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.