acid rain,precipitation possessing apH of about 5.2 or below primarily produced from the emission ofsulfur dioxide (SO₂) andnitrogen oxides (NO_x; the combination of NO and NO₂) from human activities, mostly thecombustion offossil fuels. In acid-sensitive landscapes,aciddeposition can reduce the pH of surfacewaters and lowerbiodiversity. It weakenstrees and increases their susceptibility to damage from other stressors, such asdrought, extreme cold, andpests. In acid-sensitive areas, acid rain also depletessoil of important plantnutrients and buffers, such ascalcium andmagnesium, and can releasealuminum, bound to soil particles androck, in its toxic dissolved form. Acid rain contributes to thecorrosion of surfaces exposed to airpollution and is responsible for the deterioration oflimestone andmarble buildings and monuments.

The phraseacid rain was first used in 1852 by Scottish chemistRobert Angus Smith during his investigation of rainwaterchemistry near industrial cities inEngland andScotland. The phenomenon became an important part of his bookAir and Rain: The Beginnings of a Chemical Climatology (1872). It was not until the late 1960s and early 1970s, however, that acid rain was recognized as a regional environmental issue affecting large areas of westernEurope and easternNorth America. Acid rain also occurs inAsia and parts ofAfrica,South America, andAustralia. As a global environmental issue, it is frequently overshadowed byclimate change. Although the problem of acid rain has been significantly reduced in some areas, it remains an important environmental issue within and downwind from major industrial and industrial agricultural regionsworldwide.