Review of ACP MS egusphere-2024-614 (Kulikov et al.)

General Comments

This manuscript examines the question of chemical equilibrium conditions for OH and HO2 in the mesosphere and lower thermosphere atmospheric regions (MLT). A 3-D chemical transport model is used to examine the validity of the commonly used approximation of steady-state chemical equilibrium conditions in this region as a function of altitude, latitude, and time of year. The authors consider the relative importance of the different chemical reactions involved in the production and loss of OH and HO2 to simplify the formulas of the established equilibrium criteria. These criteria are also applied to a recently published analysis of observations by the SABER/TIMED instrument.

A substantial amount of effort appears to have been devoted in the data analysis and preparation of this manuscript. In general, it is well written and explained. The topic of OH and HO2 in the MLT region is important for upper atmospheric science and there are several aspects that are still not well understood. Retrievals of atmospheric composition from remote-sensing observations in this atmospheric region still have large uncertainties and may contain inconsistencies. Examining the underlying assumptions can provide helpful insights. In that sense, the manuscript is commendable for systematically examining fundamental details related to these problems.

I am not clear on what significant new knowledge is gained. Some of the conclusions seem unsubstantiated and even unrealistic, especially the main conclusion that “the simultaneous application of OH and HO2 equilibrium to the SABER data (O3, volume emission rates at 2.0 and 1.6 um) together with the criteria (16) and (24) to control this equilibrium validity is going to retrieve all unknown HOx – Ox components (O, H, OH, and HO2), extending the altitude range of retrieval below 80 km and without external information.”

Specific Comments



* The manuscript uses solar elevation angles greater than 95° to discriminate between twilight and nighttime conditions. This is insufficient and would include twilight conditions during which steady-state nighttime conditions may not have been established yet. The convention for astronomical nighttime is 108°, and various studies have used values in the range 100-110°. In comparison, the Panka et al. (2021) SABER data set this manuscript considers indicates a nighttime cutoff angle of 105° (this is according to the available data for 2009 at the website https://saber.gats-inc.com, the Panka et al. (2021) paper does not explicitly call this angle).



* The criteria for equilibrium validity conditions require a difference of less than or equal to 10% between the species concentration calculated by the chemical transport model and that estimated using instantaneous local equilibrium between production and loss. The manuscript provides no discussion of uncertainties in the model, the observations, and several rate constants at the low MLT temperatures. To have confidence that when the equilibrium criterion is not met there is a meaningful deviation from equilibrium, all the other uncertainties must be significantly smaller than the considered range of departure from equilibrium. This does not appear to be the case given all the above uncertainties. This is a critical point that is fundamental to this research effort. The comparison with the Panka et al.(2021) data set further demonstrates this point.



* Regarding the examination of SABER remote-sensing observations and the modeling analysis by Panka et al.(2021), consideration of the relevant uncertainties would provide context and assess the results. There are substantial uncertainties in all the retrieved SABER minor constituents, [H], [O], and [O3] being the most relevant in this case. These are at best within approximately 20% and the relative errors are often largest at the lower and higher ends of the studied altitudes. With all this in mind, it seems that the data points shown in Figure 12 may well be outliers with limited significance at best. According to the manuscript (lines 406-407), “…the local height position of the OH equilibrium boundary can rise up to 87 km.” The Panka et al. data set comprises 263,432 measurements and the number of points above 80 km is no more than approximately 2% (based on about five hundred points on each of the panels of Fig. 12). The overwhelming majority of those measurements are not consistent an equilibrium height above 80 km.



* This manuscript is for a technical note. The number of figures seems rather excessive (11 figures with 155 panels). The text does a good job describing the main point of the figures. It would seem practical to include most figures in a supplementary section.



* Author Contributions: Co-authorship for the last three authors who “contributed to reviewing the article” seems somewhat unusual. Also, please note that the last initials should be AF and not AM (Line 434).



Technical Corrections

Line 19: …conditions describe the… …the top to some lower borders…

Line 21: …criteria reproduce well…

Line 22: …allow to extend previously proposed…

Line 35: …components: in particular, trace gases with short lifetimes relative to…

Line 47: …approximation has been used…

Line 79: …this approximation’s validity

Line 80: …there is no assessment of…

Line 85: …maps of the components of interest.

Line 92: …apply this approach to the analysis…

Line 100: …all excited and ground states…

Line 128: …To remove the transition regions…

Line 146: …Finally, we obtained…

Line 147: …the poorly chemical evolution…???

Line 152: …from the total sink of n.

Line 169: …Figure 1 plots…

Line 173: …see the presence of…

Line 206: …Figure 3 shows…

Line 215: …Figure 4 presents…

Line 231: …up to a 95% contribution to the equilibrium concentration.

Line 243: …major in the lower part of…

Line 256: …are different from those…

Line 261: …As a result, the…

Lines 263: The punctuation (comma, in this case) appears to be incorporated in the equation. Similar comment for other equations. A few equations do not have punctuation. This issue could be resolved at the copy-editing stage.

