A Theoretical Study of the Oxidation of Hg⁰to HgBr₂in the Troposphere

Abstract

The oxidation of elemental mercury (Hg0) to the divalent gaseous mercury dibromide (HgBr2) has been proposed to account for the removal of Hg0 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 O2. 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 Hg0 removal, both in Arctic depletion events and at lower latitudes.

Recent Advances in Atmospheric Chemistry of Mercury

“…Previous laboratory studies reported apparent rate coefficients in the range of (3.6 ± 0.9) × 10 −13 to (3.2 ± 0.3) × 10 −12 cm 3 molec −1 s −1 , at 1 atm and 298 K [19,20,22]. Theoretical studies estimated the rate constants to be from 9.8 × 10 −13 to 2.1 × 10 −12 cm 3 molec −1 s −1 , at 1 atm and 298 K [3,21,23]. Theoretical and experimental rate constants for mercury oxidation by Br and Cl atoms at 298 K have previously been compared in detail in Table A.2 by Subir et al [18].…”

Global lifetime of elemental mercury against oxidation by atomic bromine in the free troposphere

“…Laboratory kinetic studies imply that the global mean lifetime of Hg 0 is 120 -210 days against oxidation by OH [Sommar et al, 2001;Pal and Ariya, 2004a] and 60-1500 days against oxidation by O 3 [Hall, 1995;Pal and Ariya, 2004b]. However, in light of the expected rapid thermal dissociation of HgOH [Goodsite et al, 2004], Calvert and Lindberg [2005] concluded that oxidation of Hg 0 by OH is much slower than reported by the above studies and is insignificant under atmospheric conditions. Atmospheric observations constrain the residence time of total atmospheric mercury (Hg 0 + Hg(II)) to 0.5-2 years [Schroeder and Munthe, 1998], which places an upper limit on the lifetime of Hg 0 against oxidation (depending on competition between reduction and deposition of Hg(II)).…”

“…[4] Goodsite et al [2004] developed a homogeneous mechanism for Hg-Br chemistry in the troposphere based on theoretical kinetic calculations, and showed that gasphase oxidation of Hg 0 by Br atoms could explain mercury depletion events (MDEs) in the Arctic springtime boundary layer. They suggested that this mechanism would be important more generally in the marine boundary layer and on the global scale.…”

“…Lin et al [2006] suggested that Hg-Br chemistry is also significant in the upper troposphere. We present here a quantitative analysis of the global lifetime of Hg 0 against oxidation by tropospheric Br by combining the mechanism of Goodsite et al [2004] with Br concentrations from a global 3-D simulation of tropospheric bromine chemistry [Yang et al, 2005] as well as updated kinetic data. We find that oxidation by Br in the middle and upper troposphere could be an important sink for Hg 0 , and that the mechanism yields an atmospheric lifetime of Hg 0 consistent with observational constraints.…”

“…Homogeneous oxidation of Hg 0 by BrO is endothermic and has a large energy barrier, making its atmospheric relevance unlikely [Balabanov and Peterson, 2003;Tossell, 2003]. However, oxidation by Br atoms is fast [Calvert and Lindberg, 2004;Goodsite et al, 2004]. Rapid chemical cycling between BrO and Br through BrO photolysis, selfreaction, and reaction with NO, balanced by Br oxidation by O 3 , maintains Br:BrO molar ratios of 0.01 -2 in the daytime troposphere [Platt and Janssen, 1995;Yang et al, 2005].…”