August 2006
Global lifetime of elemental mercury against oxidation by
atomic bromine in the free troposphere

We calculate the global mean atmospheric lifetime of elemental mercury (Hg(0)) against oxidation by atomic bromine (Br) in the troposphere by combining recent kinetic data for the Hg-Br system with modeled global concentrations of tropospheric Br. We obtain a lifetime of 0.5-1.7 years based on the range of kinetic data, implying that oxidation of Hg(0) by Br is a major, and possibly dominant, global sink for Hg(0). Most of the oxidation takes place in the middle and upper troposphere, where Br concentrations are high and where cold temperatures suppress thermal decomposition of the HgBr intermediate. This oxidation mechanism is consistent with mercury observations, including in particular high gaseous Hg(II) concentrations in Antarctic summer. Better free-tropospheric measurements of bromine radicals and further kinetic study of the Hg-Br system are essential to more accurately assess the importance of Br as a global oxidant of atmospheric Hg(0).

The figure shows the lifetime (in days) of atmospheric Hg(0) against oxidation to Hg(II) by two-step recombination with Br atoms and OH (forming HgBr2 or HgBrOH), using rate constants from Donohoue et al. [2006] and Goodsite et al. [2004] with the bromine distribution predicted by Yang et al. [2005]. The corresponding global-mean tropospheric lifetime of Hg(0) is 510 days, assuming a uniform Hg(0) mixing ratio. Other published rate constants yield qualitatively similar distributions of Hg(0) lifetime.

BACKGROUND: Mercury is present in the atmosphere principally in its elemental form, Hg(0), which can be transported globally, as indicated by the uniformity of its atmospheric concentration in tropical and temperate regions. Hg(0) is eventually oxidized to divalent Hg(II), which may be reduced back to Hg(0) but also partitions into atmospheric water and reacts with surfaces, leading to mercury deposition and accumulation in ecosystems. Some deposited mercury is subsequently reduced and re-emitted as Hg(0). Unlike other heavy metals, mercury transits among surface reservoirs primarily through atmospheric fluxes. Therefore, understanding the atmospheric redox chemistry of mercury is critical to determining source-receptor relationships of this toxic element.

For more information see Holmes et al. [2006].