Acid precipitation has been known for centuries in locales such as London where sulfur discharged by the burning of coal produces toxic smogs; however, the problem did not assume scientific, economic, and political prominence until the early 1980s. As it transcends national boundaries, the acid rain problem has become a subject of heated controversy between otherwise friendly neighbours like the United States and Canada or Germany and the Scandinavian countries.
Scientific studies have shown that the process that results in the formation of acid rain generally begins with the discharge of sulfur dioxide and nitrogen oxides into the atmosphere. These waste gases are released by the combustion of fossil fuels by automobiles, electric power plants, and smelting and refining facilities. They also are emitted by some biological processes. The gases combine with atmospheric water vapour to form sulfuric and nitric acids. When rain or some other form of precipitation falls to the surface, it is highly acidic, frequently with a pH value of less than 4. (The term pH is defined as the negative logarithm of the hydrogen ion concentration in kilograms per cubic metre. The pH scale ranges from 0 to 14, with lower numbers representing greater acidity.) The consequent acidification of surface and subsurface waters is widely believed to have a detrimental effect on the ecology of the affected areas. Such regions as the Canadian Shield in Quebec and the Adirondack Mountains in New York are especially susceptible to contamination, because the snowpack buildup in winter allows a deadly pulse of acidic meltwater to occur during spring. As highly acidic water is toxic to many aquatic organisms, many lakes in these regions are biologically damaged. It also has been found that acid precipitation is harmful to trees and other forms of vegetation, causing foliar injury and reduction in growth.
The "cause-and-effect linkages" of the acid rain problem have been more clearly demonstrated in scientific terms than those related to ozone depletion; yet, the former has received much less direct policy action. The primary reason is the potential economic impact of efforts to remedy the problem--i.e., the enormous expenditure that would be required to control the emission of sulfur compounds from power plants, refineries, and facilities of other smoke- stack industries. There continues to be loud and angry debate as to the environmental and economic benefits of corrective action, particularly since the relationship between the discharge of potentially acidic compounds and the ultimate delivery of acid precipitation to specific geographic areas is not straightforward.