Acid deposition is the falling of acids from the atmosphere to the earth's surface. It is more commonly known as acid rain, which occurs when the acids are contained in droplets of rain. There are two major sources of acid rain: sulfur dioxide and nitrogen oxides. Sulfur dioxide contributes to about seventy percent of acid rain while nitrogen oxides provide the remaining thirty percent.
Sources of Acid Rain
The sources of sulfur in the atmosphere include coal combustion, smelting, organic decay, and ocean spray. Approximately ninety percent of atmospheric sulfur results from human activities. In the atmosphere, sulfur dioxide combines with water vapor to form hydrogen sulfite gas: SO2 + H2O → H2SO4. Next, hydrogen sulfite reacts with oxygen to form sulfuric acid, a major component of acid rain: H2SO3 + 1/2O2 → H2SO4.
The sources of nitrogen oxides include the combustion of oil, coal and natural gas, forest fires, bacterial action in soil, volcanic gases, and lighting-induced atmospheric reactions. In the atmosphere, nitrogen monoxide reacts with oxygen gas to form nitrogen dioxide gas: NO + 1/2O2 → NO2. Then, nitrogen dioxide reacts with water vapor in the atmosphere to form hydrogen nitrite and hydrogen nitrate: 2NO2 + H2O → HNO2 + HNO3. Hydrogen nitrite and hydrogen nitrate are both acidic compounds that contribute significantly to acid rain.
Effects of Acid Rain on Aquatic Ecosystems
Acid rain has a number of negative effects on both humans and the environment. It is especially harmful to aquatic ecosystems, as streams, ponds, and leaks on acidic or neutral bedrock are extremely sensitive to acidic deposition. Acid rain leaches heavy metals from bedrock, and these metals often poison fish and other aquatic species. While some species of fish can survive acidic rainfall, the rapid melting of acidic snow can result in acidic concentrations five to ten times higher than the usual acidity produced by acid rain. Acid shock produced by such melting kills fish by altering their body chemistry, reducing oxygen intake, and disrupting muscle conditions. In addition to emitting the gases that contribute to acid rain, coal combustion also releases mercury into the environment. In aquatic ecosystems, this mercury is converted to methylmercury and is absorbed by the fatty tissues of fish. Upon eating these fish, humans may suffer nervous system and kidney ailments.
Effects of Acid Rain on Soil and Plants
Equally significant are the negative effects of acid rain on soil and plants. Acid rain leeches essential plant nutrients from the soil, causing decline in plant growth. Some of the important nutrients lost include calcium, potassium, and magnesium. Acid rain also leaches metals such as lead and mercury from minerals. These metals cause harm to the plants that absorb them and to the people that eat the plants. Aluminum ions leached from the soil by acid rain damage plant roots and inhibit the plants' uptake of essential nutrients such as magnesium, potassium, and water. While most plants struggle in highly acidic soil, some species of mosses thrive, and as a result they often multiply and eliminate air from the soil. This further decreases the other plants' chances of survival in acidic conditions. Finally, acid rain reduces the health of trees and vegetation so that they are susceptible to insects, disease, drought, and severe weather.
Effects of Acid Rain on Humans
The effects of acid rain on humans also deserve consideration. Toxic metals released into the environment by acid rain may enter water supplies or accumulate in fish and crops. Acid deposition also destroys statues, headstones, buildings, and fountains. Limestone structures are especially susceptible because they dissolve easily in acidic solutions. Finally, acid rain lowers the productivity of fisheries, forests, and farms, lowering profits and causing job losses in those industries.
Ways to Control and Prevent Acid Rain
The following is a list of strategies that can contribute to the prevention and clean-up of acid rain:
- design more efficient automobile engines in order to reduce nitrogen oxide emissions
- increase efficiency of power plants that burn coal in order to reduce waste that contains sulfur dioxide and nitrogen oxide
- increase penalties on industries that do not meet air pollution guidelines
- increase tax incentives to industries that do meet guidelines
- increase funding for alternative energy sources; for example, give tax incentives to buyers of hybrid cars
- provide tax incentives to companies that use alternative energy sources
- add CaCO3 (calcium carbonate) to lakes suffering from acid deposition; calcium carbonate acts as a buffer, resisting a change in pH and lessening the negative effects of acid rain