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 Hg^II by co-sampling GOM with PBM…” – I think that no  new Hg^II is being formed but Hg^II 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 BrHg^IIO* - 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.

Although quite a number of atmospheric mercury review articles have been published over the last decade, there is still a need in a comprehensive review of the fundamental research of the atmospheric Hg cycle at a molecular level. This manuscript aims to fill this gap and it does so rather successfully. 

The review begins with an overall introduction of mercury in the environment (Ch. 1), including the threats it poses, existing challenges, and a list of previously published review articles followed by a summary of topics that are covered and not covered by the present manuscript (it would be helpful if those previous reviews are either sorted chronographically or better clustered according to their major focus points). 

The review proper begins with a brief chapter on the physical chemistry of elemental mercury (Ch. 2) followed by chapter 3 on mercury in the context of the atmospheric environment, including atmospheric measurements of mercury species and stability of atmospheric Hg(0). Kinetics, thermodynamics, and general chemistry are introduced in Ch. 4, including general fundamentals of kinetics and thermodynamics of atmospheric gas-phase reactions, surface kinetics, and aqueous redox equilibria. The redox equilibria are then expanded towards chemical properties of aqueous Hg(I) and Hg(II), comprehensive chemical equilibria data for mercury complexes with inorganic and organic ligands are presented and the speciation of Hg(II) in atmospheric waters in the context of these equilibria is discussed. Next, the manuscript presents a detailed account of the gas-phase atmospheric chemistry of mercury (Ch. 5), including not only the reactions of ground state Hg(0) in the troposphere, but also the reactions of electronically excited Hg(0) under lower stratosphere conditions. In addition to the detailed chemistry of inorganic Hg species, the chemistry of organic species, dimethylmercury and monomethylmercury is described to some extent. Following redox chemistry in the gas phase comes chapter 6 that describes red-ox transformations of mercury in the aqueous phase, including both inorganic Hg species and organic mercury. Chapter 7 on multi-phase transformations connects the processes occurring in gaseous and aqueous/particle phases. This chapter covers topics ranging from gas-particle partitioning and reactive gas uptake to reduction on surfaces, dark oxidation of Hg(0) accelerated by freeze-concentration, surface-catalyzed reduction of Hg(II) in aqueous solution, and photoreduction in precipitation, cloud and fog. Chapter 8 is very extensive and it focuses on mercury isotopic fractionation. The presence of this chapter sets this review apart from many previous review articles on atmospheric mercury. Mercury isotope fractionation measurements, despite the challenges caused by the low concentrations of atmospheric mercury, allow to obtain unique additional information about the origins of mercury present in various compartments, and associated mechanisms. This chapter begins by defining conventional mass-dependent and mass-independent fractionation, describes isotopic characteristics of mercury, and then shows how isotope fractionation can be applied to obtain additional mechanistic details during such processes as gas-phase oxidation, aqueous-phase red-ox transformation, complexation, sorption and surface-catalyzed reduction, and also air-surface Hg(0) gas exchange. The manuscript ends with a chapter describing future perspectives (this last chapter is a bit of hit-and-miss and can be improved as described in detail below). 

Overall, this is a very thorough and comprehensive review article that will undoubtedly help the scientists working on many different aspects of environmental mercury research. It can serve both as an initial reading for young researchers entering the field, such as PhD students, and also as a comprehensive collection of factual information, such as the extensive tables with gas-phase and aqueous reaction mechanisms, for seasoned mercury researchers. I started using the manuscript as a source of information for my research as I have been reading it. I believe that the manuscript should be published after several of the major and a number of smaller issues are resolved, as described below. 

The title is a bit awkward. Consider rephrasing, such as “Recent advances in understanding of complex redox transformations of mercury in the upper and lower atmosphere and its interaction with Earth surface reservoirs based on theoretical, computational, experimental, observational and isotopic studies”

It will be beneficial if the authors shape this manuscript more as a critical review. For instance, among the gas-phase kinetic studies, the studies by Donohue (2008) and Byun et al. (2010) are not really peers. I am not denigrating the effort made in the latter work, but it is based on a very complex and dirty system where rate constants had to be derive from rather indirect measurements by making a number of assumption. On the contrary, the former work is based on the absolute kinetic measurements, which have been verified using the same apparatus with known reactions (such as by measuring the rate of recombination of Cl atoms). Hence, it would be beneficial for the reader to know which work provides more accurate data by design. There are a number of other places where the authors might have indicated the confidence degree associated with different presented data. I understand that this is not always an easy task. 

