Gas-particle partitioning of atmospheric Hg(II) and its effect on global mercury deposition

Abstract

Abstract. 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 2.5 ), and temperature (T ) at five sites in North America to derive an empirical gas-particle partitioning relationship log 10 (K −1 ) = (10±1)-(2500±300)/T where K = (PBM/PM 2.5 )/RGM with PBM and RGM in common mixing ratio units, PM 2.5 in µg m −3 , and T 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 semivolatile 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).

Relative contributions of gaseous oxidized mercury and fine and coarse particle‐bound mercury to mercury wet deposition at nine monitoring sites in North America

“…Previous studies suggest that inefficient scavenging of GOM by snow could explain the lower Hg wet deposition rates in the winter [Selin and Jacob, 2008;Lombard et al, 2011]. In contrast, scavenging of particle-bound mercury by snow is efficient particularly at higher latitudes [Amos et al, 2012]. Particle scavenging by snow is theoretically more efficient than rain because of the larger surface area of snow [Wang et al, 2014;Zhang et al, 2015].…”

“…To determine C CPBM , it was assumed that the particulate Hg concentration increases with PM mass, which implies that CPBM is formed by GOM partitioning to particles. This assumption applies to gas-particle partitioning of Hg(II) and semivolatile organic compounds because the partition coefficient (K p ) is normalized by particulate matter (PM 2.5 ) and total suspended particle concentrations, respectively [Pankow, 1994;Amos et al, 2012]. Under this assumption, C CPBM can be estimated from the coarse and fine PM mass fractions using equation (3):…”

“…FPBM and CPBM concentrations could vary with point source emissions, nonpoint sources (e.g., wood burning and forest fires), and Hg(II) gas-particle partitioning processes [Y. Amos et al, 2012]. 3.2.3.…”

“…Mercury transport models have been evaluated using monitored wet deposition data; however, most models do not include coarse PBM (CPBM) in the deposition budget since particle-bound Hg is typically treated as a fine aerosol [Selin et al, 2007;Bullock et al, 2008;Pongprueksa et al, 2008;Amos et al, 2012;Holloway et al, 2012;Zhang et al, 2012a;Kos et al, 2013]. Although most model results are comparable to mercury wet deposition measurements, the omission of CPBM is another potential explanation for the overpredicted concentrations of GOM and fine PBM (<2.5 μm) in many mercury transport models [Amos et al, 2012;Holloway et al, 2012;Kos et al, 2013;Zhang et al, 2012aZhang et al, , 2012c. Measurements of CPBM indicate that the coarse Hg fraction of total particulate mercury at two sites in New Hampshire can range from 0.35 to 0.6 depending on the season and year sampled [Feddersen et al, 2012].…”

Responses of surface ozone air quality to anthropogenic nitrogen deposition in the Northern Hemisphere

“…It includes a detailed simulation of tropospheric NO x -VOC-O 3 -aerosol chemistry (Park et al, 2004;Mao et al, 2010). The model wet deposition scheme including scavenging in convective updraft and large-scale precipitation is described by Liu et al (2001) for aerosols and by Mari et al (2000) and Amos et al (2012) for soluble gases. The dry deposition parameterization for gases and aerosols follows a standard bigleaf resistance-in-series model (Wesely, 1989;Zhang et al, 2001).…”