Acid Rain

Acid rain refers to any precipitation that has a lower than normal pH resulting from evaporating gaseous pollutants mixing with water molecules and particulate matter in the atmosphere. When airborne acidic pollutants are deposited on land and in bodies of water, these pollutants can cause damage to crops, naturally occurring vegetation, wildlife, and manmade structures can occur over time.

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).

Human activities are the main cause of acid rain, and the continuous release of chemicals in the air has changed the atmospheric balance over time. Electrical utilities contribute the greatest percentage of SO2 added to the atmosphere, mostly from the combustion of coal. Electric utilities also contribute approximately one-third of the NO_x added to the atmosphere. Internal combustion engines used in automobiles, trucks, and buses contribute about half of contributing pollutants. Factory and manufacturing pollution also account for a large portion of nitrate and sulfate expulsion into the atmosphere. Natural sources such as forest fires, swamp gases, volcanoes, lightning, and microbial processes in soils contribute only 5 percent and 15 percent of the atmospheric SO2 and NO_x.

In North America, the lowest pH level indicating 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 (i.e. via the jet stream), however, can result in low pH rainfall even in areas with low population and low density of industries, as in the northeastern United States, eastern Canada, and Scandinavia.

A significant portion of acid deposition occurs in the dry form. In the United States, it is estimated that 30–60 percent 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 also 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 to maintain an adequate acidity level in the ecosystem. Acid cloud droplets can have acid concentrations of five to twenty times that in other 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, which allows a proper balance required by certain high altitude plants.

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).

Acid deposition can also adversely affect sensitive forests. Damage to vegetation and crops can occur directly, as in surface damage to a plant's leaves often via deposition of gases such as sulfur dioxide, or indirectly, changing the chemical composition of the soil which eventually poisons a plant. 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 that 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 acid rain, but the process of using these chemicals cycles back into the atmosphere worsening the acidity level of precipitation. Agriculture may have benefited from the additional sulfur that was deposited from polluted air, such that as acid deposition had declined farmers have had to add extra sulfur to soils. 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 the loss of biological productivity.

A more immediate threat to forests is the forest decline phenomenon observed in northern Europe and North America. Acidic deposition in combination with other stress factors such as ozone depletion, disease, and severe weather phenomena can lead to decline in forest productivity and, in certain cases, vegetative dieback. Acid rain alone cannot account for the observed forest decline, and acid deposition probably plays a minor role in the areas where forest decline has occurred, but it is a noted factor. Chemical changes to the ozone, also caused by human activity, are an increasing threat to forests, resulting in the tree density decrease like that found in the Sierra Nevada and San Bernardino mountains in California.

Long-term exposure of acid-sensitive materials used in building construction and monuments (e.g., zinc, marble, bronze, limestone, and some sandstone) results 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. In particular, buildings made of limestone and marble contain vulnerable calcium carbonate. Dry deposited sulfur dioxide reacts with the calcium carbonate to erode surface features. A number of famous buildings and sculptures, especially in Europe, have been damaged by acid deposition and remain the focus of restoration efforts.

Nutrient depletion due to acid rain on sensitive soils is a long-term (decades to centuries) consequence of acidic deposition. Acidity greatly accelerates the depletion of soil nutrients because of 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 because of acidic deposition.

Though acid rain does not directly impact human health, the chemicals that cause acid rain do. Prolonged exposure has been indicated in premature death from such conditions as asthma, bronchitis, and other lung-related diseases.

The U.S. Congress passed the Acid Deposition Act in 1980, which set up a long-term assessment, and created the National Acidic Precipitation Assessment Program (NAPAP), increased testing and monitoring of acid rain, the study of effects of increased acidity in ecosystems, and the examination of processes and control programs to protect national forests, monuments, and other treasures.

Decreased emissions mean positive human health benefits as well.

With regard to manmade structures, certain measures have been taken to preserve sensitive buildings and monuments throughout the world. This can include wrapping outdoor sculptures, restoring marble faces, columns, and detains on buildings, and changing construction projects to include more hardy materials capable of withstanding acid rains over long periods of time.

The decreasing emissions of sulfur dioxide from coal in the United States and Europe has meant much lower deposition of sulfuric acid although nitric acid continued to increase. While the problem was much improved in these regions, emissions in rapidly industrializing countries such as China increased sharply, meaning that problems still lie ahead.

See also Air pollution ; Clean Air Act (1963, 1970, 1990) ; Ozone .