Aqueous phase reactions of mercury with free radicals and chlorine: Implications for atmospheric mercury chemistry

The role of aqueous phase hydroperoxyl radical (HO2.), hydroxy] radical ((OH)-O-.) and chlorine (HOCl/OCl-) in the redox cycle of atmospheric mercury is investigated. The contributions of the above species to the aqueous phase Hg(II) concentration under various conditions are evaluated by a chemical kinetic model simulating the chemistry in atmospheric droplets. Based on the model results, (OH)-O-. is an important daytime oxidant for Hg-0 and can account for up to 25 % of the total oxidation rate in the aqueous phase under the model conditions in this study. In the nighttime marine atmosphere, aqueous chlorine can be the most predominant oxidant for Hg-0, contributing up to 90 % of the total oxidation rate in the aqueous phase. Increasing pH from 4.0 to 4.3 and decreasing [Cl-] from 1.0 to 0.5 mM increase [Hg(II)] in the droplets by 50 % and 26 %, respectively. Compared to aqueous SO2, HO2. can reduce Hg(II) at a significant rate. In addition, HO2.. is the only reductant balancing all the oxidation processes of Hg-0 after SO2 is depleted.