Regional scale photochemical model evaluation of total mercury wet deposition and speciated ambient mercury

Methylmercury is a known neurotoxin with deleterious health effects on humans and wildlife. Atmospheric deposition is the largest source of mercury loading to most terrestrial and aquatic ecosystems. Regional scale air quality models are needed to quantify mercury deposition resulting from complex emissions sources and physical and chemical processes that govern the fate of mercury in the atmosphere. Total mercury wet deposition estimates from multiple regional photochemical transport models applied at 12 km grid resolution over the continental United States compare well with observations (CAMx fractional error = 45%, CMAQ fractional error = 33%) despite uncertainties in global mercury emissions inventories and certain chemical transformation pathways. In addition, both CMAQ and CAMx well represent observed diel and seasonal patterns of Hg(0) and tend to exaggerate the diel patter of Hg(II) at AMNet monitor locations. The observed fraction of particulate mercury to total oxidized mercury (sum of particulate mercury and Hg(II)) is generally greater in colder months and during overnight hours. The modeling systems tend to capture these patterns but have a systematically lower fraction of particulate mercury to total oxidized mercury, especially in winter months. Annual total mercury deposition from wet and dry processes is 65% greater in CMAQ compared to CAMx over the entire modeling domain. This is largely due to higher wet deposition in CMAQ and higher dry deposition of Hg(0), which is treated as equilibrium with mercury re-emissions and not modeled in CAMx. A sensitivity using CAMx with Hg(0) dry deposition treated similar to CMAQ resulted in more comparable total mercury deposition estimates. Modeled dry deposition velocities for Hg(II) compare well with the limited experimental data, while Hg(0) dry deposition velocities are lower than published experimental data. A mercury bi-directional flux sensitivity application in CMAQ had the overall effect of reducing total mercury dry deposition and slightly improving ambient Hg(0) performance. The range of the domain wide total deposition from all model sensitivities was within 25% of the mean but exhibited larger deviations in the individual wet and dry deposition budgets. The contribution of mercury initial conditions and lateral boundary inflow conditions were tracked separately using CAMx source apportionment Initial contribution to total mercury deposition for the entire model domain falls below 5% after 2 weeks. Boundary contribution to total mercury deposition vary considerably across the continental United States, but ranges between 20 and 99% at MDN monitor locations. Published by Elsevier Ltd.