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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-1165
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
17 Jan 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
A new mechanism for atmospheric mercury redox chemistry: Implications for the global mercury budget
Hannah M. Horowitz1, Daniel J. Jacob1,2, Yanxu Zhang2, Theodore S. Dibble3, Franz Slemr4, Helen M. Amos2, Johan A. Schmidt2,5, Elizabeth S. Corbitt1, Eloïse A. Marais2, and Elsie M. Sunderland2,6 1Department of Earth & Planetary Sciences, Harvard University, Cambridge, MA, USA
2Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
3Chemistry Department, State University of New York-Environmental Science and Forestry, Syracuse, NY, USA
4Max-Planck-Institute for Chemistry (MPI-C), Department of Atmospheric Chemistry, Mainz, Germany
5Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen O, Denmark
6Department of Environmental Health, Harvard T. H. Chan School of Public Health, Boston, MA, USA
Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII controls Hg deposition to ecosystems. Here we implement a new mechanism for atmospheric Hg0 / HgII 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 Hg0 / HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant, and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII 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 ≡ Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII-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 Hg0 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 NO2 and HO2. Low Hg wet deposition observed over rural China is attributed to fast HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of global HgII deposition takes place over the oceans, reflecting the marine origin of Br and low concentrations of marine organics for HgII reduction, and most of HO2 and NO2 for second-stage HgBr oxidation.

Citation: Horowitz, H. M., Jacob, D. J., Zhang, Y., Dibble, T. S., Slemr, F., Amos, H. M., Schmidt, J. A., Corbitt, E. S., Marais, E. A., and Sunderland, E. M.: A new mechanism for atmospheric mercury redox chemistry: Implications for the global mercury budget, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1165, in review, 2017.
Hannah M. Horowitz et al.
Hannah M. Horowitz et al.
Hannah M. Horowitz et al.

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Short summary
Mercury is a toxic, global pollutant released into the air from human activities like coal burning. Chemical reactions in air determine how far mercury is transported before it is deposited to the environment. We use a 3-D atmospheric model to evaluate a new set of chemical reactions and its effects on mercury deposition. We find it is consistent with observations and leads to increased deposition to oceans, especially in the tropics. This may have implications for mercury accumulation in fish.
Mercury is a toxic, global pollutant released into the air from human activities like coal...
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