Modelling dry deposition of SO₂

Abstract

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.