Mechanisms of mercury removal by O3 and OH in the atmosphere

The mechanisms of the reactions of gaseous Hg atoms with O3 and OH radical are evaluated from current kinetic and enthalpy data. The reaction, O3+Hg→HgO+O2, is considered to be an unlikely pathway for atmospheric conditions. Considerations given here suggest that the reaction may occur with initial formation of a metastable HgO3 molecule that in laboratory experiments is the source of the HgO product observed to accumulate on the walls of the reactor (HgO3→HgO(s)+O2). Laboratory studies of the gas phase reaction, Hg+OH→HgOH (2), have been reported using relative rate measurements initiated by photodissociation of an organic nitrite in mixtures of Hg vapor with NO, air and various reference hydrocarbons. Computer simulations of this reaction system suggest that the use of reactive reference gases (e.g., cyclohexane) leads to the generation of significant ozone in these NOx–RH–air mixtures, and the resulting O3–Hg reaction can result in an over-estimate of the rate of reaction (2). Also the apparent rate coefficients for reaction (2) are highly dependent on the assumed rate coefficients of its competitive reactions of dissociation in HgOH→Hg+OH (3), and association of HgOH molecule with other free radicals present in the system: HgOH+X→XHgOH (4); X=OH, HO2, RO, RO2, NO, NO2. Reaction (4) competes successfully with HgOH decomposition for the laboratory conditions employed, and the kinetic measurements relate to the rate determining reaction, Hg+OH→HgOH in this case. However, the use of these laboratory measurements of k2 to determine the extent of Hg removal by OH in the troposphere will greatly over-estimate the importance of Hg removal by this reaction.