Together, Not Separately, OH and O₃ Oxidize Hg⁽⁰⁾ to Hg^(II) in the Atmosphere

Mercury, a highly toxic metal, is emitted to the atmosphere mostly as gaseous Hg⁽⁰⁾. Atmospheric Hg⁽⁰⁾ enters ecosystems largely through uptake by vegetation, while Hg^(II) largely enters ecosystems in oceans and in rainfall. Consequently, the redox chemistry of atmospheric mercury strongly influences its fate and its global biogeochemical cycling. Here we report on the oxidation and reduction of Hg^(I) (BrHg and HOHg radicals) in reactions with ozone and how the electronic structure of these Hg^(I) species affects the kinetics of these reactions. The oxidation reactions lead to YHgO· + O₂ (Y = Br and OH), while the reduction reactions produce Hg⁽⁰⁾, OY, and O₂. According to our calculations with CCSD(T), NEVPT2, and CAM-B3LYP-D3BJ, the kinetics of both oxidation reactions are very similar and much faster than their reduction counterparts. Past modeling of field data has supported the idea that OH and/or O₃ (rather than Br) dominates Hg^(II) production in the continental boundary layer. Almost all models invoking OH- and ozone-initiated oxidation of Hg⁽⁰⁾ assume that these reactions produce Hg^(II) in one step, despite the lack of plausible mechanisms. The two-step mechanism of formation of HOHg followed by its reaction with ozone helps reconcile modeling results with mechanistic insights.