Development and Application of a Multipollutant Model for Atmospheric Mercury Deposition

Abstract

A multipollutant model, the Community Multiscale Air Quality model paired with the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (CMAQ-MADRID), is extended to include a comprehensive treatment of mercury processes and is applied to the simulation of the atmospheric deposition of sulfate and mercury over the United States during 1996. Model performance is evaluated first by comparison with annual sulfate wet deposition data from the National Atmospheric Deposition Program?s National Trends Network; the coefficient of determination r?2 is 0.77, and the model normalized error and bias are 53% and ?8%, respectively. When actual precipitation data are used to scale the deposition fluxes, r?2 improves to 0.91 and the error and bias change to 42% and ?41%, respectively. The scaled results underscore a tendency of the model to underestimate sulfate wet deposition. Model performance for mercury wet deposition is then evaluated by comparison with data from the Mercury Deposition Network. For annual mercury wet deposition, r?2 is 0.28 and the normalized error and bias are 81% and 73%, respectively, when the modeled precipitation data are used. Model performance improves when actual precipitation data are used to scale deposition fluxes: r?2 increases to 0.41 and the error and bias decrease to 40% and 29%, respectively. The model reproduces the spatial pattern of sulfate wet deposition adequately with an increasing gradient from the upper Midwest to the Northeast, that is, from upwind to downwind of large sulfur dioxide sources in the Ohio River Valley. However, the model tends to overestimate mercury wet deposition in the Northeast downwind of these sources that also emit significant amounts of mercury. This ?Pennsylvania anomaly? may be due to a partial misrepresentation of the mercury reduction?oxidation cycle, uncertainties in the dry deposition of divalent gaseous mercury HgII, incorrect speciation of mercury emissions, and/or uncharacterized emissions in the upper Midwest.