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| 14 Feb 2025
Atmospheric Mercury: Recent advances in theoretical, computational, experimental, observational and isotopic understanding to decipher its complex redox transformations in the upper and lower atmosphere and interaction with Earth surface reservoirs
Abstract. Mercury is a volatile heavy element with no known biological function. It is present in trace amounts (on average, ~80 ppb) but is not geochemically well-blended in the Earth's crust. As a result, it sometimes occurs in extremely high concentrations (up to a few %) in certain locations. It is found along tectonic plate faults in deposits of sulfide ores (cinnabar), and it has been extensively mobilized during the Anthropocene. Mercury is currently one of the most targeted global pollutants internationally, with methylmercury compounds being particularly neurotoxic. Over 5,000 tons of mercury are released into the atmosphere annually through primary emissions and secondary re-emissions. Much of the re-emitted mercury, resulting from exchanges with surface reservoirs, is considered to be related to (legacy) human activities, as are the direct releases. Understanding the dynamics of the global Hg cycle is critical to assessing the impact of emission reductions under the UN Minamata Convention, which became legally binding in 2017. This review of atmospheric mercury focuses on the fundamental advances in field, laboratory, and theoretical studies, including six stable Hg isotope analytical methods, that have contributed fairly recently to a more mature understanding of the complexity of the atmospheric Hg cycle and its interactions with the Earth's surface ecosystem.
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RC1: 'Comment on egusphere-2024-4190', Franz Slemr, 05 Mar 2025
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The authors provide a comprehensive review of mercury reactions relevant to mercury cycling in the atmosphere: after an Introduction in Chapter 2 about the physical chemistry of elemental mercury, in Chapter 3 about mercury in the atmospheric environment, in Chapter 4 information about kinetics, thermodynamics and general chemistry, in Chapter 5 about gas-phase atmospheric Hg chemistry, in Chapter 6 about red-ox transformations in the aqueous phase, in Chapter 7 about multi-phase transformations, and in Chapter 8 about mercury isotope systematic and fractionation. The introduction provides basic information in broad outline about the atmospheric mercury cycle and the paper ends with future perspectives.
The paper is a monumental compilation of the currently available kinetic, thermodynamic, isotopic and spectroscopic data on mercury behaviour relevant for the atmosphere. It is generally well organised and written. It should be published after considering the comments below.
Because of the length of the review, a table of contents at the beginning would improve the orientation for readers looking for a specific topic.
The ever-increasing complexity of mercury behaviour in the atmosphere seems at times to obscure the fact that it is a subject, as other trace gases, to general constraints imposed by atmospheric circulation. The introduction provides a good example:
In lines 199-200 the authors report that “Hg in the stratosphere is estimated to account for about 20% of the total atmospheric Hg mass…”. No reference is given for this statement, the references at the end of the sentence relate to exchange flux between stratosphere and troposphere. This statement cannot be true. The total atmospheric mass is 5.13 x 1018 kg of which 9.06 x 1017 kg are in the stratosphere (Warneck, 1988), representing 18% of the total atmospheric mass. Even considering the uncertainties of the estimate in the above statement, 20% of total Hg burden in the stratosphere would mean the nearly same mixing ratio in the stratosphere as in the troposphere. This is not true – Slemr et al. (2018) observed always a steep Hg gradient around and above the tropopause with much lower Hg concentrations (ng m-3(STP), i.e. mixing ratios) in the stratosphere. Similar observations were made by e.g. Lyman and Jaffe (2012), Talbot et al. (2007), Radke et al. (2007). Also, it would be inconsistent with the substantial tropospheric sinks mentioned elsewhere in the introduction.
In the paragraphs, lines 193 -215 and 216-235, different lifetimes are presented and compared: the tropospheric, the chemical, the stratospheric, the lower stratospheric against surface deposition, the mid- and upper-stratospheric, and the mean atmospheric ones. This discussion needs a common denominator and/or specific qualifications. Seinfeld and Pandis (1998) define atmospheric lifetimes as
1/τ = 1/τ (reaction) + 1/τ(stratosphere) + 1/τ(ocean) + 1/τ(land) + 1/τ(wash)
What lifetime definitions are used in this paragraph, are they compatible and how do they add up to the overall atmospheric one? The definition inconsistencies in these paragraphs can be illustrated e.g. by the statement (lines 203-204) that “Based on correlations of Hg0 with N2O in the stratosphere within 4 km above the thermal tropopause, Slemr et al. (2018) provided a lifetime estimate of 74 +/- 27 yr while Lyman and Jaffe (2012) inferred a relatively short lifetime for Hg0 in intercepted descending air with stratospheric origin”. Stratospheric lifetime estimated by Slemr et al. (2018) has been derived from hundreds of CARIBIC measurements and is consistent with the definition of Seinfeld and Pandis (1998). “A relatively short Hg0 lifetime in the stratosphere of <1yr” mentioned by Lyman and Jaffe (2012) seems to be the local chemical one which cannot be compared to the lifetime according to the definition of Seinfeld and Pandis (1998).