Regarding the increased stability of HgO at lower temperatures in the stratosphere. I suggest adding “at lower temperatures and pressures”, as this reaction is collision activated. I should note that according to my estimate, the lifetime is still about 1 ms at 250 K and 0.1 atm, which is very short, making this species irrelevant.   

The “Gas to nucleation” subtitle is awkward and the message of this subsection is unclear. Do you imply that mercury oxidation products have a sufficiently low vapor pressure and sufficiently high concentration to nucleate new particles or contribute to their growth in the atmosphere? I highly doubt this proposition. We know that even elemental mercury can homogeneously nucleate new particles (J. Martens, H. Uchtmann, and F. Hensel. J. Phys. Chem. 1987, 91, 2489-2492), but this process requires extremely high supersaturations and extremely high concentrations, which are not achievable in the atmosphere. I disagree with the statement that oxidized mercury species have properties making them suitable for nucleation, e.g., not all of them ionic solids (e.g., mercuric halides) contrary to the stated. Recent studies on the homogeneous nucleation in the atmosphere, such as the experiments conducted in the CLOUD chamber with detailed measurements of the composition of both trace gases and nucleating clusters, have shown the high sensitivity of the nucleation rate to the presence of trace foreign species. For instance, amines and HOMs accelerate the nucleation of sulfuric acid by many orders of magnitude when present at a hundredth of a pptv. There is a high probability that the nucleation in the studies quoted in this subsection was due to the presence of various foreign trace species (or due to the use of very high concentrations of mercury species that have no relevance to the atmosphere). Please revise. 

Chapter 8 on isotope effects makes this review stand apart from other atmospheric mercury reviews. It has a very large amount of important information, but I found this chapter difficult to read. One reason is that I have almost no working experience with the isotopic measurements. Considering the very large number of different parameters introduced and used, this makes reading and understanding the material a nightmare. On the one hand, this is a good opportunity to inform a typical atmospheric mercury scientist, not familiar with isotopic measurements, about the existence and power of the isotope measurements tool. On the other hand, this chapter could stand as an independent review article. At a minimum, I urge the authors to add a table that summarizes the different parameters (both deltas, beta, alpha, etc.) along with their definitions, meaning, and usage. Perhaps, it would be beneficial for one of the co-authors with an intermediate knowledge of isotope fractionation (who is not an expert), to read this chapter and edit it a bit to make it more digestible for the readers who are less familiar with the isotope field, e.g., by making sure that every time a value of a parameter is mentioned, the meaning of this value is fully interpreted.  

Chapter 9 on future perspectives is a hit-and-miss. Only the second half of page 69 delivers the information a reader would expect from this concluding chapter, but the rest of this chapter is a detailed review of existing studies (with some references to challenges and possible future work). I suggest that the authors rethink the structure and contents of this chapter. It must summarize the achievements, identify the remaining challenges and gaps, propose future work, and when possible, suggest ways of addressing the challenges. Having a concluding sentence or brief paragraph would be nice, too. 

Minor

L24: Consider replacing “impact” with “threat”

L47: Consider replacing “However, it” with “Although this route was discarded, ozone”

L48: Replace “more unstable” with “less stable” 

L59-63: Sort out by year, and possibly cluster by major focus of each review. 

L87: “as the electron approaches” – the electron on which orbit?

L240: Replace “at which it occurs” with “at which the reaction occurs”

Ibid: “The rate of a reaction is determined by 240 the interaction between kinetics, a rate process, and thermodynamics that describes the energetics of the process” – This sentence is very imprecise. “kinetics” of what? For instance, for a gas phase reaction the rate depends on the number of collisions between the reactants and thermodynamics of their interactions (the change in entropy and enthalpy upon passing through the transition state).  

