Transfer to terrestrial surfaces

The processes by which atmospheric pollutants are transferred to the ground may be placed into two categories. Wet deposition, which includes all pollution reaching the surface in precipitation, and dry deposition which includes the sorption of gases and capture of particulate pollutants by terrestrial surfaces. The properties of each of the removal mechanisms are described for the gases SO2, NO2, HNO3 and the atmospheric aerosols containing sulphate and nitrate. Following a description of the processes, rates of deposition appropriate for a range of vegetation and atmospheric conditions are provided. The influence of changes in the properties of absorbing surfaces due to the presence of water or for vegetation to changes in stomatal conductance is discussed. The use of transfer resistances to describe the mechanism of deposition is then extended to provide estimates of dry deposition of SO2 on to Great Britain which are contrasted with wet deposition. The resulting maps show that over most of England, including central, eastern and southern areas, dry deposition exceeds wet deposition. For north west England, Wales and northern Scotland, particularly the large rainfall districts of the west, wet deposition exceeds dry deposition.