Line 265: Figure 9 shows …

Line 271: …reproduces many features of…

Line 276: Figure 10 plots…

Line 288: Criteria for HO2 and…

Line 290: Let us determine…

Line 295: Let us find…

Line 297: “analyzed analytically” ???

Line 300: …rewritten in the following…

Line 316: “first of all” seems redundant

Line 317: …As previously mentioned, near and above the OH…

Line 331: There is a formatting problem with the equation.

Line 335: There is a formatting problem with the equation.

Line 339: …the red line...

Line 347: We will now discuss the obtained results…

Line 348: As noted, Figs. 9-10…

Line 349: Recently, Kulikov et al. (2023) found such a feature…

Line 355: “At middle,” ???

Line 360: “From simplified Eqs. (17) and (25), it follows that…

Line 370: As a result,…

Line 372: There is a formatting problem with the equation.

Line 376: …including this additional term…

Line 378: As noted in the Introduction…

Line 379: …constitute a useful tool for retrieval of these components…

Line 384: …including improvement of existing retrieval…

Line 387: …valid for excited states…

Line 389: …lifetime is determined by the reaction…

Line 398: …the constant rates…

Line 406: …in accordance with… OR …according to…???

Line 439: Is the Acknowledgements section missing or is it the same as Financial Support?

Line 442: …and State assignment No. 0729…

Line 469: …a model study…

Line 511: SABER data…

Line 638: …averaged relative contribution…

Line 642: …averaged relative contribution…

Line 646: …averaged relative contribution…

Line 650: …averaged relative contribution…

Line 654: …averaged relative contribution…

Line 659: …averaged relative contribution…

Review of “Technical Note: Nighttime OH and HO₂ chemical equilibria in the mesosphere – lower thermosphere” by Kulikov et al.

Summary

This paper is the latest installment in a series of papers by these authors and collaborators on the equilibrium assumptions relevant to the night Ox-HOx chemistry of the MLT. The analysis is oriented toward the use of equilibrium assumptions in the development of remote sensing algorithms. As in the previous investigations, the work is thorough and is carefully documented. The technique draws heavily on their earlier work assessing ozone equilibrium but this paper is specifically focused on the variable HOx species HO₂ and OH. In the results, the authors present and explain the latitude x season x pressure bounds of when and where equilibrium is achieved.

Major comment

The approach describes, evaluates, and compares several expressions to determine equilibrium. The standard for evaluation is how well a particular equilibrium value computed from their 3-D chemical transport model agrees with the actual concentrations simulated in the same model. Winds used for transport and temperatures for chemical rate coefficients are based on temporal smoothing of once-daily values from a middle atmosphere dynamics model. My concern is that this approach excludes the transport and large temperature swings associated with tides. In the tropical MLT, vertical winds associated with the migrating diurnal tide can be quite substantial and are a leading transport process. Temperature variations of 10-20 K in a few hours are seen during some seasons. As a result of this omission, the actual variations of species concentrations that go into the analyses may be more variable than those simulated, which would affect the standard deviation criteria in Eq. (1). This omission may have led to a diagnosis of equilibrium that is more optimistic than the reality for the equatorial region.

It seems to me that the only way to quantify the importance of tides would be to perform a simulation with input dynamical fields taken more frequently. Without this, it is necessary to add some sentences that point out this omission and its possible implications for the results.

Other comments

(Figure captions) In some cases, the captions and text do not explain the figures sufficiently. In particular, the captions to Figures 1 and 9 are almost identical; the exception being a subscript “sh” in one of the terms in the Figure 9 caption. The situation for Figures 2 and 10 is similar. Please add words giving more information so the differences are more obvious.

(line 43-46) Can you provide references or more detail to support the idea that limited measurements of trace species can be used to retrieve temperature, reaction rates, chemical sources, etc? This comes across as wishful thinking that might not hold up because of multiple uncertainties in the components of the photochemical system.

(line 78-81) The two sentences (beginning “Secondly, there is no detailed numerical evaluation ..”) are confusing. This paragraph is about night ozone, which you examined in earlier papers. Have you switched the discussion to HOx without informing the reader or are you raising doubts about your 2019 and 2023 papers on ozone equilibrium?

(line 101) “this approach is tested” It seems that you test the equilibrium timescale for ground-state OH but not for the vibrationally excited states that are important components in the model of Panka et al. Please revise to make this distinction clear.

(Section 6) It was difficult to get oriented toward this analysis. Please add a sentence or more at the beginning of the section referring the reader to Eq (2) and also please reiterate the key takeaway from the discussion describing the difference between the terms “lifetime” and “local time scale”. Other readers may, like me, be unfamiliar with the distinction between these concepts and their role in your analysis.

Is the analysis of ozone equilibrium in Section 7 (lines 348-362) relevant to the present paper? If so, please explain the connection. If this paragraph remains, please provide some transition words to let the reader know when you are switching your focus from ozone to HOx.

(line 391) “one can see from Figure 8” Should this be Figure 7 or Figures 7 and 8?

Editorial comments

Throughout: I’m not sure of the journal’s style guidelines but, for me as a reader, it would be really helpful if you indicated that something is an equation when the number appears in the text. For example (line 141), replace “from 1” with “from Eq. (1)” or something similar.

