26 August 2011

Acid rain

Acid rain is a term that describes a combination of wet (rain, snow, sleet, fog and cloud water, dew) and dry (gases, dust, particles) materials that fall from the atmosphere to the surface of the earth. This natural depositing of materials is known as deposition. Normal rain has a pH (level of acidity) of about 5.6, while acid rain is defined as having a pH of less than 5.6.



Acid rain is a term that describes a combination of wet (rain, snow, sleet, fog and cloud water, dew) and dry (gases, dust, particles) materials that fall from the atmosphere to the surface of the earth. This natural depositing of materials is known as deposition. Normal rain has a pH (level of acidity) of about 5.6, while acid rain is defined as having a pH of less than 5.6.

Early history: The appearance of acid rain began with the Industrial Revolution in the mid-1800s due to the burning of fossil fuels for steam power.

British scientist Robert Angus Smith coined the term "acid rain" in 1852 to describe the correlation between the polluted air of London and the acidic precipitation in the area. In Air and Rain (1872), Smith published the findings of experiments analyzing rain water around the British Isles and determined that manufacturing towns and cities were the most acidic.

Acid rain (United States): In 1967, the Hubbard Brook Experimental Forest in the White Mountain National Forest of New Hampshire was the site of the first documented notice of acid rain in North America.

In 1980, the U.S. Congress passed the Acid Precipitation Act, establishing the National Acidic Precipitation Assessment Program (NAPAP) to investigate both causes and effects of acid deposition around the United States. NAPAP provided its first assessment to Congress in 1991. Subsequent reports to Congress have documented chemical changes in soil and freshwater ecosystems, nitrogen saturation, decreases in nutrients in the soil, episodic acidification, regional haze, and damage to monuments.

In 1990, Congress enacted the Clean Air Act, which designated the Environmental Protection Agency (EPA) as responsible for protecting and improving the nation's air quality. Title IV of the Clean Air Act details the EPA's responsibilities regarding acid rain; Title IV states that acid deposition is a problem of national and international significance and is detrimental to ecosystems, natural resources, visibility, materials, and public health.

Acid rain (worldwide): From the late 1960s through the 1970s, the Scandinavian countries promoted awareness of the dangers of acid rain and its effects on the environment. In Sweden, Svante Od©n, a professor of ecological science at the Swedish University of Agricultural Sciences, published "The Acidification of Air and Precipitation and its Consequences in the Natural Environment" in the newspaper Dagens Nyheter in October 1967. Od©n described the damaging effects of acid rain on the environment, including the deterioration of forests and fisheries. The Swedish government mobilized behind Od©n and held the United Nations Conference on the Human Environment in 1972: The purpose of this conference was to increase awareness of the long-range distribution of air pollution; it was also charged with presenting data on acid rain's impact on human health and its damage to fisheries, forests, and farms.

In 1979, the Organization of Economic Cooperation and Development drafted the Convention on Long-Range Transport of Air Pollutants (CLRTAP). CLRTAP was the first international, legally-binding attempt to control air pollution. By signing the Convention, each state was accepting responsibility for ensuring that its activities did not cause environmental damage in other countries. All major European states, the former Soviet Union, the European Commission, Canada, and the United States all signed the Convention in 1979.

In continued dedication to this issue, Sweden hosted the Stockholm Conference on Acidification in 1982; this conference included a presentation by German scientist Bernhard Ulrich. Ulrich's research demonstrated the impact of acid rain on forests, and it helped to influence the development of strict environmental domestic policies in Germany. As a result of both Scandinavia and Germany's increased interest in environmental protection, Europe became an international leader in acid rain problems.


General: Acid rain is composed of both wet (rain, snow, sleet, fog and cloud water, dew) and dry (gases, dust, particles) components that contain nitric and sulfuric acids in higher concentrations than normal. Nitrogen oxides (NOx) and sulfur dioxide (SO2) are released when fossil fuels (coal, gas, and oil) are burned. Fossil fuels are typically used to heat homes, provide power, and drive vehicles. When fossil fuels are burned, NOx and SO2 are released into the atmosphere, where they undergo chemical reactions with oxygen and water to form acidic precipitation.

