[ 1 ] An atmospheric mercury model intercomparison study has been conducted to compare three regional-scale atmospheric mercury models, CMAQ, REMSAD, and TEAM, in atightly constrained testing environment with afocus on North America. Each of these models used the same horizontal modeling grid, pollutant emission information, modeled meteorology,a nd 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,r eactive gaseous mercury,a nd 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 afactor 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 af actor 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.

[1] A previous intercomparison of atmospheric mercury models in North America has been extended to compare simulated and observed wet deposition of mercury. Three regional-scale atmospheric mercury models were tested: the Community Multiscale Air Quality (CMAQ) model, the Regional Modeling System for Aerosols and Deposition (REMSAD), and the Trace Element Analysis Model (TEAM). These models were each employed using three sets of lateral boundary conditions to test their sensitivity to intercontinental transport of mercury. The same meteorological and pollutant emission data were used in each simulation. Observations of wet deposition were obtained from the National Atmospheric Deposition Program's Mercury Deposition Network. The regional models can explain 50-70% of the site-to-site variance in annual mercury wet deposition. CMAQ was found to have slightly superior agreement with observations of annual mercury deposition flux in terms of the mean value for all monitoring sites, but REMSAD showed the best correlation when measured by the coefficient of determination (r 2 ). With the exception of one CMAQ simulation, all of the models tended to simulate more wet deposition of mercury than was observed. TEAM exceeded the observed average annual wet deposition by 50% or more in all three of its simulations. CMAQ and REMSAD were better able to reproduce the observed seasonal distribution of mercury wet deposition than was TEAM, but TEAM showed the highest correlation for weekly wet deposition samples. An analysis of model accuracy at each observation site showed no obvious geographic patterns for correlation, bias, or error. Adjusting simulated mercury deposition on the basis of the difference between observed and simulated precipitation data improved the correlation and error scores for all of the models.

Potential respiratory and dermal exposure to applicators were estimated in a ground boom spray application of 2,4-D and dicamba. Time-weighted averages for airborne herbicide residues did not exceed 2.2 microgram/cu.m. in the cabs of application vehicles allowing only minor respiratory exposure. Dermal exposure was important as relatively large amounts of 2,4-D (1.2 - 18 mg) and dicamba (0.32-6.6 mg) were rinsed from applicators' hands. Urine analysis showed that the maximum elimination of herbicides occurred between 16 and 40 h after terminating exposure. A dicamba isomer (20% of the active material in the commercial formulation) was excreted in higher concentrations than dicamba in applicators' urine suggesting different toxicokinetic properties for the two compounds.

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