Oxidation of elemental mercury by aqueous chlorine (HOCl/OCl−): Implications for tropospheric mercury chemistry

The stoichiometry and kinetics of elemental mercury (Hg 0 ) oxidation by aqueous chlorine (HOCl/OCl − ) have been investigated. The stoichiometric ratio of Hg 0 to HOCl/OCl − is found to be 1:1, the same as the electron transfer ratio. The rate constants of the oxidation are measured in a novel fashion by using chloramine (NH 2 Cl) as the free chlorine reservoir. The rate constants at room temperature (23°∼ 25°C) for the Hg 0 ‐HOCl and Hg 0 ‐OCl − are measured to be (2.09±0.06)×10 6 and (1.99±0.05)×10 6 M −1 s −1 , respectively. Based on the solubility data of chlorine, the intrinsic Henry's law constant of chlorine is calculated to be 7.61×10 −2 M atm −1 at 25°C. Model study using the kinetic data in this investigation shows that the oxidation of Hg 0 by aqueous chlorine is an important pathway contributing dissolved divalent mercury (Hg(II)) in atmospheric water, especially at higher cloud water p H when solubility of chlorine is greatly increased and before sunrise when chlorine reaches its peak concentrations in the marine troposphere.