A new mechanism for atmospheric mercury redox chemistry: Implications for the global mercury budget

Hannah M. Horowitz, Daniel J. Jacob, Yanxu Zhang, Theodore S. Dibble, Franz Slemr, Helen M. Amos, Johan A. Schmidt, Elizabeth S. Corbitt, Eloïse A. Marais, Elsie M. Sunderland
2017 Atmospheric Chemistry and Physics Discussions  
Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg<sup>0</sup>. Oxidation to water-soluble Hg<sup>II</sup> controls Hg deposition to ecosystems. Here we implement a new mechanism for atmospheric Hg<sup>0</sup>&amp;thinsp;/&amp;thinsp;Hg<sup>II</sup> redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model
more » ... (MITgcm), enabling a global 3-D representation of atmosphere-ocean Hg<sup>0</sup>&amp;thinsp;/&amp;thinsp;Hg<sup>II</sup> cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg<sup>0</sup> oxidant, and that second-stage HgBr oxidation is mainly by the NO<sub>2</sub> and HO<sub>2</sub> radicals. The resulting lifetime of tropospheric Hg<sup>0</sup> against oxidation is 2.7 months, shorter than in previous models. Fast Hg<sup>II</sup> atmospheric reduction must occur in order to match the ~&amp;thinsp;6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM&amp;thinsp;≡&amp;thinsp;Hg<sup>0</sup>&amp;thinsp;+&amp;thinsp;Hg<sup>II</sup>(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg<sup>II</sup>-organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because southern hemisphere Hg mainly originates from oceanic emissions rather than transport from the northern hemisphere. The model reproduces the observed seasonal TGM variation at northern mid-latitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but does not reproduce the lack of seasonality observed at southern hemisphere marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM-ozone relationship indicative of fast Hg<sup>0</sup> oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China, including the maximum over the US Gulf Coast driven by HgBr oxidation by NO<sub>2</sub> and HO<sub>2</sub>. Low Hg wet deposition observed over rural China is attributed to fast Hg<sup>II</sup> reduction in the presence of high organic aerosol concentrations. We find that 80&amp;thinsp;% of global Hg<sup>II</sup> deposition takes place over the oceans, reflecting the marine origin of Br and low concentrations of marine organics for Hg<sup>II</sup> reduction, and most of HO<sub>2</sub> and NO<sub>2</sub> for second-stage HgBr oxidation.
doi:10.5194/acp-2016-1165 fatcat:b7xd73tgufgqlih7xsmpemhu4m