acid rain

acid rain or acid deposition, form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids ( p H below 5.5-5.6). Produced when sulfur dioxide and various nitrogen oxides combine with atmospheric moisture, acid rain can contaminate drinking water, damage vegetation and aquatic life, and erode buildings and monuments. Automobile exhausts and the burning of high-sulfur industrial fuels are thought to be the main causes, but natural sources, such as volcanic gases and forest fires, may also be significant. It has been an increasingly serious problem since the 1950s, particularly in the NE United States, Canada, and W Europe, especially Scandinavia.

Acid rain became a political issue in the 1980s, when Canada claimed that pollutants from the United States were contaminating its forests and waters. Since then regulations have been enacted in North America and Europe to curb sulfur dioxide emissions from power plants; these include the U.S. Clean Air Act (as reauthorized and expanded in 1990) and the Helsinki protocol (1985), in which 21 European nations promised to reduce emissions by specified amounts. To assess the effectiveness of reductions a comprehensive study, comparing data from lakes and rivers across N Europe and North America, was conducted by an international team of scientists in 1999. The results they reported were mixed: while sulfates (the main acidifying water pollutant from acid rain) were lower, only some areas showed a decrease in overall acidity. It remained to be determined whether more time or a greater reduction in sulfur emissions was needed to reduce freshwater acidity in all areas. See air pollution ; forest ; pollution .

acid rain Rain that is highly acidic because of sulphur oxides, nitrogen oxides, hydrocarbons, and other air pollutants dissolved in it. Acid rain can severely damage both plant and animal life; certain lakes have lost all fish and plant life because of acid rain. The major causes of acid rain are motor vehicle emissions, industrial processes, and the burning of fossil fuels in power-stations.

acid rain Precipitation with a pH of less than about 5.0, which is the value produced when naturally occurring carbon dioxide, sulphate, and nitrogen oxides dissolve into cloud droplets. The effects of increased acidity on surface waters, soils, and vegetation are complex.