Air Pollution
Clean Air for Kids

Deposition of Air Pollutants


Air pollutants are deposited on Earth by one of two processes: wet deposition or dry deposition. Wet deposition occurs when pollutants are emitted into the atmosphere and oxidised to form an acid. Sulphur dioxide (SO2), for example, is emitted and oxidised to sulphuric acid (H2SO4). The pollutants then fall to earth as acidic precipitation. Dry deposition occurs when the acids are transformed chemically into gases and salts, and then fall to Earth. SO2, for example, is deposited as a gas and a salt.

The gases present in acid deposition are found to occur naturally in the environment. They are given off from a number of sources including volcanic eruptions and the rotting of vegetation. They become a problem when humans produce the gases in large amounts, and at high concentrations by the burning of fossil fuels. In the UK, power stations are the main source of acidic precursor emissions, contributing 65% and 21% of total suphur dioxide and nitrogen oxides emissions respectively (1999).

A Transboundary Problem

Once in the air, pollutant gases are carried by the wind, and hence deposition can take place a long distance from the source. If large quantities of acid are deposited in one place then it may have detrimental consequence for:

  • humans;
  • wildlife;
  • vegetation;
  • soils;
  • crops;
  • freshwater;
  • buildings.

Acid deposition is clearly a transboundary problem as about 8% of sulphur deposition in Germany and Sweden is of UK origin, and in Norway the figures are as high as 12 to 14%.

Problems occurring from acid deposition have been recognised in Scandinavia for a long time, but the problem has only been given attention in the UK during the last few decades. In the UK the acidity of rain is greatest in the east and least in the west. However the North West receives much higher rainfall and hence more acid deposition takes place here.

Measuring Deposition of Air Pollutants

Greater Manchester has a long history of acid deposition, with measurements being made as early as the 1850s. Despite this, acid deposition measurements are not regularly recorded in urban areas with the exception of the Greater Manchester Acid Deposition Survey (GMADS).

Regular monitoring of acid deposition takes place within the rural environment in the UK on a national basis. This network of sites is known as the secondary acid precipitation network. This monitoring began in 1983 with 59 sites, falling to 32 sites by 1991. Samples are analysed for sulphate, nitrate, ammonium and hydrogen concentrations and deposition rates are presented on an annual basis. The network is managed by the National Environmental Technology Centre (NETCEN) on behalf of the Department forEnvironment, Food & Rural Affairs (DEFRA).

In the urban environment of the UK, however, the only long term continuous monitoring of acid deposition that has taken place is for the Greater Manchester Acid Deposition Survey in the northwest of England. This covers an area of some 2000km2 and supports a population in excess of 2.8 million. The area has a diverse industrial base contributing to a wide array of large and small point sources of pollution. In terms of altitude, there is a great variation, ranging from 20m above sea level in the west to over 600m in the High Peak.

GMADS began in 1986 and ran until 1997 in collaboration with the Association of Greater Manchester Authorities. The local authorities included within the survery were Bolton, Bury, High Peak, Manchester, Oldham, Rochdale, Rossendale, Salford, Stockport, Tameside, Trafford, Warrington and Wigan. The aim of the survey was to identify the urban influence on acidity, sulphate, nitrate, ammonium and other ions present in precipitation.

The Greater Manchester Acid Deposition Survey provided the only long-term continuous survey of acid deposition in the urban environment. The data obtained from this network has been useful in enabling informed decisions to be made on local, regional and national solutions to the problem of acid deposition. A long-term trends network like GMADS is also important in tracking the response of acid deposition to the various control strategies being implemented. Within Europe more studies need to be established in order to quantify wet deposition and assess their effects in the urban environment.


Acid deposition is clearly an international problem that requires attention from all countries. By working together to keep SO2 and NOx emissions to a minimum the problems of acid rain may reduce. This would be beneficial to all and lead to a reduction in the damage to buildings, freshwater, vegetation, crops, soil, wildlife and human health.