THROUGHFALL DEPOSITION OF AMMONIUM AND SULFATE DURING AMMONIA FUMIGATION OF A SCOTS PINE FOREST

The estimation of the dry deposition of sulphur dioxide to forests is confounded by the possibility of codeposition 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 amb ient in order to test the hypothesis that increased air concentrations of NH3 would enhance the dry deposition of SO2, and the consequent am ounts of SO42- measured in throughfall below the forest canopy. Ammoni a gas, generated by evaporation of concentrated aqueous solution, was released above the canopy in proportion to wind speed when the wind di rection was between south and west. Concentrations of NH3 at canopy he ight were measured using passive diffusion tubes; throughfall was pres erved with thymol and measured weekly. Meteorological data and SO2 con centrations were recorded continuously, to permit the estimation of dr y deposition input. Deposition of NH4+ in throughfall over 8 months wa s increased by up to 40 meq m(-2) relative to 'control' sites upwind o f the NH3 release point, with largest values closest to the release po int. Deposition of SO42- in throughfall was also enhanced in the fumig ated area, by up to 20 meg m(-2), even though average ambient SO2 conc entrations were 2.3 ppbV. The results are discussed in terms of the fa ctors controlling SO2 deposition on forest surfaces, the development o f appropriate deposition models, and their relevance to using throughf all as an estimate of total S deposition.