Subtropical subsidence and surface deposition of oxidized mercury produced in the free 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) 10 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). We evaluate our two-year simulation (2013–2014) against observations of Hg(II) wet deposition as well as surface and free tropospheric observations of Hg(II), finding reasonable agreement. 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 15 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. The influence of directly emitted Hg(II) near 20 emissions sources is likely higher, but cannot be quantified by our coarse-resolution global model (2° latitude × 2.5° longitude). 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 25 into the boundary layer, while rain can scavenge Hg(II) from higher altitudes more readily. We find that