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Estimation of Speciated and Total Mercury Dry Deposition

Leiming Zhang1, Pierrette Blanchard1 and David Gay2

Mercury transport models are needed to provide estimates of dry deposition amounts at regional scales. Two model runs, one from the Community Multiscale Air Quality Modeling System (CMAQ) and another from the Global/Regional Atmospheric Heavy Metals Model (GRAHM), are analyzed to provide a first estimation of mercury dry deposition over the Great Lakes region. The modeled annual dry deposition of Hgp plus RGM is in the range of 10-40 g m-2 from CMAQ and in the range of 5-40 g m-2 from GRAHM over most of the areas south of the border. CMAQ shows a clear gradient with the highest deposition in Pennsylvania and its surrounding areas while GRAHM shows no such gradient in this region; however, GRAHM has more hot spots (> 40 g m-2) than those of CMAQ. Dry deposition of Hgp plus RGM in the areas north of the border are lower than 15 g m-2 from CMAQ and lower than 5 g m-2 from GRAHM. Modeled deposition to the water surfaces is mostly lower than 5 g m-2 from both models. The large differences in the Hgp and RGM concentrations between the measurements and the modeled values provide little confidence in the modeled dry deposition distributions in a quantitative sense. Mercury dry deposition at monitoring sites can be estimated routinely by combining monitored speciated ambient concentrations and modeled dry deposition velocities, the latter can be provided by dry deposition models with surfacelayer meteorological input from a weather forecast model. As an example, speciated dry deposition fluxes at multiple locations across eastern North America are calculated and analyzed for one year period.

1Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin St., Toronto, On., M3H 5T4, Canada
2Illinois State Water Survey, Institute of Natural Resource Sustainability, University of Illinois, 2204 Griffith Drive, Champaign, IL 61820