A sensitivity analysis on the atmospheric transformation and deposition of mercury in north-eastern USA

This paper presents the results of a sensitivity analysis on the factors that affect dry and wet deposition of atmospheric mercury (Hg), using a regional scale air quality model. Simulations were conducted for the north-eastern USA during a summer week and a winter week in 1997. Simulation results for the summer week and the winter week in general showed similar responses to changes in emission, environmental conditions, and alternative chemical mechanisms. Reduction of the ambient concentrations of soot or ozone was shown to reduce the wet deposition of Hg. When averaged over the summer and the winter week, the total deposition to the simulation domain would be reduced by 26% by reducing Hg emission from anthropogenic sources within the domain by 50%. For individual grids, however, only locations near local sources obtained noticeable reductions in ambient concentration and wet deposition due to the influence of re-emission from the natural surfaces and regional/global scale transport. The reduction in deposition would reach 36% if all Hg(II) emitted from anthropogenic sources were attached to particles. The total deposition was predicted to decrease by 22% when the gas phase Hg(II)-Hg(p) partitioning was included in the model. Only small changes in total deposition were observed by including the gas-phase ozone-Hg(0), reaction and the aqueous phase chlorine-Hg(0), reaction, and by lowering ambient concentrations of Hg(II) and Hg(p) at the upper lateral boundaries. During the summer week, Hg(II) deposition contributed 40% or more to the total deposition. The contribution increased to 70% in the winter week.