Atmospheric mercury chemistry: Sensitivity of global model simulations to chemical reactions

Abstract

The effect of new mercury (Hg) chemistry information on Hg atmospheric concentrations is investigated in a systematic manner with a global chemical transport model, taking into account current uncertainties in Hg emission and removal rates. The reactions of interest include the gas‐phase oxidation of Hg(0) by O3, the gas‐phase oxidation of Hg(0) by OH, the aqueous‐phase reduction of Hg(II) by HO2 radicals, a hypothetical gas‐phase reduction of Hg(II) by SO2, and a hypothetical pseudo‐first‐order gas‐phase reduction of Hg(II). The new kinetics of the oxidation of Hg(0) by O3 is fast and would require balancing by a commensurate reduction reaction pathway that has not been identified; it may include some heterogeneous component and should be seen as an upper limit for atmospheric applications. Eliminating the gas‐phase oxidation of Hg(0) by both O3 and OH does not lead to realistic Hg(0) concentrations even after eliminating the aqueous‐phase reduction of Hg(II) by HO2 and having a greater dry deposition rate of Hg(0). Thus gas‐phase oxidation of Hg(0) by oxidants such as O3 and/or OH is required to reproduce global Hg(0) concentration patterns. The reduction of Hg(II) by HO2 (or a reaction with a similar overall rate) is needed to balance the oxidation of Hg(0) by OH and O3 but is not needed if the gas‐phase oxidation of Hg(0) by OH is eliminated. The reduction of Hg(II) in power plant plumes can be represented by a reaction of Hg(II) with SO2; such a reaction is consistent with the global cycling of Hg. However, a first‐order reaction for Hg(II) reduction in power plant plumes is not consistent with our current understanding of the atmospheric Hg chemistry. Additional laboratory studies are recommended to address the remaining uncertainties in the atmospheric chemistry of Hg.