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

Abstract

Atmospheric deposition represents a major input of mercury to surface environments. The phase of mercury (gas or particle) has important implications for its removal from the atmosphere. We use long-term observations of reactive gaseous mercury (RGM), particle-bound mercury (PBM), fine particulate matter (PM<sub>2.5</sub>), and temperature at five sites in North America to derive an empirical gas-particle partitioning relationship log<sub>10</sub>(<i>K</i><sup>-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 Kelvin. This relationship is in 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 divalent mercury (Hg(II)). 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 snow. Model comparison to Hg observations at 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. This and other model updates, including the correction of an outstanding algorithm error, to wet deposition improve the simulation of Hg wet deposition fluxes in the US relative to the previous version of the model. The observed wintertime minimum in wet deposition fluxes is attributed to inefficient snow scavenging of gas-phase Hg(II)

Sources of carbonaceous aerosols and deposited black carbon in the Arctic in winter-spring: implications for radiative forcing

“…Wet deposition accounts globally for 77 % of annual BC deposition, consistent with the AeroCom multimodel assessment (78.6 ± 17 %) (Textor et al, 2006). The tropospheric lifetime of 210 Pb aerosol in the model is 10.4 days (Amos et al, 2011), consistent with observational constraints (Liu et al, 2001). With regard to the Arctic, our successful simulation of observations combined with the relatively good constraints on emissions (see discussion below) gives some confidence to our scavenging parameterization.…”

Benefits of mercury controls for the United States

“…1 maps these deposition differences over the contiguous United States. We model the atmospheric transport and deposition of mercury using the global, 3D landocean-atmosphere mercury model GEOS-Chem v.9-02, at 4°× 5°resolution globally and 0.5°× 0.667°resolution over the United States (22)(23)(24)(25)(26). We use net total deposition as a measure of mercury ecosystem enrichment (27).…”

Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions

“…For ASGM, we use the Emissions Database for Global Atmospheric Research gridded inventory (9). II is partitioned thermodynamically between the gas and particle phase on the basis of local temperatures and total aerosol concentration computed with a GEOS-Chem aerosol simulation (43 (45,46). Redox chemistry also takes place in the surface ocean and soil reservoirs, which receive atmospheric deposition and reemit to atmosphere through land-air and sea-air exchanges.…”