Review report on the scientific article "The boundary of nighttime ozone chemical equilibrium as an indicator of local chemistry disturbances in the polar mesopause region" by Kulikov et al.

The paper investigates variations of lower border of ozone photochemical equilibrium at nighttime conditions in MLT region at high latitudes during winter-spring period with focus on SSWs events. This knowledge is necessary for retrieving the chemical constituents based on joint observations of ozone and excited hydroxyl emission. They show that, under conditions of intense SSWs, the altitude range where the ozone equilibrium approximation works well can vary significantly depending on the phase of these events. Furthermore, this work analyses southern polar altitudes. In certain years, notable perturbations of the nighttime ozone equilibrium boundary were also identified, which, as it turned out, were unrelated to minor warmings at these latitudes.

The scientific value of this paper stems from the importance of research into the mesopause, from both a fundamental and an applied perspective. I believe that this work makes an important contribution to the study of processes in the polar mesopause and can be accepted with minor amendments based on my comments set out below.

Specific comments.

1. I fail to see why the authors felt it necessary to use data from two models, plus satellite measurements, plus reanalysis data, for this study. Can any compelling arguments be put forward for drawing on such a large number of sources of varying types?

2. If the authors nevertheless wish to include all these sources in the paper, they should compare them where possible and discuss the differences in the results based on the different data sources.

3. Figures 1-3 show time-height section, nevertheless, the x-axes are labelled as latitude.

4. Figure 1. Figure legend “WACC-X”

5. When the authors refer to ‘height’ in the article, or show various heights in the figures—including ‘z_eq’—it is unclear whether these are geometric altitudes or pseudo-altitudes (so-called ‘log-pressure altitudess’). One can only try to guess, based on the data source. This point needs to be made explicitly clear.

6. Line 225. Authors write “(geometrical altitude level z_eq)”. Then, at figures 4-5 we can see z_eq from SABER observations. But SABER shows the data on pressure levels, and corresponding altitude just approximate. How authors solve this problem and calculate geometric altitude from pressure?

7. Figures 6-9 are based on MERRA-2 reanalysis. Is it geometric altitude or log-pressure altitude?

8. During SSW events, significant pressure variations occur in the mesopause, and results expressed in isobaric coordinates may differ considerably from those expressed in geometric coordinates. I therefore advise the authors to stick to one of the two throughout the manuscript.

9. Figures 8-12. Is it MERRA-2. If - yes, it should be noted explicitly in the text, if –no, too, as well in figures legend.

10. Lines 436-437. It is well known that southern SSWs are much rarer than northern ones.

Could authors please let to know for readers, for example in Introduction, a little bit more about SSW events in Southern hemisphere? Who first identified it? Are they have identical to the Northern ones mechanism of formation? Why they are much rare? Which role plays GWs for SSWs in Southern hemisphere? If GWs play any role are they captured by MERRA-2 reanalysis?

11. Lines 443-445. “In addition, we included the temperature at the 0.1 hPa, calculated using MERRA-2 reanalysis data, as an indicator of lower mesosphere dynamics…” – could the temperature be an indicator of the dynamics?

12. Figures 13 ( as well 1, 2, 3, 4, 5, 8, 9). Dimensions should be indicated directly on the colour bars or in the figure captions.

13. Figure 13 and related text. Is the CMAM correctly reproduce SSW events? Could authors please give a reference, as well references that CMAM generally correctly reproduce dynamics at 80° N?

14. Why authors do not use WACCM-X or MERRA-2 and jump to CMAM for the only one last figure?

15. Lines 542-543. “Thus, we can assume that the boundary can be used to monitor significant disturbances in both the Ox–HOx components and the airglows generated by physicochemical processes involving them.” –may be vice versa? Because volume emissions observable parameters, and chemical constituents and consequently ozone photochemical equilibrium lower boundaries can be only calculated from these observations?

16. Could authors please explain in more details how they calculate O2(b1) and O(1S).

It seem to me they use approximations of McDade et al. (1986). But this paper is not cited.

If my assumption is true, then, it is necessary to note explicitly in the text, which of McDade parameters they use, because McDade give several sets of fitting coefficients for O2(b1) and O(1S) calculations.

17. It is necessary let to know in more details how was calculated OH*. Which assumptions and set of coefficients were utilized?

18. Lines 551-556. “As at mid- and low latitudes, the NOCE criterion clearly indicates the local position and thickness of the transition zone separating deep and weak photochemical oscillations of O and H caused by the diurnal variations in solar radiation.”

- I didn’t see anything in this article about low latitudes;

- Where exactly is the point made in this sentence demonstrated?

- How do deep photochemical oscillations differ from weak ones?

-“ caused by the diurnal variations in solar radiation” – but in this work authors are in polar night area at nighttime conditions!

19. Lines 532-533. “the NOCE boundary rapidly drops down up to 72-74 km”;

     Lines 560-561. “Immediately after such SSWs, the NOCE boundary rapidly descends to 72–74 km and can remain there for a long time”

      – but at Figure 13 we can see 76-77 km.

