MODELING ATMOSPHERIC CONCENTRATIONS OF MERCURY AND DEPOSITION TO THE GREAT-LAKES

Regional patterns of seasonal and annual average air concentrations and cumulative deposition of mercury to the Great Lakes basin are calculated with the ASTRAP model. The model treats the Hg system as three chemical components: elemental (Hg-0), particulate (Hg-p), and gaseous divalent (Hg-II). Primary anthropogenic emission inventories (i.e. emissions resulting from current activities) include surface and elevated sources of each of the three Hg species for eastern North America. Natural and secondary anthropogenic emissions (i.e. reemission of Hg deposited or released during earlier anthropogenic activity) over the United States and Canada are estimated by defining an emission term for Hg-0 that varies with latitude and season. Global background concentrations of Hg-0 and Hg-p are specified to average annually 1.0 and 0.01 ng m(-3), respectively. Rates of parameterizations of wet and dry removal are very rapid for Hg-II, intermediate for Hg-p, and very slow for Hg-0. Because of the disparate removal efficiencies, estimates of deposition resulting from anthropogenic emissions are critically dependent upon the speciation of emissions and, for the rapidly depositing Hg-II, the assumptions about effective stack heights. Integration of the Hg deposition field over the Great Lakes produces estimates of direct atmospheric loading from primary anthropogenic emissions of 1.44 and 2.46 t Hg yr(-1) by wet and dry deposition, respectively. Estimates of direct loading from natural and secondary anthropogenic emissions of Hg-0 over the continent are 0.09 and 0.15 t Hg yr(-1) for wet and dry deposition, respectively, while the corresponding contributions from the global background are estimated to be 0.15 and 0.39 t Hg yr(-1), respettively. Although they constitute only 16% of estimated anthropogenic emissions, emissions of Hg-II contribute 78% of the current direct anthropogenic deposition to the Lakes, or 65% of the deposition df Hg from all sources. Revolatilization of Hg-0 from the Lakes is estimated to lie between 2.3 and 13.7 t Hg yr(-1). Thus, revolatilization may well be greater than the direct atmospheric loading of all Hg species to the Lakes, 4.7 t Hg yr(-1).