Wet and dry deposition monitoring in Southeastern Michigan

Abstract

Wet and dry deposition as collected by a bucket were measured at two sites in southeastern Michigan for two years. The precipitation had an average pH of 4.27 and a SO 2− 4 to NO − 3 ratio of 2.0. Particulate dry deposition velocities of 0.6 cm s −1 for SO 2− 4 and NO − 3 and > 2 cm s −1 for Cl −, Ca 2+, Mg 2+,Na + and K + were calculated. The ambient particle composition, dry bucket collection and wet deposition were compared at two sites, one urban and the other rural. Higher ambient particle concentrations and dry deposition rates were measured at the urban site than the rural site, indicating the influence of local emissions. However, local emissions had no effect on the wet deposition concentrations. The influence of more distant source regions was examined by separating the precipitation events by wind direction. The events from the south and east had the highest SO 2− 4 to NO − 3 ratios, which corresponded to the areas with the highest sulfur emissions. NO − 3 showed no directional dependence. Wet deposition was examined for the effect of storm type and seasonal trends. Contrary to a recent study on Long Island, we found higher concentrations of H +, SO 2− 4 and NH + 4 in winter rain compared to snow. The wet deposition concentrations of H +, SO 2− 4, and NH + 4 were highest in the summer, while only Na + and Cl − concentrations were highest in the winter, presumably due to winter road salting. The total deposition of acidic ions was highest in the summer and lowest in the winter, due both to lower concentrations and lower precipitation volumes in the winter. The dry deposition as collected by a bucket accounted for 1 % of total H + deposition, 21 % of SO 2− 4 deposition, 27% of NO − 3 deposition, 50% of Cl − deposition and 61 % of Ca 2+ deposition.