Nested-grid simulation of mercury over North America
We have developed a new nested-grid mercury (Hg) simulation over North\nAmerica with a 1/2A degrees latitude by 2/3A degrees longitude\nhorizontal resolution employing the GEOS-Chem global chemical transport\nmodel. Emissions, chemistry, deposition, and meteorology are\nself-consistent between the global and nested domains. Compared to the\nglobal model (4A degrees latitude by 5A degrees longitude), the nested\nmodel shows improved skill at capturing the high spatial and temporal\nvariability of Hg wet deposition over North America observed by the\nMercury Deposition Network (MDN) in 2008-2009. The nested simulation\nresolves features such as higher deposition due to orographic\nprecipitation, land/ocean contrast and and predicts more efficient\nconvective rain scavenging of Hg over the southeast United States.\nHowever, the nested model overestimates Hg wet deposition over the Ohio\nRiver Valley region (ORV) by 27%. We modify anthropogenic emission\nspeciation profiles in the US EPA National Emission Inventory (NEI) to\naccount for the rapid in-plume reduction of reactive to elemental Hg\n(IPR simulation). This leads to a decrease in the model bias to -2.3%\nover the ORV region. Over the contiguous US, the correlation coefficient\n(r) between MDN observations and our IPR simulation increases from 0.60\nto 0.78. The IPR nested simulation generally reproduces the seasonal\ncycle in surface concentrations of speciated Hg from the Atmospheric\nMercury Network (AMNet) and Canadian Atmospheric Mercury Network\n(CAMNet). In the IPR simulation, annual mean gaseous and\nparticulate-bound Hg(II) are within 140% and 11% of observations,\nrespectively. In contrast, the simulation with unmodified anthropogenic\nHg speciation profiles overestimates these observations by factors of 4\nand 2 for gaseous and particulate-bound Hg(II), respectively. The nested\nmodel shows improved skill at capturing the horizontal variability of Hg\nobserved over California during the ARCTAS aircraft campaign. The nested\nmodel suggests that North American anthropogenic emissions account for\n10-22% of Hg wet deposition flux over the US, depending on the\nanthropogenic emissions speciation profile assumed. The modeled percent\ncontribution can be as high as 60% near large point sources in ORV. Our\nresults indicate that the North American anthropogenic contribution to\ndry deposition is 13-20%.