Acid deposition is the fallout of acidic compounds in solid, liquid, or gaseous forms. Wet deposition occurs as precipitation while dry deposition is the fallout of particulate matter. Acid rain is the best-known form of acid deposition. The term “acid rain” was coined by British chemist Robert Angus Smith (1817–1884) who, in 1872, published Air & Rain: The Beginnings of a Chemical Climatology. Since then, acid rain has become an increasingly used term for rain, snow, sleet, or other precipitation that has been polluted by acids such as sulfuric and nitric acids.

When gasoline, coal, or oil are burned, their waste products of sulfur dioxide and nitrogen dioxide combine in complex chemical reactions with water vapor in clouds to form acids. The United States alone discharges 40 million metric tons of sulfur and nitrogen oxides into the atmosphere. This, combined with natural emissions of sulfur and nitrogen compounds, has resulted in severe ecological damage. Hundreds of lakes in North America (especially northeastern Canada and United States) and in Scandinavia are so acidic that they cannot support fish life. Crops, forests, and building materials, such as marble, limestone, sandstone, and bronze, have been affected as well, but the extent is not as well documented. However, in Europe, where so many living trees are stunted or killed, the new word “Waldsterben” (forest death) has been coined to describe this new phenomenon.

In 1990, amendments to the U.S. Clean Air Act contained provisions to control emissions that cause acid rain. It included the reductions of sulfur dioxide emissions from 19 million tons to 9.1 million tons annually and the reduction of industrial nitrogen oxide emissions from 6 to 4 million tons annually.