Our current understanding of major chemical and physical processes affecting mercury dynamics in the atmosphere and at the air-water/terrestrial interfaces

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The predictions of atmospheric chemical models are limited by the accuracy of our understanding of the basic physical and chemical processes that underlie the models. In this work we review the current state of our knowledge of the chemical processes that transform atmospheric mercury species via gas and aqueous phase reactions and the physical processes of deposition. We concur with the conclusions of other recent reviews that our understanding of the basic chemistry that controls mercury is incomplete and the experimental data either limited or nonexistent. In spite of this recent experimental and theoretical studies of mercury reaction kinetics have clarified some issues. Observations in Polar Regions suggest that Hg0 can undergo fast oxidation in the presence of elevated levels of bromine compounds. Both experimental and theoretical studies suggest that the recombination of Hg0 with Br atoms is sufficiently fast to initiate this oxidation process. However there is a large uncertainty in the value of the rate coefficient for this recombination reaction and in the fate of the reaction product, HgBr. Most global mercury models incorporate reactions of Hg0 with OH and O3. Based on the most recent high level ab-initio calculations of the stability of HgO it appears that neither of these reactions is likely to play a significant role in mercury oxidation. The most important aqueous oxidation for Hg0 appears to be reaction with O3 however that there has only been one determination of the Hg + O3 reaction rate constant in the aqueous phase. Aqueous phase reduction of oxidized mercury via reaction with HO2 is the only significant reduction reaction in current models but now seems unlikely to be significant. Again this suggests that the chemistry controlling mercury transformation in current models requires significant modification.