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Abstract Title: Atmospheric Fate of HOHgO: the Hg(II) Product of OH-Initiated Oxidation of Hg(0)
Presenter Name: Darshi Hewa Edirappulige
Company/Institution: SUNY College of Environmental Science and Forestry
Session: Atmospheric Hg cycling: Source and Emissions
Co-Authors: Darshi Hewa Edirappulige,Camille K. Beckett,Illena J. Kirby,Theodore Dibble

Abstract Information :

It is important to understand the oxidation and reduction reactions of mercury in the atmosphere to be able to predict when and where the mercury will enter ecosystems. Also, once deposited, GOM (Gaseous Oxidized Mercury) is more readily converted into toxic methylmercury than is GEM (Gaseous Elemental Mercury). Atomic bromine had been considered as essentially the sole oxidant of Hg(0) in the gas phase. Recently, however, we found a valid mechanism for OH?-initiated oxidation of GEM. This mechanism relies on the reaction between XHg? (X=Br, OH) and O3 to make XHgO? + O2. This rapidly oxidizes Hg(I) to Hg (II) and enables OH? to initiate about 1/3 of the global net oxidation of GEM to GOM. OH?-initiated oxidation of GEM occurs primarily via this two-step process: (1) ?OH + Hg ? HOHg? (2) HOHg? + O3 ? HOHgO? + O2 This makes HOHgO? a major intermediate in mercury oxidation in the atmosphere. Consequently, we are using computational chemistry to characterize reactions of HOHgO? with atmospheric trace gases. 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 in its reactions with atmospheric trace gases. HOHgO? abstracts hydrogen from CH4 with a modest barrier (4.0 kcal mol-1) to produce Hg(OH)2 with a rate constant of ~2 x 10-14 cm^3 /(molecule s) at 298 K. This reaction and the reactions of HOHgO? with C2H4, CH2O, NO, NO2 maintain Hg in the (+2) oxidation state. By contrast, the reaction CO + 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.

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