the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Bromine and Iodine in Atmospheric Mercury Oxidation
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.
Status: open (until 14 Mar 2025)
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CC1: 'Comment on egusphere-2025-116', Henrik Skov, 31 Jan 2025
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There is a wrong citation:
ref. 27. Goodsite, M. E., Skov, H., Christensen, J. H., Heidam, N. Z., Jensen, B., and W˚ahlin, P.: Fate of elemental mercury in the Arctic during atmospheric mercury depletion episodes and the load of atmospheric mercury to the Arctic. Antarctic springtime depletion of atmospheric mercury, Environmental Science & Technology, 38, 2373–2382, 2004.
Should be:
Skov, H. Christensen, J. Goodsite, M.E. Heidam, N.Z. Jensen, B. Wåhlin, P. and Geernaert, G. (2004) “The fate of elemental mercury in Arctic during atmospheric mercury depletion episodes and the load of atmospheric mercury to Arctic” ES & T. vol. 38, 2373-2382.
Citation: https://doi.org/10.5194/egusphere-2025-116-CC1 -
CC2: 'Reply on CC1', Svend L. Bager, 01 Feb 2025
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Thanks for letting us know. We will make sure the correction is included.
Citation: https://doi.org/10.5194/egusphere-2025-116-CC2
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CC2: 'Reply on CC1', Svend L. Bager, 01 Feb 2025
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RC1: 'Comment on egusphere-2025-116', Anonymous Referee #1, 12 Feb 2025
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Bager et al. investigate the reactions between Hg(I) (primarily HgBr, but also HgI) and various atmospheric species (I, Br, BrO, ClO, IO, NO₂, and HO₂) leading to the formation of Hg(II) or Hg(0) using computational methods. They propose that these reactions could play a dominant role in mercury oxidation during Arctic Mercury Depletion Events (AMDEs) in spring.
However, both experimental and theoretical studies (Shat et al., 2021; Gómez Martín et al., 2023) have firmly established that the reaction between HgBr and O₃ is the primary pathway to Hg(II), with a measured rate constant of k(HgBr + O₃) = 7.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹.
During extreme AMDEs, ozone mixing ratios may drop below 1 ppb. Even at such low levels, given that the reaction between HgBr and O₃ is orders of magnitude faster than the reactions considered in this study, these alternative pathways would have negligible impact.
For example, assuming 10 ppt of Br and 1 ppb of O₃:
Species
Mixing Ratio (ppt)
Concentration (molecule cm⁻³)
k (cm³ molecule⁻¹ s⁻¹)
k' (s⁻¹)
Br
10
2.50 × 10⁸
8.00 × 10⁻¹³
2.00 × 10⁻⁴
O₃
1000
2.50 × 10¹⁰
7.50 × 10⁻¹¹
1.88
This indicates that the removal rate of HgBr by O₃ is still four orders of magnitude faster than its removal by Br.
To reach a point where these rates are comparable:
Species
Mixing Ratio (ppt)
Concentration (molecule cm⁻³)
k (cm³ molecule⁻¹ s⁻¹)
k' (s⁻¹)
Br
10
2.50 × 10⁸
8.00 × 10⁻¹³
2.00 × 10⁻⁴
O₃
0.1
2.50 × 10⁶
7.50 × 10⁻¹¹
1.88 × 10⁻⁴
Thus, ozone mixing ratios would need to drop to sub-ppt levels—an extremely unlikely scenario, even in the most extreme AMDEs.
Rather than questioning the computational methods used in this study, my concern is the atmospheric relevance of its conclusions. The authors appear to have overlooked key aspects of mercury atmospheric chemistry and have overstated the implications of their results.
Since ACP is a journal focused on atmospheric chemistry and physics, this paper does not meet the necessary standards for publication in this venu
Citation: https://doi.org/10.5194/egusphere-2025-116-RC1 -
RC2: 'Comment on egusphere-2025-116', Anonymous Referee #2, 15 Feb 2025
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This paper describes high level electronic structure calculations to explore the oxidation pathways for elemental Hg(0) by reactions with Br and I atoms. The objective is to assess the importance of I relative to Br in the atmospheric oxidation of Hg to Hg(II). The ab initio calculations are carried out at a high level of theory, and rate coefficients are then estimated using variational transition rate theory. Overall, the study makes some potentially useful contributions to understanding Hg oxidation.
However, there is at least one major problem to address. Table VIII compares results from the present study with previous work. Although the temperature and pressure is not stated (!), one assumes it is 298 K and 1 bar. The reaction HgBr + Br --> HgBr2 is a recombination reaction. According to the authors, this has a second-order rate coefficient of 8.8e-13 cm3 molecule-1 s-1, and hence a third-order rate coefficient of ~ 4e-32 cm6 molecule-2 s-1. This seems extremely slow for a reaction that is exothermic by 71 kcal mol-1 (Table I) and involves exclusively very heavy atoms, so must have a high density of rovibrational states in the HgBr2 adduct. Strangely there is no discussion as to why this reaction is so unexpectedly slow (by around 2 orders of magnitude). Inspection of Table VIII shows an earlier estimate from Goodsite et al. that is close to the HgBr + Br capture rate, which is closer to what would be expected.
Can the authors demonstrate that their TST approach yields rate coefficients for any of these Hg reactions that agree well with experiment? There do not appear to be any examples in Table VIII.
Other points include:
The references are in the format of a physical chemistry journal, not ACP!
All calculated energies are quoted to 0.01 kcal mol-1 which implies a level of accuracy that is certainly not warranted.
Page 3 discusses observations of IO in the Arctic, but fails to cite the first observations made (at Hudson Bay): Mahajan, A. S., M. Shaw, H. Oetjen, K. E. Hornsby, L. J. Carpenter, L. Kaleschke, X. Tian-Kunze, J. D. Lee, S. J. Moller, P. Edwards, R. Commane, T. Ingham, D. E. Heard, and J. M. C. Plane (2010), Evidence of reactive iodine chemistry in the Arctic boundary layer, J. Geophys. Res. 115, art. no.: D20303.
Page 4: the original mechanism for Br oxidation was formulated by Goodsite et al (not Goodsite).
Page 5: in the computational details, there is no mention of how spin-orbit coupling is included (particularly important for I)
Page 6: no mention of the temperature at which G is calculated (presumably 298 K)
Citation: https://doi.org/10.5194/egusphere-2025-116-RC2
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