Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere Without Violating Physical Laws

In 2005, Calvert and Lindberg wrote that the use of laboratory-derived rate constants for OH + Hg(0) “…to determine the extent of Hg removal by OH in the troposphere will greatly over-estimate the importance of Hg removal by this reaction.” The HOHg• intermediate formed from OH + Hg will mostly fall apart in the atmosphere before it can react. By contrast, in laboratory experiments, Calvert and Lindberg expected HOHg• to react with radicals (whose concentrations are much higher than in the atmosphere). Yet almost all models of oxidation of Hg(0) ignore the argument of Calvert and Lindberg. We present a way for modelers to include the OH + Hg reaction while accounting quantitatively for the dissociation of HOHg•. We use high levels of quantum chemistry to establish the HO-Hg bond energy as 11.0 kcal/mole, and calculate the equilibrium constant for OH + Hg = HOHg•. Using the measured rate constant for association of OH with Hg, we determine the rate constant for HOHg• dissociation. Theory is also used to de...