Acidification of rain and snow may seem to be a recent environmental pollution problem. However, the phenomenon has been known for over a century since it was first noticed that buildings, trees and plants were damaged if they were downwind of chemical factories discarging acid fumes. The damage was mostly confined to periods of rainfall because of the removal of air-borne pollutants by rain doplets. The air was also polluted with the smoke from the individual coal fires in all the houses, as well as from small power stations adjacent to the towns and cities that burned coal to produce electricity.
The main acidifying gases are sulphur dioxide (SO2) and various oxides of nitrogen such as nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2). These are collectively referred to as NOx. The SO2 originates mostly from power stations while road traffic is the main source of NOx. These gases undergo a series of chemical reactions with cloud water and sunlight to form sulphuric and nitric acids.
Rainfall is naturally slightly acidic (about pH 6.0) because it dissolves carbon dioxide from the air to form carbonic acid. If rainfall has a pH of less than 5.6 it is regarded as being polluted with acid gases. Very polluted rain has a pH value of 3.0 or less.
