The North American Mercury Model Intercomparison Study (NAMMIS): Study description and model-to-model comparisons

An atmospheric mercury model intercomparison study has been conducted to compare three regional-scale atmospheric mercury models, CMAQ, REMSAD, and TEAM, in a tightly constrained testing environment with a focus on North America. Each of these models used the same horizontal modeling grid, pollutant emission information, modeled meteorology, and boundary conditions to the greatest extent practical. Three global-scale atmospheric mercury models were applied to define three separate initial condition and boundary condition (IC/BC) data sets for elemental mercury, reactive gaseous mercury, and particulate mercury air concentrations for use by the regional-scale models. The monthly average boundary concentrations of some mercury species simulated by the global models were found to vary by more than a factor of 10, especially at high altitudes. CMAQ, REMSAD, and TEAM were each applied three times, once for each IC/BC data set, to simulate atmospheric mercury transport and deposition during 2001. This paper describes the study design and shows qualitative model-to-model comparisons of simulation results on an annual basis. The air concentration patterns for mercury simulated by the regional-scale models showed significant differences even when the same IC/BC data set was used. Simulated wet deposition of mercury was strongly influenced by the shared precipitation data, but differences of over 50% were still apparent. Simulated dry deposition of mercury was found to vary between the regional-scale models by nearly a factor of 10 in some locations. Further analysis is underway to perform statistical comparisons of simulated and observed mercury wet deposition using weekly and annual sample integration periods.