What causes acid rain?

Acid deposition is a general term that includes more than simply acid rain. Acid deposition primarily results from the transformation of sulphur dioxide (SO₂) and nitrogen oxides into dry or moist secondary pollutants such as sulphuric acid (H₂SO₄), ammonium nitrate (NH₄NO₃) and nitric acid (HNO₃). The transformation of SO₂ and NOx to acidic particles and vapours occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. Acidic particles and vapours are deposited via two processes - wet and dry deposition. Wet deposition is acid rain, the process by which acids with a pH normally below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry deposition takes place when particles such as fly ash, sulphates, nitrates, and gases (such as SO₂ and NOx), are deposited on, or absorbed onto, surfaces. The gases can then be converted into acids when they contact water.

What does acid mean?

An acid is a substance with a sour taste that is characterized chemically by the ability to react with a base to form a salt. Acids turn blue litmus paper (also called pH paper) red. Strong acids can burn your skin.

What is pH?

A pH scale is used to measure the amount of acid in a liquid-like water. Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH." This scale is used to measure the acidity of rain samples.