Bromine and Iodine in Atmospheric Mercury Oxidation

Abstract

We investigate the atmospheric oxidation of mercury Hg(0) by halogens, initiated by Br and I to yield Hg(I), and continued by I, Br, BrO, ClO, IO, NO2 and HO2 to yield Hg(II) or Hg(0), using computational methods with a focus on the creation of data for determining the impact of rising iodine levels. We calculate reaction enthalpies and Gibbs free energies using the Coupled Cluster singlets, doublets, and perturbative triplets method (CCSD(T)) with the ma-def2-TZVP basis set and effective core potential to account for relativistic effects. Additionally, we investigate the reaction kinetics using variational transition state theory based on geometric scans of bond dissociations at the CASPT2/ma-def2-TZVP level. We compare the results obtained from the CASPT2 and CCSD(T) methods to help define the uncertainty. Our results provide insights into the mechanisms of these reactions, and the data produced get us closer to determining iodine's impact on mercury depletion events and on the atmosphere as a whole. The reaction *HgBr + Br* -> HgBr2 was found to be twice as fast as HgI* + I* -> HgI2, with reaction rate coefficients of 8.8x10-13 and 4.2x10-13 cm3molecule-1s-1 respectively. The BrHg* + BrO* -> BrHgOBr reaction was about 7.2 times faster than the *HgI + IO* -> IHgOI reaction with their rates being 3.3x10-14 and 4.6x10-15 cm3molecule-1s-1 respectively. We investigate the Hg*XOY (X and Y being halogen) complexes. From the reactions investigated including iodine, the reaction with the most plausible chance of impacting the mercury lifetime in the atmosphere is HgI* + I* -> HgI2.