Review report on the scientific article "Boundary of nighttime ozone chemical equilibrium in the mesopause region: long term evolution from 20-year satellite observations" by Kulikov et al.

The airglow measurements are used to determine atomic oxygen and atomic hydrogen, which are very difficult to measure by other methods. Number of investigations of minor chemical constituent distributions in mesopause are based on the assumption about photochemical equilibrium of ozone. Thus, solid knowledge on the spatial-temporal distributions of ozone photochemical equilibrium are desirable.

Main idea of the paper is to study the relation of ozone photochemical equilibrium boundary with O and H spatio-temporal variability basing on the 3D chemical transport. Moreover, authors propose investigation the spatio-temporal evolution of the ozone photochemical equilibrium boundary in 2002-2021 from SABER/TIMED data.

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 revision.

Comments.

1. The title does not accurately reflect the essence of what has been done in this article. One might get the impression that the equilibrium boundary was taken from satellite data. In reality, this characteristic was found using SABER data. Should be corrected, for example: "...: long-term evolution of the boundary determined (OR derived) using 20-year satellite observations".
2. I would like to note that the nighttime ozone equilibrium boundary was investigated in previous papers of the authors using 3D MLT modelling. There the criterion was also proposed, which in this work is applied to determine the altitude position of this boundary, using already the data of real measurements. It is well known that any model is an idealized representation of the real reality and, in principle, may not take into account some important features of the natural object, poorly formulated mathematically. In this regard, a very important question that needs to be clarified at least in the discussion. Can the authors present any other indications of the nighttime ozone equilibrium boundary in the SABER profiles or in the O and H profiles reconstructed from these data?
3. There are quite a large number of figures (20) with different number of panels (from 1 to 20) in the article with a relatively small volume of text. For better structuring of the article, some of the figures should be omitted or merged, for example: (4, 8, 13 and 17), (6 and 15), (7 and 16), (10, 12, 19 and 20).
4. Figures 1-3 show variations of O and H normalised to some mean daily values. It is not clear what these values are. If these are averages over the entire range of altitudes, then the figures should show known maximums of O and H, but this is not present in the figures. Apparently different daily average O and H values were used for each altitude. Please clarify this issue. Also, the figures show white spots where the normalised concentrations appear to fall below 10-6. Apparently the range of variation shown needs to be increased.
5. Lines 315-317. The authors write "Basing on daytime O and H distributions in the mesopause region obtained in Kulikov et al. (2022), we calculated O/H in summer and winter." It would be nice to provide figures, which confirm that "this ratio at middle latitudes is remarkably less than in winter".
6. I have my doubts that quite a few of the many instances in which the articles are mentioned correctly. Furthermore, there are a number of questions about the use of English expressions. Therefore I strongly recommend checking the text of the article with the help of a professional translator.