Acid rain usually originates in urban areas, but it is frequently transported by wind to rural areas hundreds or thousands of miles away. The acidic precipitation then falls onto rural lands and water sources.

Dry deposition refers to dry materials that have combined with acidic components and have then settled on surfaces. Dry deposition is difficult to measure and is estimated at anywhere from 20% to 80% of total acidic buildup in the landscape.

Sulfur dioxide (SO2): All raw materials, including coal, crude oil, and ore, are rich in common metals (such as lead, silver, and zinc ores) and contain SO2 gases. These gases form when materials containing sulfur are burned. When these gases attach to other airborne particles, they may become very harmful to human and animal health as well as to the environment. Human activities, however, are not the only source of SO2 gases. SO2 emissions are also produced by volcanic eruptions, the bacterial production of dimethyl sulfide, anaerobic decay, and other natural processes.

The amount of SO2 produced by natural processes is one-third the amount of SO2 produced by humans. Human activities in northern Europe, in the eastern United States, and in some parts of China result in SO2 emissions that are 10 times higher than natural emissions. Annual worldwide emissions of SO2 were estimated in 1990 at over 70 million tons, with global trends toward increased emissions due to the use of fossil fuels in Asia. If the use of fossil fuels in Asia continues at the current rate, emissions of SO2 gas may double. From 1980 to 2000, emissions in Europe from land-based SO2 sources fell from 53 to 15 million tons annually. Further, 40% of these emissions came from 100 land-based sources, 80 of which were power plants. While the overall trend is towards decreased SO2 emissions, the amount of SO2 produced by some European countries increased from 1980 to 2000. In Turkey, emissions increased by one-third, from 1.59 to 2.11 million tons.

Nitrogen oxides (NOx): NOx are highly reactive gases containing nitrogen and oxygen in varying amounts, most of which are colorless and odorless. Both nitrogen monoxide (NO) and nitrogen dioxide (NO2) are nitrogen oxides. They are produced as the result of a combustion reaction between nitrogen and oxygen. A combustion reaction is one in which all substances in a compound are combined with oxygen, and heat is released. The amount of NOx gases that are formed during a combustion reaction is determined by the temperature of the reaction; the higher the temperature, the greater the amount of NOx formed.

According to the World Health Organization (WHO), the critical level at which plant damage occurs due to NO2 is set at 30 micrograms per cubic meter annually; this level is typically exceeded in urban environments and near roads that have heavy traffic.

NO2 mixes with airborne particles, forming a reddish-brown layer over many urban areas. NO2 is a harmful pollutant shown to contribute to ground-level ozone, deteriorating water quality, acid rain, and global warming.

Coal and oil combustion from power plants and vehicle emissions are the main sources of NOx in most countries, resulting in about 35 million tons of nitrogen (N) annually worldwide. The emission of NOx gases is concentrated in the industrialized parts of the world. In Europe, emissions of NOx declined from 23 million tons in 1990 to just under 16 million tons in 2000. Of the 23 million tons of NOx, about 50% came from road traffic, 20% from combustion plants, and 15% from other sources. International shipping also contributes to NOx emissions: In 2000, it accounted for about four million tons of N.


Surface waters and aquatic organisms: Acid rain significantly impacts both surface waters and aquatic life. Acidic deposition occurs directly over aquatic ecosystems, such as marshes, and it may flow into rivers, lakes, and streams after first being deposited on or in forests, buildings, and fields. The biggest impact of acid rain occurs in areas where watershed soils and drainage are incapable of neutralizing the acid as quickly as it is deposited. Areas that have limestone rock are better able to neutralize acid and may not be impacted by acid rain. Limestone contains calcium carbonate that neutralizes strong acids by forming water, carbon dioxide, and calcium salts. Areas with granite rock, however, may not be able to neutralize the acid. Lakes that become acidified may be unable to support healthy species, particularly fish.

When acidic snow melts after the winter, then lakes, rivers, and especially streams may have much lower pH levels (indicating higher levels of acidity) than normal; this situation is called episodic acidification. Acid rain results in the leaching of nutrients and metals, including aluminum, from the soil, and it transports these materials through runoff and groundwater into streams and lakes.

