Abstract
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%).
Highlights
< 5 % of global mercury resides in reactive form, this fraction is subject to more rapid chemical reactions and faster deposition to the Earth surface (Lindberg et al, 2007)
Simulated wet deposition from CMAQ-Hg is compared with observations from the Mercury Deposition Network (MDN) on an annual and seasonal basis in Table 1, where Fig. 1 shows the locations of the Great Lakes Region (GLR) measurement sites included in these calculations
Spring shows the lowest model error at 42 %, and the highest R2 value at 0.54, two to three times higher than any other season. These results show similar skill to previous studies comparing regional models to MDN results over the US, biases differ among studies, and within single studies among sensitivity tests (Bullock et al, 2009; Lin et al, 2007; Seigneur et al, 2003)
Summary
< 5 % of global mercury resides in reactive form, this fraction is subject to more rapid chemical reactions and faster deposition to the Earth surface (Lindberg et al, 2007). While the chemical stability of different forms of particulate mercury are not fully understood (Amos et al, 2012), the sum of reactive gaseous mercury (RGHg) and particulate mercury (PHg) is often called reactive mercury (RHg) due to its relatively short lifetime in the atmosphere compared to gaseous elemental mercury (Hg0). Zhang et al (2012), both of which compare multiple regional chemical transport models with ground based measurements of speciated ambient mercury in the context of wet and dry deposition. Both studies find that the regional models overestimate RHg relative to observations, and that treatment of mercury deposition is a major source of divergence among model simulations It is possible that the adequate simulation of wet deposition (dominated by RHg) may be due to compensating errors in deposition or other chemical processes
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