A regional scale modeling study of atmospheric transport and transformation of mercury

A modeling study was conducted on the transport, transformation, and deposition of atmospheric Hg. Three mercury species, Hg(0), Hg(II), and Hg(p), were considered. A three-dimensional regional scale air quality model was developed based on the framework of SAQM (Chang et al., 1996) by formulating cloud processes, air-surface exchange of elemental Hg, and atmospheric mercury chemistry. Preliminary validation was conducted by comparing simulation results with observations. The model was then applied to a portion of the northeast US for a summer week and a winter week in 1997. ^ Based upon research results in the literature, the traditional resistance model for chemical dry deposition was extended in this study to a bi-directional Hg flux approach. Calculated values in a portion of the northeast US during a summer week in 1995 were within a factor of two from published data, suggesting that the parameterization of the exchange processes was acceptable. This method was then used in the 1997 simulations. ^ The newly developed model simulates transport and chemical reactions of Hg. Three types of cloud were included: precipitating clouds, co-existing non-precipitating clouds, and fair weather clouds. The simulation results of ambient Hg concentration and Hg concentration in precipitation were compared with measurements made in Connecticut and other locations. The comparison indicated that the model was promising in predicting gaseous Hg concentrations, and acceptable in predicting Hg concentration in precipitation. The summer week application showed that the model was capable of predicting hourly concentration and deposition fields of the three Hg species, as well as in-cloud transformation of Hg(0) by each of the three cloud types. ^ A sensitivity analysis was conducted to examine the ambient concentrations and dry and wet deposition in response to changes in emission, environmental conditions, and inclusion of additional chemical reactions. The results indicated that the Hg deposition was sensitive to ambient concentrations of ozone and soot particles, but less sensitive to point-source emissions. Total Hg deposition depended on the fraction of ambient mercury bound to particles, and Hg(II) deposition had more impact on the total deposition than Hg(0) and Hg(p). ^