Atmospheric Fate of HOHgO, the Hg(II) Product of OH-initiated Oxidation of Hg(0)

Mercury is released into the atmosphere as gaseous elemental mercury (GEM). GEM is the most abundant form of mercury in the atmosphere. However, GEM deposition is somewhat slow in comparison to deposition of gaseous oxidized mercury (GOM). Hence, it is important to understand the oxidation and reduction reactions of mercury in the atmosphere to understand when and where the mercury will enter ecosystems. Also, once deposited, GOM is more readily converted than GEM into toxic methylmercury. OH-initiated oxidation of GEM occurs primarily via this two-step process: (1) OH + Hg HOHg (2) HOHg + O3 HOHgO + O2 HOHg falls apart rapidly, but due to the high abundance of ozone and the high rate constant for reaction (2), OH is effective in initiating oxidation of Hg(0). HOHgO, the product of reaction (2), has never been characterized in laboratory studies, let alone in fieldwork. Consequently, we are using computational chemistry to characterize reactions of HOHgO in the atmosphere. In this study, we report on the reactions of HOHgO with CH4, C2H4, and CH2O as models for HOHgO reactions with a range of volatile organic compounds. We also present results for HOHgO reactions with NO, NO2, and CO. We exploit a quantum chemistry approach that has worked well for similar problems in mercury chemistry. Our results can be summarized by stating that HOHgO largely mimics the reactivity of OH radicals. More specifically, abstracts hydrogen from CH4 with a modest barrier (4.0 kcal mol-1) to produce Hg(OH)2. HOHgO reacts readily with C2H4 by addition to a carbon atom to produce HOHgOCH2CH2. The addition of HOHgO to NO and NO2 appeared to be barrierless, so these will have high rate constants. HOHgO reacts with CH2O via two pathways to produce either Hg(OH)2 or HOHgOCH2O. All these reactions of HOHgO maintain Hg in the (+2) oxidation state. By contrast, the reaction between CO and HOHgO reduces mercury to the (+1) state by making HOHg and CO2. This work will help improve the modeling of atmospheric mercury and may shed light on the identity of GOM compounds.