nostran Rheingold ISBN
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40. ^ Clean Air Act Amendments of 1990, 42 U.S. Code 7651

External links

• National Acid Precipitation Assessment Program Report - a 98-page report to Congress (2005)
• Acid rain for schools
• Acid rain for schools - Hubbard Brook
• Acid Rain Retirement Fund environmental education links about acid rain
• United States Environmental Protection Agency - New England Acid Rain Program (superficial)
• Acid Rain (more depth than ref. above)
• U.S. Geological Survey - What is acid rain?
• Acid rain analysis - freeware for simulation and evaluation of titration curves and pH calculations
• CBC Digital Archives – Acid Rain: Pollution and Politics
• Larssen, Thorjørn et al. “Acid Rain in China”. Environmental Science and Technology, 40:2, 2006, pp 418-425.
• Acid Rain: A Continuing National Tragedy - a report from The Adirondack Council on acid rain in the Adirondack region (1998)
• Assortment of Summaries on Acid Rain
• Acid rain linked to decline of the wood thrush, a songbird of the Eastern Forest
• Trouble in the Forest, a 1988 documentary hosted by David Suzuki
• What Happens to Acid Rain?
• Acid Rain and how it affects fish and other aquatic organisms

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