The Control of SO2 Dry Deposition on to Natural Surfaces by NH3 and its Effects on Regional Deposition

Two years of continuous measurements of SO2deposition fluxes to moorland vegetation are reported. The mean flux of 2.8 ng SO2 m-2 s-1 is regulated predominantly by surface resistance (r c) which, even for wet surfaces, was seldom smaller than 100 s m-1. The control of surface resistance is shown to be regulated by the ratio of NH3SO2 concentrations with an excess of NH3 generating the small surface resistances for SO2. A dynamic surface chemistry model is used to simulate the effects of NH3 on SO2 deposition flux and is able to capture responses to short-term changes in ambient concentrations of SO2, NH3 and meteorological conditions. The coupling between surface resistance and NH3/SO2 concentration ratios shows that the deposition velocity for SO2 is regulated by the regional pollution climate. Recent long-term SO2 flux measurements in a transect over Europe demonstrate the close link between NH3/SO2 concentrations and rc (SO2). The deposition velocity for SO2 is predicted to have increased with time since the 1970s and imply a 40% increase in v d at a site at which the annual mean ambient SO2 concentrations declined from 47 to 3 g m-3 between 1973 and 1998.