Subtropical subsidence and surface deposition of oxidized mercury produced in the free troposphere

Abstract

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). We evaluate our 2-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 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 US, South America, South Africa, and Australia is produced in the upper and middle troposphere, whereas 26-66 % of surface Hg(II) over the eastern US, Europe, and East Asia, and South Asia is directly emitted. The influence of directly emitted Hg(II) near emission 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 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 Mercury Deposition Network 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 the dry subtropical subsidence regions. Hg(II) present in these regions accounts for 74 % of Hg(II) at 500 hPa over the continental US and more than 60 % of the surface Hg(II) over high-altitude areas of the western US. Globally, it accounts for 78 % of the tropospheric Hg(II) mass and 61 % of the total Hg(II) deposition. During the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign, the contribution of Hg(II) from the dry subtropical regions was found to be 75 % when measured Hg(II) exceeded 250 pg m −3 . Hg(II) produced in the upper and middle troposphere subsides in the anticyclones, where the dry conditions inhibit the loss of Hg(II). Our results highlight the importance the subtropical anticyclones as the primary conduits for t...

Spatial Patterns and Temporal Changes in Atmospheric-Mercury Deposition for the Midwestern USA, 2001–2016

“…This analysis suggests that local and regional sources, rather than exclusively continental or global Hg emissions likely contributed to the extreme episodes and at least in part, to the spatial patterns of precipitation-Hg deposition in the study area. Other research attributed substantial contributions from local and regional Hg emissions sources to precipitation-Hg deposition in the Midwestern USA [19,20,54,55]. According to Holmes et al [56] and Kaulfus et al [57], warm weather, convective thunderstorms are believed to increase Hg deposition and this may be the case for many of the extreme episodes examined in this study.…”

“…According to Holmes et al [56] and Kaulfus et al [57], warm weather, convective thunderstorms are believed to increase Hg deposition and this may be the case for many of the extreme episodes examined in this study. Such storms can incorporate additional GOM formed in the troposphere from GEM originating at local and regional sources [55], in addition to effective below-cloud and in-cloud scavenging of GOM, as described by Lynam et al [58]. …”

Understanding factors influencing the detection of mercury policies in modelled Laurentian Great Lakes wet deposition

“…Finally, Gratz et al 37 propose that interannual variability in local meteorology-particularly precipitation amount and type-can mask the influence of emissions in wet deposition concentration at a remote northeastern site. Shah et al 41 , using a modelling approach, reach similar conclusions on the contribution of precipitation to variability in wet deposition, while also highlighting the importance of meteorological factors that affect the production and export of divalent mercury to free tropospheric air, like subtropical anticyclones. Mao et al 42 also point to the contribution of large-scale circulation patterns, such as the North Atlantic Oscillation, on wet deposition trends in the Adirondack region.…”

“…Interannual variability in meteorology-including in temperature, precipitation volume, and precipitation type-can impact mercury chemistry and transport, with implications for wet deposition. 37,41 We simulate "meteorological years" 2005-2012 using GEOS-5 data (i.e., historical variability), while holding the "emissions year" constant at 2005, resulting in an identical emissions trajectory as in the "Policy Only" case (i.e. emissions only change due to policy).…”

“…These include: uncertainties in both magnitude and speciation of emissions inventories; 8 decreasing influence of local/regional sources given increasing global background concentration of atmospheric mercury, driven by emissions growth in Asia; 32 and meteorological and climatological variability. 37,41,42 Zhang et al 8 find better agreement between modelled and observed twenty-year (1990-2010) trends in elemental mercury and mercury wet deposition in North America and Europe after revising emissions inventories to take into account decreasing emissions from commercial products and artisanal and small scale gold mining, and changes in flue gas speciation due to adoption of air pollution control devices. Weiss-Penzias et al 32 , based on their interpretation of spatial patterns in observed wet deposition and concentration patterns, suggest that the recent positive trends in the central and western US may be due increases in the transpacific transport of mercury in tropospheric air masses, which have larger influences over these regions.…”