L290: I believe that instead of Pankow, 2007 the reference should be to Mao, et al 2021.

L303: “These positive potentials indicate”

L356-357: Replace M with Hg in chemicals formulas, as you focus solely on one metal – mercury – in this review

L370: Replace “it is mainly in hexavalent form” with “the sulfur it is mainly present in hexavalent form”  

L371-373: “Whose presence in AOM is not universal”, “which is not relevant in this context”, 

“questionable to apply speciation by geochemical equilibrium modeling”.  The meaning is not clear. Why is it questionable? In which context? For what specific reason?

L377: Replace “speciation is controlled” with “speciation is represented”

L380: What does “It” refer to?

L385: What do you mean by “less constrained”?

Table 1: Elemental mercury is shown as a ligand in the first row. Does it indeed complex with Hg2+? Is it just complexation or a redox reaction? 

L401: Clarify what you mean by “in the electronic ground state of atmospheric importance”. Perhaps “in the electronic ground state” would be sufficient. 

Table 2: Are photolysis rates calculated using the global annual average photon flux? There is an extra O2 among the products in Reactions G22b and G22c. 

L542: “och”?

L559: Replace “obsolete” with a more appropriate word

L580: Clarify what you mean by “speciated”

L584-586: Simplify the sentence, e.g., “HgX can neither abstract hydrogen atoms from volatile organic compounds nor add to double bonds”.

L889: Rewrite, e.g., “Although atmospheric generally more stable than Hg(I) species, Hg(II) species are still labile and the atmospheric pool”

L592: “indicated”

L596: I suggest to clarify that BrHgY are molecules while XHgO are radicals, e.g., “Mixed compounds such as BrHgIIY molecules (Y= ONO, OOH, OH, OCl, OBr etc.) and XHgIIO● radicals (X = Br, OH)”

L607: “computer-assisted theoretical calculations” – be more specific

L609: Did you mean that “Photolysis of BrHgONO forms NO and BrHgO”?

L614: What do tilde means in O~O~H?

L636: Consider revising “The enthalpy of thermal decay of HgO is weakly endothermic”, as for diatomic molecules thermal dissociation is always endothermic. Did you mean “only weakly endothermic”?

L646: “marginal” – did you mean “scarce”?

L647: Rephrase “computational calculations”

Figure 6: add a citation to the source of this figure.

Figure 7: It looks pretty but highly overloaded and messy, making it hard to read. Consider removing molecular structures, keeping only the formulas. Spectra look aesthetically nice, but also complicate the figure. Aim at striking a balance between prettiness and legibility. The same applies to several following figures. In this figure caption, I would not put NO2, BrO and OH as abundant radicals, as their concentrations are vastly different. 

L739: “elemental oxygen”

L760: Instead of asterisk provide the specific state of O2

L775: “less energetic than the reactants” – awkward, consider revising. 

L789: Is it the excited HgO?

L819: “automatically measured” is confusing. Is “automatically” important here? 

Figure 10: The pathway resulting in polymerization of HgO is inconsistent with its extremely short lifetime and extremely low concentration under typical atmospheric conditions, even in the stratosphere. 

L861: I highly doubt that the MMHg+ cation can be volatilized. Did you mean a species with its counter-anion, e.g., MMHgCl?  

Table 4: W17 appears to show a wrong reaction mechanism

L1025: Is this a dark reaction? What do you mean by “divergent”? Divergent in what sense? 

Rxn10: flip the second structure horizontally

L1081: “extremely strongly”

L1130-1131: This sentence is bizarre. Clarify why that literature is irrelevant to the atmosphere. 

L1181: Define “FF”

L1229: “its freezing point” – water freezing point? 

L1240: As written, this contradict the information given in the previous paragraph (L1235) that adsorption reduces the energy required for photoreduction. Consider rephrasing. 

L1251: A second order rate constant is given for a photolytic process. Why? Is this reaction limited by the rate of the complex formation? Does it depend on light intensity and wavelength? Please clarify. 

L1273: Replace “means” with “tweezer”

L1363: “Parenthesized”?

L1204 and L1407: Consider sticking to a single term (NFS vs NVF). 

L1581: “Antarctica are interpreted”