The National Surface Water Survey (NSWS) is directly related to the National Acid Precipitation Assessment Program; it monitors the chemical composition of surface water in the United States. The NSWS found that acid rain was responsible for the acidity of 75% of acidic lakes and 50% of acidic streams; it also identified several regions that are more susceptible to acid rain, including the northeastern United States. In addition, acidification of lakes is a problem in Scandinavia: Nearly 14,000 Swedish lakes are affected, causing damage to both plant and animal life.

Forests: Acid rain impacts forests, particularly those at high elevations. Acid rain's impact is especially severe when it is combined with other stress-inducing factors, such as air pollution, droughts, disease, and insects. Acid rain that falls onto the forest ground and runs through the forest in streams may be buffered by the soil. However, if the soil is unable to buffer the effects of acid rain, it too becomes acidic.

Researchers and foresters have noted that acid rain results in slower growth, injury, and in extreme cases, the death of forests. In general, however, trees are not directly killed by acid rain. Instead, a combination of factors enables acid rain to weaken trees. Aluminum hydroxide, an insoluble, nontoxic compound, is normally present in soil. When acid rain decreases the pH of soil to a reading less than 5.0, aluminum hydroxide dissolves. The aluminum ions that are released interfere with the uptake of nutrients by trees. Nutrients (such as calcium and magnesium) and beneficial minerals in the soil are dissolved and washed away by acid rain. Unlike rivers, trees may be damaged even if the surrounding soil is well buffered.

A study in Taiwan of fog and precipitation in a mid-land forest found a disproportionate amount of nitrogen oxides (NOx) and sulfur dioxide (SO2) in the fog, as compared with bulk precipitation. Researchers concluded that the use of fertilizers (composed of ammonium sulfate and animal manure) in mountain agriculture resulted in major depositions in the forest ecosystem.

The threat to forests appears to be greatest where lakes are also seriously threatened, such as in eastern Canada, the northeastern United States, and northern Europe. In Canada, central Ontario, southern Quebec, and the Atlantic, forests experience about twice the tolerable level of acid rain. The level of acid deposition that is tolerable in nature varies from one ecosystem to the next, and is sometimes called the critical load. The United Nations Economic Commission for Europe in 1991 defined critical load as, "The highest deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on ecosystems' structure and function."

Soil: Although acidic precipitation results in the accumulation of sulfur and nitrogen in soil, after the precipitation falls, the affected soil may be able to neutralize the acidic deposits. Neutralizing ability of the soil is dependent on multiple factors, including the type of soil, its thickness, and both weather and water flow patterns. Soil that is frozen, composed of quartz (sand), or has no bases present may not be able to neutralize acid; basic soils, however, which are rich in limestone and calcium carbonate, are able to neutralize acidic deposition. Typical evergreen forests that are found in the northeastern United States, Canada, and Europe, are often naturally, slightly acidic. Vegetation in acidic soil may retain sulfate and nitrate ions, immobilizing the acid. Deeper soil may retain larger amounts of sulfate and nitrate ions.

Materials: Both wet and dry deposition of acidic particles results in damage to materials: Metals such as bronze may corrode, and paint and stone (such as marble and limestone) may deteriorate, resulting in decreased values of buildings, bridges, statues, monuments, and tombstones. The Statue of Liberty has been, in part, damaged by acid rain. Westminster Abbey in England has had ©10 million in renovations due to damage from acid rain.

Environmental fallout is a term used in the auto industry to describe damage to automobiles by acid rain, decaying insects, bird droppings, pollen, and tree sap. Studies have demonstrated that acid rain may scar automotive coverings, and analyses of damaged areas of exposed test panels indicate high sulfur levels. However, it is difficult to quantify how much of the damage is the fault of acid rain and how much is due to other factors, such as improper paint application or incorrect paint formulations.

Visibility: SO2 and NOx lead to the formation of sulfates and nitrates in the atmosphere, which contributes to decreased visibility; as a result, we cannot see as far or as clearly through the air. In the eastern United States, 50-70% of visibility reduction is due to sulfate particles.

