Surface deposition of oxidized mercury dominated by production in the upper and middle troposphere

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 10 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 15 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– 20 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 25 layer, while rain can scavenge Hg(II) from higher altitudes more readily. We find that 55% of the Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-145, 2017 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 7 March 2017 c © Author(s) 2017. CC-BY 3.0 License.