ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-591-2012 Gas-particle partitioning of atmospheric Hg(II) and its effect on global mercury deposition Amos H. M. 1 Jacob D. J. 1 2 Holmes C. D. 3 Fisher J. A. 1 Wang Q. 2 Yantosca R. M. 2 Corbitt E. S. 1 Galarneau E. 4 Rutter A. P. 5 Gustin M. S. 6 Steffen A. 4 Schauer J. J. 7 Graydon J. A. 8 Louis V. L. St. 8 Talbot R. W. 9 Edgerton E. S. 10 Zhang Y. 11 Sunderland E. M. 2 12 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA School of Engineering and Applied Sciences, Harvard University, Boston, MA, USA Department of Earth System Science, University of California, Irvine, CA, USA Air Quality Research Division, Environment Canada, Toronto, Ont., Canada Civil and Environmental Engineering Department, Rice University, Houston, TX, USA Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, NV, USA Department of Civil and Environmental Engineering, University of Wisconsin, Madison, WI, USA Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA Atmospheric Research & Analysis, Inc., Cary, NC, USA Department of Atmospheric Sciences, University of Washington, Seattle, USA Department of Environmental Health, Harvard University, Boston, MA, USA 11 01 2012 12 1 591 603 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/591/2012/acp-12-591-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/591/2012/acp-12-591-2012.pdf

Atmospheric deposition of Hg(II) represents a major input of mercury to surface environments. The phase of Hg(II) (gas or particle) has important implications for deposition. We use long-term observations of reactive gaseous mercury (RGM, the gaseous component of Hg(II)), particle-bound mercury (PBM, the particulate component of Hg(II)), fine particulate matter (PM<sub>2.5</sub>), and temperature (<i>T</i>) at five sites in North America to derive an empirical gas-particle partitioning relationship log<sub>10</sub>(K<sup>&minus;1</sup>) = (10±1)–(2500±300)/<i>T</i> where <i>K</i> = (PBM/PM<sub>2.5</sub>)/RGM with PBM and RGM in common mixing ratio units, PM<sub>2.5</sub> in μg m<sup>−3</sup>, and <i>T</i> in K. This relationship is within the range of previous work but is based on far more extensive data from multiple sites. We implement this empirical relationship in the GEOS-Chem global 3-D Hg model to partition Hg(II) between the gas and particle phases. The resulting gas-phase fraction of Hg(II) ranges from over 90 % in warm air with little aerosol to less than 10 % in cold air with high aerosol. Hg deposition to high latitudes increases because of more efficient scavenging of particulate Hg(II) by precipitating snow. Model comparison to Hg observations at the North American surface sites suggests that subsidence from the free troposphere (warm air, low aerosol) is a major factor driving the seasonality of RGM, while elevated PBM is mostly associated with high aerosol loads. Simulation of RGM and PBM at these sites is improved by including fast in-plume reduction of Hg(II) emitted from coal combustion and by assuming that anthropogenic particulate Hg(p) behaves as semi-volatile Hg(II) rather than as a refractory particulate component. We improve the simulation of Hg wet deposition fluxes in the US relative to a previous version of GEOS-Chem; this largely reflects independent improvement of the washout algorithm. The observed wintertime minimum in wet deposition fluxes is attributed to inefficient snow scavenging of gas-phase Hg(II).

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