Acid Rain

Synonyms

Acid deposition; Acid precipitation

Definition

The US Environmental Protection Agency (EPA) refers to acid rain as a broad term that encompasses acidic particles and vapors deposited through dry and wet deposition. Wet deposition occurs in wet precipitation (such as rain, sleet, hail, snow, fog, or mist), with a pH normally less than 5.6. Half of atmospheric acidity falls to the ground as dry deposition (such as fly ash, sulfates, nitrates, and gases), where acidity is incorporated into dust and smoke, and then falls dry onto natural and human-made surfaces, including buildings. When gases come into contact with water, they become acids (such as sulfuric and nitric acids).

Sources: Natural (as from volcanoes and decaying vegetation) and anthropogenic (as from emissions into the atmosphere from human activities, including combustion). Primary anthropogenic emissions are of sulfur dioxide (SO₂) and nitrogen oxides (NO _x ) from the burning of fossil fuels. Energy production...