What is Acid Rain?

            Acid rain is caused when the sulfur and nitrogen oxides in air pollution combine with atmospheric moisture to produce highly acidic precipitation. Unpolluted rainfall is slightly acidic, with a natural pH of about 5, due to naturally occurring constituents of the atmosphere, such as carbon dioxide, but acid rain has a pH of about 4. Acid precipitation includes rain, snow, hail, and fog. The term acid deposition refers to both acid precipitation and tiny dust particles formed from the pollutants in acid rain. Acid rain has destroyed plant and animal life in lakes, damaged forests and crops, and caused extensive erosion of structures built of stone.
             Sulfur and nitrogen oxides (often abbreviated SOx and NOx, respectively) are common by-products of fossil-fuel combustion. SOx forms from sulfur impurities in fuels, especially coal and some petroleum products. NOx forms from nitrogen impurities in fuels and from nitrogen in the air used for combustion. Coal- and oil-fired electric power plants are major sources of SOx emissions (see power, generation and transmission of); NOx comes from electricity production and gasoline-powered vehicles.
             Acidic precipitation is widespread, because when these emissions are in the form of gases, they can be carried for thousands of kilometers by winds. Portions of eastern North AmericaÑincluding southeastern Canada, New England, and the mid-Atlantic regionÑhave suffered significant effects due to heavy automobile traffic in the Northeast and the concentration of coal-fired electricity generators in the midwestern region of the United States. In Europe the northern countries, especially Scandinavia, and the former Communist countries in the east have also been affected. In some areas of central Africa, use of heavy fuel oils to generate electricity produces acid rain. In China, coal is the most widely used energy source, and consequently, there is a serious acid rain problem.