Air pollution

Acid emission and deposition processes

Acid deposition

The primary pollutants that contribute to surface water acidification are generally emitted as the gases sulphur dioxide (SO2), nitrogen oxides (e.g. NO and NO2, commonly referred to as NOx) and ammonia (NH3). The acidifying gases (SO2, NO, & NO2) react with oxygen in the atmosphere forming sulphuric (H2SO4) and nitric acids (HNO3-). During oxidation gaseous sulphur dioxide is transformed to sulphate (SO42-), a particulate, while nitric acid may be present as a gas or within particles or cloud droplets. The particles are removed from the atmosphere either by direct contact with the ground or in the form of rain or snow, which gives rise to the term 'acid rain'.

The processes of emission, transformation and deposition of acidifying compounds are illustrated in the diagram above.

You can find out more about acid deposition in the UK at the UK Pollutant Deposition web site maintained by CEH.