Scientists studying the effects of acid rain are more apt to talk about acid deposition. Both terms refer to acids produced by chemical reactions of pollutants in the atmosphere and the return of those acids to Earth's surface in rain, snow, other forms of precipitation, and dry particu-lates (minute particles). Most acid deposition is formed from emissions of sulfur dioxide (SO2), chiefly from the smoke-stacks of coal-burning power plants and factories, and nitrogen oxide (NOx), from auto exhaust and industrial sources. In the air, SO2 and NOx oxidize into sulfate and nitrate particles. If water vapor is present, the particles may be changed into sulfuric acid and nitric acid. Pollutant particles can stay in the atmosphere for weeks and be carried hundreds of miles by prevailing winds.
The symbol pH is used to indicate the degree of acidity or alkalinity of a solution (usually water, as in a lake or a swimming pool). On a scale of 0 to 14, ``pure'' water has a pH of 7. Values under 7 indicate acidity; those over that value denote alkalinity. Water at pH 6 is 10 times more acidic than water at pH 7. Few natural bodies or streams of water have a ``neutral'' pH.
Acidification increases the solubility of heavy metals -- including aluminum, cadmium, lead, manganese, mercury, and zinc -- which are naturally present in soil and water. Higher levels can kill aquatic life.
Ozone (O3), a form of oxygen that occurs naturally but also is a byproduct of air pollution, is believed to have a role in damaging trees.--