Acid Rain

Acid rain is the term used in the popular press that is equivalent to acidic deposition as used in the scientific literature. Acid deposition results from the deposition of airborne acidic pollutants on land and in bodies of water. These pollutants can cause damage to forests as well as to lakes and streams.

The major pollutants that cause acidic deposition are sulfur dioxide (SO₂) and nitrogen oxides (NO_x) produced during the combustion of fossil fuels. In the atmosphere these gases oxidize to sulfuric acid (H₂SO₄) and nitric acid (HNO₃) that can be transported long distances before being returned to the earth dissolved in rain drops (wet deposition), deposited on the surfaces of plants as cloud droplets, or directly on plant surfaces (dry deposition). Electrical utilities contribute 70% of the 20 million tons (21 million metric tons) of SO₂ that are annually added to the atmosphere. Most of this is from the combustion of coal. Electric utilities also contribute 30% of the 19 million tons of NO_x added to the atmosphere, and internal combustion engines used in automobiles, trucks, and buses contribute more than 40%. Natural sources such as forest fires, swamp gases, and volcanoes only contribute 1–5% of atmospheric SO₂. Forest fires, lightning, and microbial processes in soils contribute about 11% to atmospheric NO_x. In response to air quality regulations, electrical utilities have switched to coal with lower sulfur content and installed scrubbing systems to remove SO₂. This has resulted in a steady decrease in SO₂ emissions in the United States since 1970, with a 18–20% decrease between 1975 and 1988. Emissions of NO_x have also decreased from the peak in 1975, with a 9–15% decrease from 1975 to 1988.

A commonly used indicator of the intensity of acid rain is the pH of this rainfall. The pH of non-polluted rainfall in forested regions is in the range 5.0–5.6. The upper limit is 5.6, not neutral (7.0), because of carbonic acid that results from the dissolution of atmospheric carbon dioxide. The contribution of naturally occurring nitric and sulfuric acid, as well as organic acids, reduces the pH somewhat to less than 5.6. In arid and semi-arid regions, rainfall pH values can be greater than 5.6 due the effect of alkaline soil dust in the air. Nitric and sulfuric acids in acidic rainfall (wet deposition) can result in pH values for individual rainfall events of less than 4.0.

In North America, the lowest acid rainfall is in the northeastern United States and southeastern Canada. The lowest mean pH in this region is 4.15. Even lower pH values are observed in central and northern Europe. Generally, the greater the population density and density of industrialization the lower the rainfall pH. Long distance transport, however, can result in low pH rainfall even in areas with low population and low density of industries, as in parts of New England, eastern Canada, and in Scandinavia.

A very significant portion of acid deposition occurs in the dry form. In the United States, it is estimated that 30–60% of acidic deposition occurs as dry fall. This material is deposited as sulfur dioxide gas and very finely divided particles (aerosols) directly on the surfaces of plants (needles and leaves). The rate of deposition depends not only on the concentration of acid materials suspended in the air, but on the nature and density of plant surfaces exposed to the atmosphere and the atmospheric conditions(e.g., wind speed and humidity).

Direct deposition of acid cloud droplets can be very important especially in some high altitude forests. Acid cloud droplets can have acid concentrations of five to 20 times that in wet deposition. In some high elevation sites that are frequently shrouded in clouds, direct droplet deposition is three times that of wet deposition from rainfall.

Acid deposition has the potential to adversely affect sensitive forests as well as lakes and streams. Agriculture is generally not included in the assessment of the effects of acidic deposition because experimental evidence indicates that even the most severe episodes of acid deposition do not adversely affect the growth of agricultural crops, and any long-term soil acidification can readily be managed by addition of agricultural lime. In fact, the acidifying potential of the fertilizers normally added to cropland is much greater than that of acidic deposition. In forests, however, long-term acidic deposition on sensitive soils can result in the depletion of important nutrient elements (e.g., calcium, magnesium, and potassium) and in soil acidification. Also, acidic pollutants can interact with other pollutants (e.g., ozone) to cause more immediate problems for tree growth. Acid deposition can also result in the acidification of sensitive lakes and with the loss of biological productivity.

Long-term exposure of acid sensitive materials used in building construction and in monuments (e.g., zinc, marble, limestone, and some sandstone) can result in surface corrosion and deterioration. Monuments tend to be the most vulnerable because they are usually not as protected from rainfall as most building materials. Good data on the impact of acidic deposition on monuments and building material is lacking.

Nutrient depletion due to acid deposition on sensitive soils is a long-term (decades to centuries) consequence of acidic deposition. Acidic deposition greatly accelerates the very slow depletion of soil nutrients due to natural weathering processes. Soils that contain less plant-available calcium, magnesium and potassium are less buffered with respect to degradation due to acidic deposition. The most sensitive soils are shallow sandy soils over hard bedrock. The least vulnerable soils are the deep clay soils that are highly buffered against changes due to acidic deposition.

The more immediate possible threat to forests is the forest decline phenomenon that has been observed in forests in northern Europe and North America. Acidic deposition in combination with other stress factors such as ozone, disease and adverse weather conditions can lead to decline in forest productivity and, in certain cases, to dieback. Acid deposition alone cannot account for the observed forest decline, and acid deposition probably plays a minor role in the areas where forest decline has occurred. Ozone is a much more serious threat to forests, and it is a key factor in the decline of forests in the Sierra Nevada and San Bernardino mountains in California.

The greatest concern for adverse effects of acidic deposition is the decline in biological productivity in lakes. When a lake has a pH less than 6.0, several species of minnows, as well as other species that are part of the food chain for many fish, cannot survive. At pH values less than about 5.3, lake trout, walleye, and smallmouth bass cannot survive. At pH less than about 4.5, most fish cannot survive (largemouth bass are an exception).

Many small lakes are naturally acidic due to organic acids produced in acid soils and acid bogs. These lakes have chemistries dominated by organic acids, and many have brown colored waters due to the organic acid content. These lakes can be distinguished from lakes acidified by acidic deposition, because lakes strongly affected by acidic deposition are dominated by sulfate.

Lakes that are adversely affected by acidic deposition tend to be in steep terrain with thin soils. In these settings the path of rainwater movement into a lake is not influenced greatly by soil materials. This contrasts to most lakes where much of the water that collects in a lake flows first into the groundwater before entering the lake via subsurface flow. Due to the contact with soil materials, acidity is neutralized and the capacity to neutralize acidity is added to the water in the form of bicarbonate ions (bicarbonate alkalinity). If more than 5% of the water that reaches a lake is in the form of groundwater, a lake is not sensitive to acid deposition.

An estimated 24% of the lakes in the Adirondack region of New York are devoid of fish. In one third to one half of these lakes this is due to acidic deposition. Approximately 16% of the lakes in this region may have lost one or more species of fish due to acidification. In Ontario, Canada, 115 lakes are estimated to have lost populations of lake trout. Acidification of lakes, by acidic deposition, extends as far west as Upper Michigan and northeastern Wisconsin, where many sensitive lakes occur and there is some evidence foracidification. However, the extent of acidification is quite limited.

Bresser, A. H., ed. Acid Precipitation. New York: Springer-Verlag, 1990.

Mellanby, K., ed. Air Pollution, Acid Rain and the Environment. New York: Elsevier, 1989.

Turck, M. Acid Rain. New York: Macmillan, 1990.

Wellburn, A. Air Pollution and Acid Rain: The Biological Impact. New York: Wiley, 1988.

Young, P. M. Acidic Deposition: State of Science and Technology. Summary Report of the U. S. National Acid Precipitation Program. Washington, DC: U. S. Government Printing Office, 1991.

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