What Causes Acid Rain?

The main contributors to acid rain are sulfur dioxide (SO₂) and nitrogen oxides (NO_x; the combination of NO and NO₂), which are released into the air primarily through the combustion of such fossil fuels as coal, oil, and natural gas. These pollutants are emitted by power plants, vehicles, and industrial facilities, and can travel long distances in the atmosphere, affecting regions far from the original source of pollution. Once in the atmosphere, the gases react with water, oxygen, and other chemicals to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids then mix with cloud moisture, lowering the pH of precipitation and falling to the ground as acid rain.

Acid rain has a number of harmful effects in both natural and human-made environments. While human activities are the primary sources of the pollutants, natural sources can also contribute to acid rain, albeit to a much lesser extent. Volcanic eruptions, for example, can release sulfuric acid, nitric acid, and hydrochloric acid (HCl), and lightning can produce nitrogen oxides. However, these natural sources typically have a more localized impact.

(Read Britannica’s essay “What Happened to Acid Rain?”)