Abstract
Acid rain, more properly termed acid deposition, is the input of acidifying compounds from the atmosphere to the earth's surface. It is comprised primarily of sulfuric and nitric acids that are formed when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOX) react in the atmosphere with water, oxygen, and other compounds. Chlorine emissions can also lead to hydrochloric acid, but it usually only accounts for a small fraction of acid rain. The source of sulfur dioxide and nitrogen oxides is principally from the burning of fossil fuels by electric utilities, industries, and motor vehicles. Acid deposition occurs in two forms: wet deposition (rain, snow, sleet, and hail), dry deposition (particles, gases, and vapor. Acidity is measured in units called pH on a scale from 1 to14. A pH of 7 is considered neutral, a pH lower than 7 is acidic, and a pH greater than 7 is alkaline. Natural rainwater is already somewhat acidic with a pH as low as 5.0. In general, low pH levels in lakes and streams create conditions that harm fish and other aquatic organisms. Similarly, low pH conditions change forest soil chemistry, degrading growing conditions for some tree species. The deposition of acidic compounds back to earth has a negative impact on aquatic and terrestrial ecosystems, public health, visibility and materials and structures.