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Mercury (Hg) is emitted to the atmosphere mainly asvolatile elemental Hg0. Oxidation to water-soluble HgII plays amajor role in Hg deposition to ecosystems. Here, we implement a newmechanism for atmospheric Hg0 ∕ HgII redox chemistry in theGEOS-Chem global model and examine the implications for the globalatmospheric Hg budget and deposition patterns. Our simulation includes a newcoupling of GEOS-Chem to an ocean general circulation model (MITgcm),enabling a global 3-D representation of atmosphere–ocean Hg0 ∕ HgIIcycling. We find that atomic bromine (Br) of marine organobromine origin isthe main atmospheric Hg0 oxidant and that second-stage HgBr oxidationis mainly by the NO2 and HO2 radicals. The resulting chemicallifetime of tropospheric Hg0 against oxidation is 2.7 months, shorterthan in previous models. Fast HgII atmospheric reduction must occurin order to match the  ∼  6-month lifetime of Hg againstdeposition implied by the observed atmospheric variability of total gaseousmercury (TGM  ≡  Hg0 + HgII(g)). We implement this reductionin GEOS-Chem as photolysis of aqueous-phase HgII–organic complexes inaerosols and clouds, resulting in a TGM lifetime of 5.2 months againstdeposition and matching both mean observed TGM and its variability. Modelsensitivity analysis shows that the interhemispheric gradient of TGM,previously used to infer a longer Hg lifetime against deposition, ismisleading because Southern Hemisphere Hg mainly originates from oceanicemissions rather than transport from the Northern Hemisphere. The modelreproduces the observed seasonal TGM variation at northern midlatitudes(maximum in February, minimum in September) driven by chemistry and oceanicevasion, but it does not reproduce the lack of seasonality observed at southernhemispheric marine sites. Aircraft observations in the lowermoststratosphere show a strong TGM–ozone relationship indicative of fastHg0 oxidation, but we show that this relationship provides only a weaktest of Hg chemistry because it is also influenced by mixing. The modelreproduces observed Hg wet deposition fluxes over North America, Europe, andChina with little bias (0–30 %). It reproduces qualitatively the observedmaximum in US deposition around the Gulf of Mexico, reflecting a combinationof deep convection and availability of NO2 and HO2 radicals forsecond-stage HgBr oxidation. However, the magnitude of this maximum isunderestimated. The relatively low observed Hg wet deposition over ruralChina is attributed to fast HgII reduction in the presence of highorganic aerosol concentrations. We find that 80 % of HgII depositionis to the global oceans, reflecting the marine origin of Br and lowconcentrations of organic aerosols for HgII reduction. Most of thatdeposition takes place to the tropical oceans due to the availability ofHO2 and NO2 for second-stage HgBr oxidation

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