Lagrangian modeling of mercury air emission, transport and deposition: An analysis of model sensitivity to emissions uncertainty

The Regional Lagrangian Model of Air Pollution (RELMAP) is used to simulate the emission, transport and diffusion, chemical transformation, and wet and dry deposition of elemental mercury gas, divalent mercury gas and particulate mercury from various point- and area-source types. A version of the RELMAP mercury model is used which separately simulates the transport and deposition of emissions from the seven point-source categories modeled. A mercury air emission inventory developed by the U.S. EPA Office of Air Quality Planning and Standards for the continental United States is used. This inventory provides estimates of the total mercury mass emitted, but does not define the chemical and physical forms of these emissions. A wide variety of emission speciations, combinations of chemical and physical forms, are evaluated with the new RELMAP mercury model. Average annual concentrations and wet and dry deposition totals are simulated for a complete calendar year. The results of the simulation are used to estimate the quantity of mercury deposited to soils and water bodies based on a variety of assumed emission speciation for the seven point-source types. An analysis of the modeling results provides some quantitative information about the sensitivity of modeling results to uncertainty in the mercury emissions speciation of modeled industrial activities, but a qualitative assessment is the primary goal. The model is shown to be most sensitive to mercury emissions speciation uncertainty related to the burning of waste materials and electrical power generation.