An assessment of atmospheric mercury in the Community Multiscale Air Quality (CMAQ) model
1Nelson Institute Center for Sustainability and the Global Environment (SAGE), University of Wisconsin-Madison, Madison, WI, USA
2Public Policy Center (PPC), University of Iowa, Iowa City, IA, USA
3Civil and Environmental Engineering, Rice University, Houston, TX, USA
4Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI, USA
Abstract. Quantitative analysis of three atmospheric mercury species – gaseous elemental mercury (Hg0), reactive gaseous mercury (RGHg) and particulate mercury (PHg) – has been limited to date by lack of ambient measurement data as well as by uncertainties in numerical models and emission inventories. This study employs the Community Multiscale Air Quality Model version 4.6 with mercury chemistry (CMAQ-Hg), to examine how local emissions, meteorology, atmospheric chemistry, and deposition affect mercury concentration and deposition the Great Lakes Region (GLR), and two sites in Wisconsin in particular: the rural Devil's Lake site and the urban Milwaukee site. Ambient mercury exhibits significant biases at both sites. Hg0 is too low in CMAQ-Hg, with the model showing a 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, with annual average biases >250%. Performance metrics for RHg are much worse than for mercury wet deposition, ozone (O3), nitrogen dioxide (NO2), or sulfur dioxide (SO2). Sensitivity simulations to isolate background inflow from regional emissions suggests that oxidation of imported Hg0 dominates model estimates of RHg at the rural study site (91% of base case value), and contributes 55% to the RHg at the urban site (local emissions contribute 45%). Limited evidence on the lifetime of RHg transported to the rural site suggests that modeled dry deposition rates are too high, possibly compensating for the erroneously high RHg values.