
Pure water has a pH value of 7 (neutral), but rainwater falling in the atmosphere always contains impurities. The atmosphere contains natural acids including CO2 (a weak acid); nitric acid produced naturally from NOx emitted by lightning, fires, and soils; and sulphuric acid produced by the oxidation of sulphur gases from volcanoes and the biosphere. It also contains natural bases, including ammonia (NH3) emitted by the biosphere and calcium carbonate (CaCO3) from suspended soil dust. CO2 alone at natural levels would result .....