Human health: Acid rain indirectly impacts human health. People cannot be harmed from walking or swimming in acid rain; however, the SO2 and NOx gases that cause acid rain may result in increased illness and even premature death due to heart and lung disorders.


Surface waters and aquatic life: Episodic acidification periods may result in mass fish kills, as fish cannot survive in low-pH water. Mass fish mortalities have been documented in both the salmon and trout populations of Norway.

However, episodic acidification is not the only source of declining fish populations. Fish populations that are not killed off due to episodic acidification may suffer from stress due to chronically decreased water pH. Nutrients and metals that are leached from the soil and transported in groundwater and runoff may also have damaging effects on aquatic organisms. Aluminum in particular is highly toxic to aquatic life, especially the young.

Low water pH may also result in the inability of female fish to reproduce, an inability of the spawn to survive in the acidic waters, developmental problems in the spawn, as well as lower body weights and smaller size of organisms.

Some fish are more sensitive to an acidic environment and disappear more quickly, including smallmouth bass, brook trout, and salmon. As the numbers of fish decline, the animals that feed on them, such as loons and other water birds, may also decline. Most fish cannot survive when the water pH falls below 5.2. The pH of surface waters may vary greatly from one body of water to the next, but generally the pH ranges from 6 to 8.

Forests: The impact of acid rain on trees varies depending on the region and the acidity of the rain. When acid rain increases aluminum ions in the soil while decreasing beneficial nutrients (such as calcium and magnesium), the health of trees is negatively impacted. Tree growth may slow and even stop completely; some trees may eventually stop producing leaves.

Acid cloud droplets, fog, and vapor leech nutrients from the trees, resulting in destruction of their leaves and needles. Destruction of foliage may decrease a tree's ability to withstand cold, prevent germination and reproduction, and make the tree more susceptible to disease and insects.

Soil: Acid deposition has several effects on soil, including depletion of calcium and other base cations (elements or ions with a positive charge that are able to neutralize acids); acid deposition may also transport aluminum ions into soil water, and increase the accumulation of sulfur and nitrogen in the soil. Accumulation of sulfur and nitrogen may lead to runoff into groundwater, thereby acidifying nearby streams and lakes. In addition, acidified soil may damage trees, as discussed above.

Materials: Dry deposition not only causes destruction but may also dirty buildings, leading to an increased need for maintenance. The Environmental Protection Agency's (EPA) Acid Rain Program in the United States works to limit sulfur dioxide (SO2) emissions; it thereby decreases SO2 damage to materials.

To reduce the impact of acid rain on automobiles, some manufacturers apply acid-resistant paints, at a cost of about five dollars per new vehicle.

Air quality: The EPA's Acid Rain Program is expected to improve visual range in the eastern United States by about 30%. Research has been able to estimate the monetary value that national park visitors place on visibility; consequently, the Acid Rain Program's SO2 reduction may be worth over one billion dollars annually by 2010.

Human heath: Airborne particles of sulfate and nitrate may lodge deeply in the lungs, causing inflammation and tissue damage. Studies have shown a relationship between high levels of fine sulfate and nitrate particles and increased illness and premature death from heart and lung disorders, such as asthma and bronchitis.

These gases have been regulated under the Clean Air Act in order to reduce the amount released from power generation. The EPA's Acid Rain Program, which is to be fully implemented by 2010, aims to reduce levels of both sulfate and nitrate particles in order to decrease the impact of acid rain on human health. The Acid Rain Program has an estimated value of $50 billion annually, due to decreases in mortality rates, hospital admissions, and emergency room visits. In Canada, a 50% reduction in SO2 is expected to prevent 550 premature deaths annually, 1,520 emergency room visits, and 210,070 asthma symptom days. These health benefits are valued from between $500 million to $5 billion annually for Canada.


General: Acid rain presents a serious global threat to the environment and human health. Acid rain is directly associated with the destruction of aquatic environments, forests, and organisms that are dependent on these ecosystems. Additionally, acid rain has detrimental effects on human health. Environments damaged by acid deposition need to be restored to improve the viability of organisms.

