Acid Rain

Large regions are affected by acidic precipitation in North America, Europe, and elsewhere. A relatively small region of eastern North America is known to have experienced acidic precipitation before 1955, but this has since expanded so that most of the eastern United States and southeastern Canada is now affected.

Interestingly, the acidity of precipitation is not usually greater close to large point-sources of emission of important gaseous precursors of acidity, such as smelters or power plants that emit SO₂ and NO_x. This observation emphasizes the fact that acid rain is a regional phenomenon, and not a local one. For instance, the acidity of precipitation is not appreciably influenced by distance from the world's largest point-source of SO₂ emissions, a smelter in Sudbury, Ontario. Furthermore, when that smelter was temporarily shut down by a labor dispute, the precipitation averaged pH 4.49, not significantly different from the pH 4.52 when there were large emissions of SO₂.