20. Figure 12 partly duplicates Figures 10–11. I believe that, without detracting for the paper, Figures 8–12 could be reorganized.

21. Figure 13 shows the temporal variation of the vertical profiles of O and the volume emissions during SSW 2009. It can be seen that, jointly with rising of altitude the concentration of atomic oxygen and emission rates are declining. In this context, it is worth noting the results of a recent study [Hu et al., ‘Influence of sudden stratospheric warming with elevated stratopause on the hydroxyl in the polar middle atmosphere’, Atmos. Chem. Phys., https://doi.org/10.5194/acp-25-18431-2025, 2025], in which a similar effect was detected.

Please, check other published works which show similar effect.

22. Line 149. «4.1·10−¹⁰ hPa» -> «4.1·10⁻¹⁰ hPa».

23. Seems to me that the English of the paper should be corrected with native English speaker or professional English editor.

This manuscript follows on from several papers by the first author and collaborators that discuss the vertical domain where ozone is in equilibrium. These papers have made valuable contributions, particularly for emphasizing the limitations of using photochemistry assumptions to interpret observations in the MLT. The new contributions from the present paper are modest. Basically, this work shows examples of applying the ideas developed in the previous papers to the winter polar region.

This review discusses two concerns about the manuscript. First is that there are many instances where the text lacks clarity. These are itemized under Minor Comments but the cumulative effect is that the manuscript is difficult to follow. Second is that the manuscript is long and sometimes repetitive and should be substantially shortened.

Major Comments

The manuscript is longer than it needs to be. The development and discussion of the equations for different ways of looking at NOCE can be grouped together and made more compact. Repetition from earlier papers can be minimized. The year-by-year discussion of NH variability (lines 317-432; almost four pages) does not contribute to the understanding of the results. Identification of a few patterns that repeat over the 24 years sampled would be more valuable for the reader than a blow-by-blow rundown of which variations occurred on which dates of individual years, without some focus on the general developments.

The substantial WACCM-X diurnal variations of O and H below 80 km in polar winter darkness (Figure 3) are surprising. Can you provide a mechanism beyond dismissing this as “a combined effect of atmospheric dynamics and chemical relaxation”?

Minor Comments

1. Did you run WACCM-X to generate the results shown? If yes, please provide more information about the type of run (model version? free-running or constrained? output frequency?). If not, please give a source and reference for the output and a justification for the choice of the specific year.
2. Please be clear about what is shown both in the text and, especially, in the figure captions. I.e., discussion and captions for all figures should indicate the sources for the values shown: which model or which dataset. See captions to Figures 4 and 5, for example.
3. Equation 8 is not useful. Please use symbols in the equation and define their numerical values as necessary. As written, it is hard-wired to parameter choices used by Mlynczak et al. (2013, 2018), which are not universally accepted. In particular, their high rate for quenching by O₂ in the 2018 paper could affect the overall morphology of quantities derived using this variable.
4. As a related matter, the assumption that quenching by O is unimportant is less robust in polar regions where transport leads to a strong seasonal maximum in the winter O concentration down as far as 80 km (Wang et al. 2023; see their Figure 1). The evaluation at l. 217-221 should be checked specifically for polar winter conditions.
5. Do the calculations account for a seasonal change in O₂ mixing ratio?
6. (p. 7) What is the physical meaning of VER_min?
7. The first paragraph of Section 6, along with Eq. (9) and (10), is not clear. The cited paper (Kulikov et al. 2023a) is not included in the reference list. Why does the lifetime depend on the local time and the elapsed time since nightfall? In what way does a calculation of the lifetimes lead to improved physical interpretation?
8. (l. 495-496) “altitude dependence of the reaction rates” This is misleading if one uses the customary meaning of “reaction rates” to be the k in your equations. Perhaps you could just leave this off, as in “The sharp change in the diurnal oscillations of O and H above 80 km in Figure 1 is caused by the altitude dependence of the chemical lifetimes of these components.”
9. What is the purpose of Figure 13? It shows that the location of the NOCE varies with time during dynamical events, as already shown in many figures of the paper. It also indicates that there are simultaneous perturbations in airglow in the levels above threre. The manuscript does not indicate that there is predictive capability in either direction.

Editorial Comments

• (l. 14; l. 115) “deep and weak” Does this mean deep in vertical depth and weak in magnitude? Or did you mean “strong and weak”?
• (l. 224) “the relationship at 70-75 km is inverse” What do you mean?
• (l. 253) “the lower mode” What does this mean?
• (l. 309) “travelling” -> “propagating”
• (l. 435; 442) “above” -> “poleward of”

Reference

Wang, J. C., Yue, J., Wang, W., Qian, L., Jones, M., Jr., & Wang, N. (2023). The lower thermospheric winter-to-summer meridional circulation: 2. Impact on atomic oxygen. Journal of Geophysical Research: Space Physics,128, e2023JA031684. https://doi.org/10.1029/2023JA031684