The potential influence of iodine-containing compounds on the chemistry of the troposphere in the polar spring. II. Mercury depletion

Simulations of atomic mercury depletion episodes in the polar spring are now based largely on Br2 and Br-atom initiated chemistry. Chlorine is believed to contribute little to the observed depletion. The role, if any, that the presence of iodine compounds play in Hg-atom depletion is unknown at present. The theoretically predicted instability of the HgI species suggests that I-atom reactions with mercury may be an unimportant loss process. However, iodine atoms react rapidly with ozone to develop IO radicals that interact with BrO radicals to enhance Br- and I-atom concentrations, so an indirect influence of iodine compounds on Hg removal might be expected. Computer simulations are described in this study that test this hypothesis using the homogeneous portion of the chemistry of the mercury depletion in the troposphere. Conditions are chosen equivalent to the 1300–1400 h on a clear day (17 March) at the location of Barrow, Alaska (Atmos. Environ. 37 (2003) 4467). Small amounts of reactive trace gases, representative of the Arctic spring, are present initially with typical background levels of Hg (0.24 ppt) and 50 ppb of O3. The simulations show that gaseous atomic mercury depletion in typical Br2 and BrCl mixtures can be enhanced significantly by the presence of small amounts of iodine-containing compounds (I2, CH2I2, CH2IBr, CH2ICl, IBr, and ICl). The major initial product of the possible mercury reactions is HgBr. The subsequent coupling reactions of this species with Br, BrO, Cl, ClO, I, IO, and OH radicals are expected to lead to a variety of reactive gaseous mercury-containing products.