Acid Rain

Also called acid precipitation or acid deposition, acid rain is precipitation containing harmful amounts of nitric and sulfuric acids formed primarily by sulfur dioxide and nitrogen oxides released into the atmosphere when fossil fuels are burned. It can be wet precipitation (rain, snow, or fog) or dry precipitation (absorbed gaseous and particulate matter, aerosol particles or dust). Acid rain has a pH below 5.6. Normal rain has a pH of about 5.6, which is slightly acidic. The term pH is a measure of acidity or alkalinity and ranges from 0 to 14. A pH measurement of 7 is regarded as neutral. Measurements below 7 indicate increased acidity, while those above indicate increased alkalinity.

Develops when sulphur oxides (SOx) and nitrogen oxides (NOx), released by the combustion of fossil fuels (particularly coal), combine with moisture in the atmosphere to form sulphurous, sulphuric, nitrous and nitric acids. Because SOx and NOx are gases and because the formation of acid rain takes tie, acid rain damage often occurs far from the source of the problem.