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MODELING ASSESSMENT OF TRANSPORT AND DEPOSITION PATTERNS OF MERCURY AIR EMISSIONS FROM THE U.S. AND CANADA
Citation:
Bullock, O R. MODELING ASSESSMENT OF TRANSPORT AND DEPOSITION PATTERNS OF MERCURY AIR EMISSIONS FROM THE U.S. AND CANADA. Presented at 5th International Conference on Mercury as a Global Pollutant, Rio de Janeiro, Brazil, May 23-28, 1999.
Impact/Purpose:
The goal of this research is to develop and test appropriate chemical and physical mechanisms for use in EPA's Models-3 chemical/transport models. These models will be addressing issues of tropospheric photochemistry, fine particles, toxic and semi-volatile substances, and acid deposition. As such, scientifically credible mechanisms for atmospheric gas- and aqueous-phase chemistry as well as heterogeneous chemistry, applicable to the particular pollutant regimes must be included in Models-3.
Description:
In December 1997, the U.S. EPA submitted the Mercury Study Report to Congress which included a regional-scale modeling assessment of the transport and deposition of U.S. air emissions of mercury. This modeling was performed with a modified version of the Regional Lagrangian Model of Air Pollution (RELMAP). Canadian mercury emissions data were not obtained in time to be included in the simulations described in the report to the U.S. Congress. Thus, reported comparisons of deposition modeling results to observed values in the northern portions of the continental U.S. were of limited scientific value. An inventory of Canadian air emissions of mercury was obtained in late 1997 from Environment Canada which has now been included in a new RELMAP simulation of annual mercury deposition patterns.
These new Canadian emissions data have been simulated using the same version of the RELMAP and the same estimates of source-dependent emission speciation as were used for U.S. emissions in the previous study. New mercury wet deposition modeling results obtained using both U.S. and Canadian emission inventories have been compared with observed values in various locations within the continental U.S. These new modeling results have also been compared to the previously reported results from U.S. emissions only to obtain a rough bi-national apportionment of mercury deposition across the continental U.S. and southern Canada.
Preliminary modeling results show that the addition of the Canadian emissions inventory data increases RELMAP simulated wet deposition by 20% or more in only a few areas of the U.S. immediately adjacent to the Canadian border. The locations for which observed wet deposition data were compared to modeling results in the Mercury Study Report to Congress were not significantly affected by the Canadian emissions to the simulation. Conversely, a rather large area of Canada north of the Great Lakes and in the Maritime Provinces does appear to have a significant portion of simulated mercury wet deposition attributable to U.S. sources.
Eulerian modeling frameworks are now being developed at the U.S. EPA and other research organizations to more accurately treat global-scale and upper atmospheric transportable mercury. However, comparison of these new modeling results with long-term observations of wet deposition shows that the simpler Lagrangian framework of the RELMAP is capable of simulating regional-scale cross-boundary transport of mercury to very useful scientific accuracies of about 40% over most of the U.S. and Canada.