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

The role of aqueous phase hydroperoxyl radical (HO2.), hydroxyl radical (.OH) 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 is an important daytime oxidant for Hg0 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 Hg0, 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 Hg0 after SO2 is depleted.