Primary effects of changes in meteorology vs. anthropogenic emissions on mercury wet deposition: A modeling study

In this study we investigated the respective contributions of changing meteorology and anthropogenic emissions to Hg wet deposition in New York State (NYS), which illustrated quantitatively how the variations in meteorological conditions modified the response of Hg wet deposition to anthropogenic emission reductions in NYS. A regional chemical transport model, CMAQ-newHg-Br (Ye et al., 2018), was employed driven by four sets of meteorological fields (2004, 2005, 2007, and 2010) and two anthropogenic emission inventories (NEI, 2005 and 2011). Four major findings emerged from this study. First, simulated Hg wet deposition flux in NYS was greatly affected by variation in meteorological conditions, ranging from a 91% decrease to a factor of 5 increase in monthly Hg wet deposition, whereas it hardly responded (2%) to a near doubling of emissions in the northeastern U.S. Second, the precipitation amount dominated interannual and seasonal variations of Hg wet deposition over all other meteorological variables, with total (= convective + non-convective) precipitation explained 60% of Hg wet deposition variance in an urban environment compared to convective precipitation alone explaining 65% in a rural area. Third, large-scale atmospheric circulation influenced regional transport greatly leading to out-of-state anthropogenic emissions explaining at least 80% of Hg wet deposition changes in a NYS urban environment. Fourth, higher Hg concentrations in precipitation and higher atmospheric reactive mercury concentrations in urban areas were related to calm conditions due likely to regional build-up of emissions, whereas in rural areas photochemical oxidation of gaseous elemental mercury was a more important source of reactive mercury. Solar radiation variance was found to explain 41–55% of the variance in simulated reactive mercury concentrations. Given that more than 95% of the US is rural lands and the majority of water bodies occur in rural areas, a realistic chemical mechanism for photochemical gaseous elemental mercury oxidation is paramount to accurate simulation of reactive mercury formation and Hg wet deposition in the US.