An assessment of atmospheric mercury in the Community Multiscale Air Quality (CMAQ) model

Abstract

Quantitative analysis of three atmospheric mercury species - gaseouselemental mercury (Hg-0), reactive gaseous mercury (RGHg) andparticulate mercury (PHg) - has been limited to date by lack of ambientmeasurement data as well as by uncertainties in numerical models andemission inventories. This study employs the Community Multiscale AirQuality Model version 4.6 with mercury chemistry (CMAQ-Hg), to examinehow local emissions, meteorology, atmospheric chemistry, and depositionaffect mercury concentration and deposition the Great Lakes Region(GLR), and two sites in Wisconsin in particular: the rural Devil's Lakesite and the urban Milwaukee site. Ambient mercury exhibits significantbiases at both sites. Hg-0 is too low in CMAQ-Hg, with the model showinga 6% low bias at the rural site and 36% low bias at the urban site.Reactive mercury (RHg = RGHg + PHg) is over-predicted by the model, withannual average biases > 250%. Performance metrics for RHg are muchworse than for mercury wet deposition, ozone (O-3), nitrogen dioxide(NO2), or sulfur dioxide (SO2). Sensitivity simulations to isolatebackground inflow from regional emissions suggests that oxidation ofimported Hg-0 dominates model estimates of RHg at the rural study site(91% of base case value), and contributes 55% to the RHg at the urbansite (local emissions contribute 45%).