Alternative energy: Alternative energy sources may also help decrease the amount of fossil fuels being burned to generate power. Nuclear power, hydropower, wind energy, geothermal energy, and solar energy are all options. Currently, nuclear and hydropower are the most commonly used alternative energy sources in the United States.

Wind, solar, and geothermal power have not yet been developed enough to be economical, but continued research may improve these methods and make them viable energy sources.

Environmental Protection Agency (EPA): The Acid Rain Program, created through the 1990 Clean Air Act Amendments, has established goals of lowering annual emissions of sulfur dioxide (SO2) from the electric power industry to half of 1980 levels, starting in 2010. Sulfur dioxide (SO2) levels will be capped at 8.95 million tons, and the levels of nitrogen oxides (NOx) will be set at the 2000 annual U.S. emissions level of 6.1 million tons. To achieve these reductions, a two-phase plan was developed for fossil-fuel fired power plants. Part of this plan includes tightening annual emissions limits imposed on large, higher-emitting power plants, as well as setting restrictions for smaller, cleaner plants. In total, 110 power plants were included in Phase I of the program.

The EPA also sets allowances authorizing an industrial source to emit a certain amount of SO2 per year. If the company reduces its emissions of SO2 to a level below the amount it is allowed, the company may sell its allowances or retain them for use at a later date. Emissions may be reduced by a variety of methods, and trading provides incentives for companies to reduce their emissions. If the EPA decreases the allowances given each year, companies will be forced to continually reduce their SO2 emissions. Further, the ability to sell allowances encourages companies to develop new ways to conserve energy and control emissions.

Worldwide: In 1991, researchers in Sweden built a large, plastic roof over a forest ecosystem on the Swedish west coast and observed a number of changes in a short period of time. Acidic precipitation was caught by the roof, and precipitation that was 'clean' then rained over the area until 2001. In the course of a few years, the outflow of sulfur and toxic aluminum from the area into surface water dropped markedly. The pH of run-off water and the concentration of base cations (elements or ions with a positive charge that are able to neutralize acids), however, remain unchanged. The soil recovery progressed much more slowly, and aquatic organisms may have the benefit of decreased deposition only when the soil has begun to recover. From this study, the Swedish researchers concluded that in most places, a 70% reduction in sulfur deposition may result in decreased acidification. However, the researchers also concluded that a 70% reduction would not be enough to allow soil recovery.

The Chinese government has implemented a series of policies and regulations to control SO2 emissions. However, China's ability to achieve its environmental goals has been stunted by rapid economic growth. Attempts have been made more recently to link environmental quality to performance assessment. There is still much room for China to integrate environmental protection into its economic process.

In areas where soil acidification is in advanced stages, reductions in emissions need to be even greater than the already agreed-upon levels. The recovery of soil in places with extensive damage may take centuries.


This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (

  • Air Pollution & Climate Secretariat.
  • Camargo JA, Alonso A. Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment. Environ Int. 2006 Aug;32(6):831-49. View abstract
  • Gao C, Yin H, Ai N, et al. Historical analysis of SO2 pollution control policies in China. Environ Manage. 2009 Mar;43(3):447-57. View abstract
  • Driscoll CT, Lawrence GB, Bulger AJ, et al. Acidic Deposition in the Northeastern United States: Sources and Inputs, Ecosystem Effects, and Management Strategies. BioScience. 2001 Mar.51;(3):180-198.
  • Encyclopedia of Earth.
  • Environment Canada.
  • Hubbard Brook Ecosystem Study.
  • Lawrence GB, Roy KM, Baldigo BP, et al. Chronic and episodic acidification of Adirondack streams from acid rain in 2003-2005. J Environ Qual. 2008 Oct 23;37(6):2264-74. View abstract
  • Liang YL, Lin TC, Hwong JL, et al. Fog and precipitation chemistry at a mid-land forest in central Taiwan. J Environ Qual. 2009 Feb 25;38(2):627-36. View abstract
  • Natural Standard: The Authority on Integrative Medicine.
  • Singh A, Agrawal M. Acid rain and its ecological consequences. J Environ Biol. 2008 Jan;29(1):15-24. View abstract
  • U.S. Environmental Protection Agency (EPA).

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