Acid rain is a general term describing the pollution that occurs when acids fall out of the atmosphere (mass of air surrounding Earth). The principal pollutants that produce acids in the atmosphere are sulfur dioxide (SO₂) and nitrogen oxides, like nitrogen oxide (NO) and nitrogen dioxide (NO₂). These compounds combine with water in the atmosphere to form sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Acid rain has significantly affected the waters that flow into lakes and rivers, as well as the lakes and rivers themselves. In turn, the plants and animals that depend on lakes, rivers and oceans are harmed by acid rain.

When describing acid rain, scientists use the more precise term acid deposition. Scientists distinguish between two types of acid deposition: dry and wet. Dry deposition includes acidic gases and solid particles containing sulfuric and nitric acid that settle out of the air...