Homogeneous gas-phase reaction of Hg° with H2O2, 03, CH3I, AND (CH3)2S: Implications for atmospheric Hg cycling

In order to better define the cycling of mercury in the global troposphere, we have studied over the past four years the homogeneous gas-phase reaction of elemental mercury (Hg0) with hydrogen peroxide (H2O2), 03, methyl iodide (CH3I), and dimethyl sulfide ((CH3)2S or DMS) in FEP Teflon® reactors. For the first time, we report laboratory measurements of the reaction rate constant of Hg0 with H202. Results which were at or below our detection limit suggest this reaction must have a rate constant of <8.5 x 10−19 cm3 molecule−1 s−1. With 8.5 x 10−19 cm3 molecule−1 s−1 as an upper limit rate constant, mean H202 concentrations of 1.0 ppb would yield a global average Hg0 residence time of 1.5 years, similar to that calculated by reaction with 30 ppb 03. Methyl iodide or (CH3)2S are not geochemically important in the cycling of gaseous Hg0. New research should focus on both the homogeneous and heterogeneous reactions of Hg0 with Cl2 and other reactive CI species, OH, HO2 and organic peroxyradicals (RO2).