Minor comments:
Line 58: Perhaps a reference Jiskra et al. (2018) should be added here.
Line 65: What do you mean with ”reductionist”?
Line 71: “compilation” is perhaps a better word than “tabulation”.
Line 102: Why “filtered”? The AAS method will measure elemental mercury even with some aerosols as long as there is enough light coming through. LIDAR technique shows that aerosol poses no problem. In fact, back scattering on aerosol is the basis of the LIDAR techniques.
Line 113: Awkward wording – “Since gold does not trap only Hg0 but…” would be perhaps better.
Line 121: “…the risk of artifact formation of HgII by co-sampling GOM with PBM…” – I think that no new HgII is being formed but HgII from GOM and PBM is being co-measured, later called RM.
Line 153-154: Awkward wording: “… is not recommended because it cannot be applied in multi-stage atmospheric pressure systems…” perhaps better.
Line 155: “…of ambient GOM species…” may be perhaps better.
Line 290: “Hinshelwood” instead of “Hinselwood”.
Table 1, caption: “are” instead “is”
Table 3: In reactions G24 and G25 appears BrHgIIO* - where does it come from? Is the oxidation stage of Hg consistently described? Please check the consistency of all chemical formulas, in the tables and in the text.
Line 453: “…determined by Donohue…”
Line 466: Please define what RR studies are? If that means “reaction rate” it could be replaced “kinetics studies”. Or omit it altogether – the type of studies is given by the context.
Line 538: decreases with decreasing temperature?
Line 542: “and” instead “och”
Line 574: “they” – Dibble et al or Castro Pelaez et al?
Lines 714-715: But above the O3 maximum in the stratosphere the reactions of excited Hg atom become important? See section 5.1.6. Perhaps the Section 5.1.5 should be merged with 5.1.6 to avoid misunderstanding.
Line 765: “Hg(3P)” – Hg(3P1) or Hg(3P0) or both?
Line 983: “bis-sulphite complex is thermally stable” perhaps better.
Sections 6 and 7: Interaction of HgCl2 with H2SO4 droplets would be interesting because they constitute the major aerosol particles in the stratospheric Junge layer. Are any data available? Any comment on this? Here or in the Section 9?
Line 1215: “Brunauer-Emmett-Teller” instead of “BET” perhaps better for non-specialists.
Line 1239: “aerosolized”?
Lines 2110-2111: Reliable measurement of Hg0 in wet deposition? Perhaps “…despite generally reliable measurements of Hg0 in air and HgII in wet deposition.” would be less ambiguous. These measurements may not be sufficient for verifying model studies in detail but they provide constraints with which the model results have to comply.
References: The titles of the papers are sometimes written with capital letters, sometimes without. Please homogenize.
References
Lyman, S.N., and Jaffe, D.A.: Formation and fate of oxidized mercury in the upper troposphere and lower stratosphere, Nature Geosci., 5, 114-117, 2012.
Seinfeld, J.H. and Pandis, S.N., Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley & Sons, New York, 1997, pp 1098.
Radke, R.F., Friedli, H.R., and Heikes, B.G.: Atmospheric mercury over NE Pacific during spring 2002: Gradients, residence time, upper troposphere lower stratosphere loss, and long-range transport, J. Geophys. Res., 112, D19305, doi:10.1029/2005JD005828, 2007.
Slemr, F., Weigelt, A. Ebinghaus, R., Bieser, J., Brenninkmeijer, C.A.M., Rauthe-Schöch, A., Hermann, M., Martinsson, B.G., van Velthoven, P., Bönisch, H., Neumaier, M., Zahn, A. and Ziereis, H.: Mercury distribution in the upper troposphere and lowermost stratosphere according to measurements by the IAGOS-CARIBIC observatory: 2014 - 2016, Atmos. Chem. Phys., 18, 12329-12343, 2018.
Talbot, R., Mao, H., Scheuer, E., Dibb, J, and Avery, M.: Total depletion of Hg0 in the upper troposphere – lower stratosphere, Geophys. Res. Lett., 34, L23804, doi:10.1029/2007GL031366, 2007.
Warneck, P., Chemistry of the Natural Atmosphere, Academic Press, San Diego, 1988, pp 14.
Citation: https://doi.org/10.5194/egusphere-2024-4190-RC1
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