(line 51) “of a critical parameters” -> “of critical parameters”

(line 66) Sentence beginning “First …” is not clear. Is this what you mean? “First, there are no clear criteria indicating the conditions under which the equilibrium conditions are satisfied?”

(line 71) “is too short varying” -> “varies”

(line 83) “to correct search of” -> “to correctly search for”

(line 100) “exited” -> “excited”

(line 128) “we took into account the local time” -> “we use only local times”

(line 147) “the poorly chemical evolution” It is not clear what this means.

(line 170) “dashed area” -> “stippling” Also this sentence would fit better in the figure caption than the main text.

(line 173) “present” -> “presence”

(line 263) apostrophe in denominator

(line 288) “criterions” -> “criteria”

(line 290, 295) “Let” is an awkward word here. How about replace the first instance (line 290) with “First, we” and the second (line 295) with “Then”

(line 310, 343) “in zero approximation” do you mean “in the zeroth order approximation”?

(line 351) “whichwas” -> “which was”

(line 352-355) This sentence is too long and convoluted and the point being made is not clear. What are “nighttime evolution times”? In the next sentence, what does “At middle” refer to?

(line 360) “It is follows” -> “It follows”

(line 426) “is going to retrieve” Do you mean “allows the retrieval of”?

Review report on the scientific article "Technical Note: Nighttime OH and HO₂ chemical equilibria in the mesosphere – lower thermosphere" by Kulikov et al.

Main idea of the paper is to study the photochemical equilibrium and its boundary for OH and HO₂. Moreover, authors derived the criterion of photochemical equilibrium for both constituents. This can be utilized for retrieving of these minor species, which are hardly measured by other techniques, based on common volume observations of ozone and volume emissions of excited hydroxyl. In addition, the current work contains potentially useful information on the most important chemical reactions that determine the balance of OH and HO₂ over a wide range of space and time.

This is an important work showing how one should detect and use the equilibrium conditions of chemically active minor species with small lifetimes. Despite the fact that the assumption on photochemical equilibrium has been used for quite a long time to solve various problems related with chemical processes in the atmosphere, this question has only recently begun to be given due attention. The use of analytical criteria allows one to somehow control the possible error due to the violation of equilibrium. For example, as was shown in previous works of the authors (i.e., Kulikov et al., JGR, 2018, https://doi.org/10.1002/2017JD026717) for nighttime ozone, its concentration can deviate from the equilibrium values by several orders of magnitude.

The research is scientifically valuable. The theoretical part is presented in the paper very convincingly. The methods and approaches are correct. On my opinion this work bring deep insights on chemistry of MLT region and knowledge essential for measurements. On my opinion it should be accepted in ACP after minor revisions.

Comments.

1. By the meaning, construction, and titles the work is “Technical Note”, however, the number of figures seems excessive for this type of publication. I recommend the authors to consider reducing them. On my opinion, it is possible to remove a number of panels in Figs. 3-8 and merge the remaining panels. However, the complete figures for major reactions with all 12 panels for each month could be presented in Supporting Information if authors assume them important to a potential reader. In addition, Figure 11 could be moved in Supporting Information and Figure 12 removed at all. This modification may help better focus this work.

2. OH equilibrium boundary retrieving based on the data of Panka et al. (2021), presented in the Discussion, does not give much. The authors themselves say that most of these data do not allow us to determine the local altitude position of the boundary of the OH equilibrium region because they are cut off at 80 km. In addition, the authors have previously raised the question of the correctness of the Panka et al. approach for O retrieving below 85 km (that is based on proportionality atomic oxygen concentration to the ratio between the volume emission rates of OH* measured at 2.05 and 1.6 μm), see the discussion to the Kulikov et al. (2023, https://doi.org/10.5194/acp-23-14593-2023). It is therefore possible that the data of Panka et al. (2021) are in principle unsuitable for determining the OH equilibrium boundary. Although it is worth noting that even the small number of points presented in Fig. 12 show a seasonal and latitudinal dependence similar to the model curves in Fig. 10. However, I would suggest that this section be deleted and addressed in a separate paper along with the claimed reduction of O, H, OH, and HO₂ from SABER/TIMED data or some other data.

3. Lines 152-156. It seems to me that for the sake of clarity, it is necessary to specify where this statement comes from, and if we need accuracy not 10% but, for example, 1%, what would the criterion look like?

4. Lines 227-229. I think that in Supporting Information it would be appropriate to show the interesting features noted for the reactions H+O₃→OH+O₂ and HO₂+O→OH+O₂ at the 100-130 km altitudes.

5. Lines 424-427. The authors should add a couple of sentences clarifying their claim: "The simultaneous application of OH and HO₂ equilibrium conditions to the SABER data (O₃, volume emission rates at 2.0 and 1.6 μm) together with the criteria (16) and (24) to control this equilibrium validity is going to retrieve all unknown HOx - Ox components (O, H, OH, and HO₂), extending the altitude range of retrieval below 80 km and without external information."

6. I recommend to check the text of the work with the help of a professional editor, as I am not sure about the correctness of wording of some sentences.