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

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Horowitz, Hannah M. ; Jacob, Daniel J. ; Zhang, Yanxu ; Dibble, Theodore S. ; Slemr, Franz ; Amos, Helen M. ; Schmidt, Johan A. ; Corbitt, Elizabeth S. ; Marais, Eloïse A. ; Sunderland, Elsie M. (2017)

Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg<sup>0</sup>. Oxidation to water-soluble Hg<sup>II</sup> plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg<sup>0</sup> ∕ 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 (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg<sup>0</sup> ∕ 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 chemical 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  ∼  6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM  ≡  Hg<sup>0</sup> + 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 midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric 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 with little bias (0–30 %). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO<sub>2</sub> and HO<sub>2</sub> radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg<sup>II</sup> reduction in the presence of high organic aerosol concentrations. We find that 80 % of Hg<sup>II</sup> deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg<sup>II</sup> reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO<sub>2</sub> and NO<sub>2</sub> for second-stage HgBr oxidation.
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