Acid rain is a form of precipitation containing heavy concentration of sulfuric and nitric acids. The term is also commonly applied to snow, sleet and hail that manifest similar acidification. Such precipitation has become an increasingly serious environmental problem in many areas of North America and Europe. Although this form of pollution is most severe in and around large urban and industrial areas, substantial amounts of acid precipitation may be transported great distances.
The process that results in the formation of acid rain generally begins with emissions into the atmosphere of sulfur dioxide and nitrogen oxide. These gases are released by automobiles, certain industrial operations and electric power plants that burn such fossil fuels as coal and oil. The gases combine with water vapor in clouds to form sulfuric and nitric acids. When precipitation falls from the clouds, it is highly acidic, having a pH value of about 5.6 or lower. At several locations in the United States and Western Europe, pH values between 2 and 3 have been recorded. In areas such as Los Angeles and San Francisco, fog is often 10 or more times as acidic as the local precipitation.
Precipitation and fog of high acidity contaminate lakes and streams; they are particularly harmful to fish and other aquatic life in regions with thin soil and granitic rock, which provide little buffering to acidic inputs. It also has been discovered that aluminum is leached from the soil in such regions subjected to acid precipitation, and that dissolved aluminum seems to be extremely toxic to aquatic organisms. All forms of acid precipitation have been found to damage various kinds of vegetation, including agricultural crops and trees, chiefly by inhibiting nitrogen fixation and leaching nutrients from foliage. In addition, these pollutants can corrode the external surfaces of buildings and other man-made structures.