It is imperative to understand the redox chemistry of mercury in the atmosphere to be able to predict when and where the mercury will enter ecosystems. Previously, atomic bromine was considered as almost the sole oxidants of Hg(0) in the gas phase. Recently, however, we found a mechanism by which OH-initiated oxidation of Hg(0) can contribute almost as much as bromine, despite the propensity of HOHg● to dissociate. This oxidation first oxidizes Hg(0) to Hg(I) and then to Hg(II): OH + Hg → HOHg● + O3 k1([M], T) HOHg● + O3 → HOHgO● + O2 k2(T) The above mechanism relies on the high atmospheric abundance of O3 and a large rate constant for k2 (~5 × 10-11 cm3 molecule-1 s-1) to oxidize HOHg● to HOHgO● in competition with HOHg● dissociation. No experimental data exists on the Hg(II) species, HOHgO●, formed in these reactions. Computations previously showed that it is too stable to thermally decompose. We use computational chemistry to characterize its reactions with traces gases, exploiting a quantum chemistry approach that has worked well for similar problems in mercury chemistry. In summary, HOHgO● largely mimics the reactivity of OH● in reactions with NO, NO2, CH4, C2H4, HCHO, and CO. The rate constant for the reaction, HOHgO● + CH4 → Hg(OH)2 + ●CH3 was found as ~2.5 x 10-14 cm3 molecule-1 s-1 at 298 K. Reactions of HOHgO● with NOx, alkanes, alkenes, and aldehydes maintain mercury as Hg(II). By contrast, reaction of HOHgO● with CO and O3 reduces mercury to Hg(I): HOHgO● + CO → HOHg● + CO2 HOHg● + O3 → HOHg● + 2 O2 These results will help to improve the modeling of atmospheric mercury and may shed light on the identity of gaseous oxidized mercury compounds.

Atmospheric fate of HOHgO: The Hg(II) product of OH-initiated oxidation of Hg(0)

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