Normal rainfall is slightly acidic because carbon dioxide in the atmosphere dissolves in the raindrops to produce hydrogen ions. Yet, normal rainfall is not considered acid rain. Acid rain is a form of air pollution in which airborne acids produced by electric utility plants and other sources fall to Earth in distant regions. The corrosive nature of acid rain causes widespread damage to the environment. The problem begins with the production of sulfur dioxide and nitrogen oxides from the burning of fossil fuels, such as coal, natural gas, and oil, and from certain kinds of manufacturing.
Sulfur dioxide and nitrogen oxides react with water and other chemicals in the air to form sulfuric acid, nitric acid, and other pollutants. These acid pollutants reach high into the atmosphere, travel with the wind for hundreds of miles, and eventually return to the ground by way of rain, snow, or fog, and as invisible dry forms. Damage from acid rain has been widespread in eastern North America and throughout Europe, and in Japan, China, and Southeast Asia. Acid rain leaches nutrients from soils, slows the growth of trees, and makes lakes uninhabitable for fish and other wildlife.
In cities, acid pollutants corrode almost everything they touch, accelerating natural wear and tear on structures such as buildings and statues. Acids combine with other chemicals to form urban smog, which attacks the lungs, causing illness and premature deaths. The process that leads to acid rain begins with the burning of fossil fuels. Burning, or combustion, is a chemical reaction in which oxygen from the air combines with carbon, nitrogen, sulfur, and other elements in the substance being burned. The new compounds formed are gases called oxides.
When sulfur and nitrogen are present in the fuel, their reaction that occurs with oxygen yields sulfur dioxide and various nitrogen oxide compounds. In the United States, 70 percent of sulfur dioxide pollution comes from power plants, especially those that burn coal. In Canada, industrial activities, including oil refining and metal smelting, account for 61 percent of sulfur dioxide pollution. Nitrogen oxides enter the atmosphere from many sources, with motor vehicles emitting the largest share43 percent in the United States and 60 percent in Canada.
Once in the atmosphere, sulfur dioxide and nitrogen oxides undergo complex reactions with water vapor and other chemicals to yield sulfuric acid, nitric acid, and other pollutants called nitrates and sulfates. The acid compounds are carried by air currents and the wind, sometimes over long distances. When clouds or fog form in acid-laden air, they too are acidic, and so is the rain or snow that falls from them. Acid pollutants also occur as dry particles and as gases, which may reach the ground without the help of water.
When these dry acids are washed from ground surfaces by rain, they add to the acids in the rain itself to produce a still more corrosive solution. The combination of acid rain and dry acids is known as acid deposition. The acids in acid rain react chemically with any object they contact. Acids are corrosive chemicals that react with other chemicals by giving up hydrogen atoms. The acidity of a substance comes from the abundance of free hydrogen atoms when the substance is dissolved in water.
Rain, snow, or fog with a pH below 5. 6 is considered acid rain. When bases mix with acids, the bases lessen the strength of an acid (see Acids and Bases). This buffering action regularly occurs in nature. Rain, snow, and fog formed in regions free of acid pollutants are slightly acidic, having a pH near 5. 6. Alkaline chemicals in the environment, found in rocks, soils, lakes, and streams, regularly neutralize this precipitation.
But when precipitation is highly acidic, with a pH below 5. naturally occurring acid buffers become depleted over time, and natures ability to neutralize the acids is impaired. Acid rain has been linked to widespread environmental damage, including soil and plant degradation, depleted life in lakes and streams, and erosion of human-made structures. Acid rain can best be curtailed by reducing the amount of sulfur dioxide and nitrogen oxides released by power plants, motorized vehicles, and factories.
