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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-145
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
07 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Surface deposition of oxidized mercury dominated by production in the upper and middle troposphere
Viral Shah and Lyatt Jaeglé Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
Abstract. Oxidized mercury (Hg(II)) is chemically produced in the atmosphere by oxidation of elemental mercury and is directly emitted by anthropogenic activities. We use the GEOS-Chem global chemical transport model, with gaseous oxidation driven by Br atoms, to quantify how surface deposition of Hg(II) is influenced by Hg(II) production at different atmospheric heights. We tag Hg(II) chemically produced in the lower (surface–750 hPa), middle (750–400 hPa) and upper troposphere (400 hPa–tropopause), in the stratosphere, as well as directly emitted Hg(II). A two-year simulation (2013–2014) reproduces the spatial distribution and seasonal cycle of Hg(II) surface concentrations and Hg wet deposition observed at the Atmospheric Mercury Network (AMNet) and the Mercury Deposition Network (MDN) stations over the United States to within 21 %, but displays a 46 % underestimate of wet deposition observed at the European Monitoring and Evaluation Programme (EMEP) stations. We find that Hg(II) produced in the upper and middle troposphere constitutes 91 % of the tropospheric mass of Hg(II) and 91 % of the annual Hg(II) wet deposition flux. This large global influence from the upper and middle troposphere is the result of strong chemical production coupled with a long lifetime of Hg(II) in these regions. Annually, 77–84 % of surface level Hg(II) over the western U.S., South America, South Africa, and Australia is produced in the upper and middle troposphere, whereas 26–66 % of surface Hg(II) over the eastern U.S., Europe, East Asia, and South Asia is directly emitted. Over the oceans, 72 % of surface Hg(II) is produced in the lower troposphere, because of higher Br concentrations in the marine boundary layer. The global contribution of the upper and middle troposphere to the Hg(II) dry deposition flux is 52 %. It is lower compared to the contribution to wet deposition because dry deposition of Hg(II) produced aloft requires its entrainment into the boundary layer, while rain can scavenge Hg(II) from higher altitudes more readily. We find that 55 % of the spatial variation of Hg wet deposition flux observed at the MDN sites is explained by the combined variation of precipitation and Hg(II) produced in the upper and middle troposphere. Our simulation points to a large role of Hg(II) present in the dry subtropical subsidence regions, which account for 74 % of Hg(II) at 500 hPa over the continental U.S., and more than 60 % of the surface Hg(II) over high-altitude areas of the western U.S. During the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign, the contribution of these dry regions was found to be 75 % when measured Hg(II) exceeded 250 pg m−3. Our results highlight the importance of the upper and middle troposphere as key regions for Hg(II) production and of the subtropical anticyclones as the primary conduits for the production and export of Hg(II) to the global atmosphere.

Citation: Shah, V. and Jaeglé, L.: Surface deposition of oxidized mercury dominated by production in the upper and middle troposphere, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-145, in review, 2017.
Viral Shah and Lyatt Jaeglé
Viral Shah and Lyatt Jaeglé
Viral Shah and Lyatt Jaeglé

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Short summary
We use a model of mercury chemistry and transport in the atmosphere along with mercury observations to learn that most of the oxidized mercury falling to the Earth’s surface is chemically produced at high altitudes in the atmosphere. Much of this oxidized mercury is brought down by descending air in the subtropics. Our findings imply that mercury chemistry happening in these high-altitude regions of the atmosphere is an important link between global emissions and surface deposition of mercury.
We use a model of mercury chemistry and transport in the atmosphere along with mercury...
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