Preprints
https://doi.org/10.48550/arXiv.2401.10053
https://doi.org/10.48550/arXiv.2401.10053
31 Jan 2025
 | 31 Jan 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Bromine and Iodine in Atmospheric Mercury Oxidation

Svend L. Bager, Luna Zamok, Stephan P. A. Sauer, and Matthew S. Johnson

Abstract. We investigate 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 focus on 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 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 both 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 cm3 molecule−1 s−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 cm3 molecule−1 s−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.

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Svend L. Bager, Luna Zamok, Stephan P. A. Sauer, and Matthew S. Johnson

Status: open (until 14 Mar 2025)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2025-116', Henrik Skov, 31 Jan 2025 reply
    • CC2: 'Reply on CC1', Svend L. Bager, 01 Feb 2025 reply
Svend L. Bager, Luna Zamok, Stephan P. A. Sauer, and Matthew S. Johnson
Svend L. Bager, Luna Zamok, Stephan P. A. Sauer, and Matthew S. Johnson

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Short summary
In this work we have studied by means of quantum chemical calculations at the CCSD(T) or CASPT2 level the kinetics of reactions of both bromine and iodine compounds with Hg, which are discussed to lead to the atmospheric oxidation of Hg. The particular interest is in the question whether the reactions with iodine compounds are as fast as the corresponding reactions with bromine compounds and therefore could also contribute to mercury depletion events.
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