An assessment of atmospheric mercury in the Community Multiscale Air Quality (CMAQ) model at an urban site and a rural site in the Great Lakes Region of North America
Quantitative analysis of three atmospheric mercury species - gaseous\nelemental mercury (Hg-0), reactive gaseous mercury (RGHg) and\nparticulate mercury (PHg) - has been limited to date by lack of ambient\nmeasurement data as well as by uncertainties in numerical models and\nemission inventories. This study employs the Community Multiscale Air\nQuality Model version 4.6 with mercury chemistry (CMAQ-Hg), to examine\nhow local emissions, meteorology, atmospheric chemistry, and deposition\naffect mercury concentration and deposition the Great Lakes Region\n(GLR), and two sites in Wisconsin in particular: the rural Devil's Lake\nsite and the urban Milwaukee site. Ambient mercury exhibits significant\nbiases at both sites. Hg-0 is too low in CMAQ-Hg, with the model showing\na 6% low bias at the rural site and 36% low bias at the urban site.\nReactive mercury (RHg = RGHg + PHg) is over-predicted by the model, with\nannual average biases > 250%. Performance metrics for RHg are much\nworse than for mercury wet deposition, ozone (O-3), nitrogen dioxide\n(NO2), or sulfur dioxide (SO2). Sensitivity simulations to isolate\nbackground inflow from regional emissions suggests that oxidation of\nimported Hg-0 dominates model estimates of RHg at the rural study site\n(91% of base case value), and contributes 55% to the RHg at the urban\nsite (local emissions contribute 45%).