Acid rain, more properly called wet acid deposition, is caused by the emission of various pollutants to the air. The principle sources are the smokestacks of fossil fuel power plants, other industrial facilities and automotive exhausts. These gases combine with oxygen and water vapor in the air to form sulfuric and nitric acids. The acids fall to the earth in two forms of deposition. If the acids descend as sulfate or nitrate particles, it is labeled dry deposition.
Acidic deposition can affect a wide variety of ecosystems.