a1 Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian, EH26 0QB, U.K.
a2 TNO Institute of Environmental Sciences, PO Box 6011, 2600JA AG Delft, The Netherlands
a3 Atmospheric Turbulence and Diffusion Division, National Oceanic and Atmospheric Administration, PO Box 2456, Oak Ridge, TN 37831, U.S.A.
The deposition of reactive gases on terrestrial surfaces is one of the primary mechanisms by which pollutant gases are removed from the atmosphere. The chemical properties of the gases (SO2, NO2, HNO3, HCl) and of the absorbing surfaces lead to differing rates of exchange and controlling processes. The most reactive gases, HNO3, HCl (and for many surfaces NH3) exhibit negligible surface resistances; deposition velocities (Vg) appropriate for short vegetation ranging from 2 to 5 cm s−1, for forests Vg may approach 10 cm s−1. The large rates of deposition for NH3 on moorland and forests lead to annual inputs, in areas with large atmospheric concentrations of NH3 (≥ 5 μg NH3 m−3), ranging from 20 to 60 kg N ha−1. The net exchange of NH3 over cropland, attributable to deposition during vegetative growth and emission of NH3 during senescence, is less well known but believed to be small.
The co-deposition of NH3 and SO2 on external surfaces of plant canopies is believed to enhance SO2deposition with reported deposition velocities over short vegetation of 2.0 cm s−1.
Rates of cloud droplet deposition to vegetation have been shown to be very similar to rates of momentum deposition (i.e. Vt ≈ ram−1). These findings provide the basis for estimates of cloud deposition inputs of major ions to upland Britain where they may contribute up to 30% of the wet deposited sulphur and nitrogen.