Pollution in the form of acids and acid-forming compounds (such as sulfur dioxide [SO2] and oxides of nitrogen [NOx]) can deposit from the atmosphere to the Earth’s surface. This acid deposition can be either dry or wet. Wet deposition is more commonly known as acid rain. Acid rain can occur anywhere and, in some areas, rain can be 100 times more acidic than natural precipitation. Acid deposition can be a very serious regional problem, particularly in areas downwind from high SO2- and NOx-emitting sources (e.g., coal burning power plants, smelters, and factories). Acid deposition can have many harmful ecological effects in both land and water systems. While acid deposition can damage tree foliage directly, it more commonly stresses trees by
changing the chemical and physical characteristics of the soil. In lakes, acid deposition can kill fish and other aquatic life.