A theoretical study of the oxidation of Hg0 to HgBr2 in the troposphere

The oxidation of elemental mercury (Hg-0) to the divalent gaseous mercury dibromide (HgBr2) has been proposed to account for the removal of Hg-0 during depletion events in the springtime Arctic. The mechanism of this process is explored in this paper by theoretical calculations of the relevant rate coefficients. Rice-Ramsberger-Kassel-Marcus (RRKM) theory, together with ab initio quantum calculations where required, are used to estimate the following: recombination rate coefficients of Hg with Br, I, and O; the thermal dissociation rate coefficient of HgBr; and the recombination rate coefficients of HgBr with Br, I, OH, and O-2. A mechanism based on the initial recombination of Hg with Br, followed by the addition of a second radical (Br, I, or OH) in competition with thermal dissociation of HgBr, is able to account for the observed rate of Hg-0 removal, both in Arctic depletion events and at lower latitudes.