The estimation of the dry deposition of sulphur dioxide to forests is confounded by the possibility of co-deposition of SO2 with NH3 on leaf surfaces. A sector of Scots pine forest was selectively fumigated with NH3 to give average concentrations up to 15 ppbV (nL L–1) above ambient, in order to test the hypothesis that increased air concentrations of NH3 would enhance the dry deposition of SO2, and the consequent amounts of SO4 2– measured in throughfall below the forest canopy. Ammonia gas, generated by evaporation of concentrated aqueous solution, was released above the canopy in proportion to wind speed when the wind direction was between south and west. Concentrations of NH3 at canopy height were measured using passive diffusion tubes; throughfall was preserved with thymol and measured weekly. Meteorological data and SO2 concentrations were recorded continuously, to permit the estimation of dry deposition input. Deposition of NH4 + in throughfall over 8 months was increased by up to 40 meq m–2 relative to control sites upwind of the NH3 release point, with largest values closest to the release point. Deposition of SO4 2– in throughfall was also enhanced in the fumigated area, by up to 20 meq m–2, even though average ambient SO2 concentrations were 2.3 ppbV. The results are discussed in terms of the factors controlling SO2 deposition on forest surfaces, the development of appropriate deposition models, and their relevance to using throughfall as an estimate of total S deposition.\u
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