The contribution of dry deposited ammonia and sulphur dioxide to the composition of precipitation from continuously open gauges

A series of experiments using bulk precipitation collectors of the type used in the UK precipitation chemistry network measured the amounts of NH4+, SO42- and other ions that could be washed from funnels (diameter 15 cm) exposed to a wide range of NH3 and SO2 concentrations over periods from hours to days. In dry conditions, the average deposition flux of NH3 was between 50 and 120 nmol NH4+ funnel(-1)d(-1) (0.1-0.3 kg N ha(-1) yr(-1)), and was independent of the concentration of NH3. Dry deposition of NH3 to Wet funnels at small NH3 concentrations was almost 5 times that to dry funnels under the same conditions (average 240 nmol funnel(-1) d(-1); 0.7 kg ha(-1) yr(-1)), and increased with increasing NH3 concentrations. The amount of NH4+ ions remaining on the funnel surface was inversely proportional to the vapour pressure deficit during the experiment. This result was interpreted as a dependence on the duration of surface wetness, with greater deposition of NH4+ when evaporation rates of surface water were small. The amount Of SO2 deposited on funnel surfaces was closely related to the amount of NH3 deposited, in both wet and dry conditions, but was not strongly correlated with the SO2 concentration. At low NH3 and SO2 concentrations the average deposition to dry funnels was 70 nmol SO4 funnel(-1)d(-1) (0.5 kgha(-1)yr(-1)), and to wet funnels was approximately 2.5 times larger. The results are interpreted in terms of the balance between the rate of evaporation of surface water, and the rate of oxidation Of SO2, which leads to the 'fixing' of NH4+ ions on the surface as involatile salts. It is predicted that dry deposition of NH3 to funnel surfaces across the UK Secondary Network could account for as much as one-half of the measured bulk wet deposition at sites where wet deposition of NH4-N is small. The amount of dry deposition depends on how long and how often funnel surfaces are wetted by rain or dew, and on the air concentrations of NH3. These predictions are based on funnels being wetted only once per day. More frequent wetting would increase the contribution from dry deposition, and the consequent overestimate of wet deposition of NH4-N across the UK by using data obtained from bulk collectors. To some extent this overestimate may be offset by microbial degradation and loss of NH4-N in weekly bulk precipitation samples during collection and storage. (C) 2002 Elsevier Science Ltd. All rights reserved.