Homogeneous gas-phase reaction of Hg0 with H2O2, O3, 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 (HgO) with hydrogen peroxide (H2O2), O-3, methyl iodide (CH3I), and dimethyl sulfide ((CH3)(2)S or DMS) in FEP Teflon(R) reactors. For the first time, we report laboratory measurements of the reaction rate constant of HgO with H2O2. Results which were at or below our detection limit suggest this reaction must have a rate constant of <8.5 x 10(-19) cm(3) molecule(-1) s(-1). With 8.5 x 10(-19) cm(3) molecule(-1) s(-1) as an upper limit rate constant, mean H2O2 concentrations of 1.0 ppb would yield a global average Hg-0 residence time of 1.5 years, similar to that calculated by reaction with 30 ppb O-3. Methyl iodide or (CH3)(2)S are not geochemically important in the cycling of gaseous HgO. New research should focus on both the homogeneous and heterogeneous reactions of HgO with Cl-2 and other reactive CI species, OH, HO2, and organic peroxyradicals (RO2). (C) 1998 Elsevier Science Ltd. All rights reserved.