Acid rain is caused by an increase in the atmospheric concentration of SO2 and NO2.

Acid rain refers to the precipitation with a pH of less than 5. It is a mixture of H2SO4 and HNO3, the ratio of the two acids vary depending on the relative quantities of sulphur oxides and nitrogen oxides present in the atmosphere. These oxides are mainly produced by combustion of fossil fuels, smelters, industries, power plants, automobile exhausts etc. SO2 and NO2, when present in large quantities dissolved in water vapour form sulphuric acid and nitric acid which dissolve in rain water resulting in acid rain (H2SO4) and (HNO3) which in turn causes great damage to forests and vegetation.