Inputs of trace gases, particles and cloud droplets to terrestrial surfaces

Synopsis 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 (SO 2 , NO 2 , HNO 3 , HCl) and of the absorbing surfaces lead to differing rates of exchange and controlling processes. The most reactive gases, HNO 3 , HCl (and for many surfaces NH 3 ) exhibit negligible surface resistances; deposition velocities ( V g ) appropriate for short vegetation ranging from 2 to 5 cm s −1 , for forests V g may approach 10 cm s −1 . The large rates of deposition for NH 3 on moorland and forests lead to annual inputs, in areas with large atmospheric concentrations of NH 3 (≥ 5 μg NH 3 m −3 ), ranging from 20 to 60 kg N ha −1 . The net exchange of NH 3 over cropland, attributable to deposition during vegetative growth and emission of NH 3 during senescence, is less well known but believed to be small. The co-deposition of NH 3 and SO 2 on external surfaces of plant canopies is believed to enhance SO 2 deposition 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. V t ≈ r am −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.