Modelling dry deposition of SO<sub>2</sub>

The goals of this paper are to briefly describe experimental methods that are used to measure SO2 dry deposition, and to discuss the physical, biological and chemical processes that control SO2 deposition fluxes over vegetation and to describe how these fluxes are modelled. The predominant pathway for gaseous SO2 uptake to dry vegetation is via turbulent transfer through the atmosphere surface boundary layer and molecular diffusion through the leaf laminar boundary layer and the stomata. The soil surface is a significant, but weaker sink for sulphur, especially when frozen or covered with snow. The appreciable solubility of SO2 causes its uptake to be enhanced greatly in the presence of moisture on leaves and the soil. The aqueous uptake of SO2, however, causes the pH of a solution to decrease which in turn produces a reduction in the solubility of SO2. Neutralising species (ammonia, inner plant species) may cancel this reduction. A method is proposed to estimate local scale dry deposition fluxes of SO2 in Europe. The method combines long-range transport modelling results, land use and surface specific data and an inferential approach.