Review of "The MIPAS global climatology of BrONO 2 2002–2012: a test for stratospheric bromine chemistry"

by Hoepfner and colleagues.

This is a very nice paper describing a well-executed study capitalizing on a valuable new dataset.  I am very happy to recommend this manuscript for publication in ACP, and have a few minor suggestions for improvements detailed below.

My only broad comment relates to the discussion of uncertainty in the MIPAS BrONO2 observations as it relates to model discrepancies.  This is just a suggestion, but presumably the approach used to quantify uncertainty in MIPAS BrONO2 measurements has been similarly applied to other MIPAS products, for which a larger set of validation/correlative observations exist.  For example, if the MIPAS ozone, N2O, CH4, etc. observations are shown to agree with other observations to a degree consistent with the estimated uncertainty in the MIPAS products, does that not speak to the robustness of the uncertainty budget approach?  I recognize that, to coin the authors' phrasing, such an argument does not constitute an "unambiguous" assessment, but it could be an additional point to bolster the authors' arguments that the discrepancies are unlikely to be dominated by measurement uncertainties.

Specific comments.  Most of these are simply wording suggestions.

Line 5: "... into sunlight *observations* and observations in the dark..."

Line 15: insert "upper stratospheric" before "BrONO2"?

Line 18: "...the lower stratosphere, a simulated production of BrONO2 that is too low during day as well as strongly..."

Line 19: "additionally" -> "in addition"

Line 24: wonder if "smallest" might be better than "lowest" here.

Line 34 and others: The reactions have the molecules in italics, but Equation 1 (page 24) has them in upright font.  It would be nice to be consistent (probably more of an issue for the copy editor/typesetter).  Similarly, would be good to be consistent on whether the y in Br_y is in italics or not.

Line 36: Not being a kineticist, I was unclear whether some of the numbers quoted on this page for reaction rate uncertainties should have units.  From the discussion later in the paper, it is clear that these numbers are indeed "factors".  Might it be good to say something like "(i.e. 20% uncertainty)" or something like that for this first one, to make that clearer.  Again, feel free to ignore this if what you have is the widely accepted practice.

Line 49: I think "Finally" might be better than "At last"

Line 64: "contributing" -> "contribute"

Line 66: comma needed after "changes" I think.

Line 68: Put commas before and after "in consequence"

Line 81: "years" -> "year"

Line 95: "retrieval" -> "retrievals".  Also, "has" -> "have"

Line 99: "shortly" -> "briefly"

Line 103: comma needed after averaging.

Figure 1 caption: I suggest "...from four 3-day periods, two during P1 (top) and two during P2 (bottom), for both dark (left) and sunlit (right) conditions"

Line 182: "following" -> "subsequent years".  Also, "until" -> "through"

Line 183-184: I suggest: "The coverage at lower altitudes is determined by the lower limit of 15 km, chosen to confine the retrievals to the stratosphere, and by the presence of high-altitude clouds and the scan pattern of MIPAS (which is mainly a factor in the tropics)." Or something like that (move the parenthetical point at the end earlier if it only applies to the clouds).

Line 189: Insert "a" between "as" and "supply"

Line 190: "at PSC particles" -> "on PSC surfaces"? (unless the reactions take place inside the PSC particles, in which case "in PSCs" might be simpler)

Line 237: insert "of BrONO2" before "might".  Also ".... might be the emission scenario used for organic bromine species, taken from Warwick..."

Line 242: "at" -> "on"? (see above though)

Line 266/267: move "quite well" to the end of the sentence.

Line 273: Perhaps change "those altitudes" to "the 30 km region" to be that bit clearer.  Also, comma needed after "errors"

Line 274: Comma needed after "negative"

Line 278/279: Suggest you move "still" from the start of the sentence to before "appeared"

Line 280:  Suggest comma after "simulations"

Line 285: Suggest: "The test the sensitivity of modeled BrONO2 to the production..."

Line 286: "which" -> "that"

Line 304: "extend" -> "extent"

Line 368: "are" -> "is"

Line 373/374: Suggest "a too efficient" -> "an overly efficient".

Line 374: Also, comma needed after "model"

Line 399: Suggest "In case" be changed to "If it is the case that".  Also "they" -> "these inconsistencies".

Line 481: Funny spacing in NOy

Line 561: Funny spacing in Bry

(Note, I didn't to a comprehensive check of the references, it's just that those two caught my eye).

The paper describes the first satellite-derived stratospheric BrONO2 global climatology based on MIPAS-Envisat infrared spectra. Most notably, the BrONO2 dataset has a well-resolved vertical, latitudinal and temporal resolution, that allows for a comprehensive understanding of the BrONO2 seasonality as well as the different BrONO2 abundance between day- and night-time conditions. The methodology clearly describes the uncertainties and errors associated with the satellite retrieval, as well as the dominant chemical processes that affect BrONO2 formation and destruction in the stratosphere. The satellite climatology is analyzed by means of a comprehensive comparison with the EMAC model, which presents an overall good performance in reproducing the BrONO2 seasonality as well as the day/night cycle. The major differences observed between the model and the satellite retrievals are identified and discussed in detail, highlighting the most probable causes of the model-observation disagreement. Based on this, I believe the paper should be published in ACP after the following minor issues are revised.

Main Issues:

• The total stratospheric Bry value of 21.2 ± 1.4 pptv of Bry at mid-laitudes has been estimated for the 40-60 S and 40-60N but without considering the 10° latitudinal bins used in previous sections. Wouldn’t it help to evaluate if there is any Stratospheric Bry trend and/or sink by means of computing Bry at different latitudes and heights?. In particular, Section 4 (Fig. 12) would benefit of a discussion comparing the almost negligible trend in MIPAS Bry trend between 1996-2007 with the reported value in WMO of -0.16 ± 0.07 ppt for the 2004-2014 period.
• The authors suggests that the low modeled polar BrONO2 during winter is caused by a low bias in NO2 abundance due to missing mesospheric NO2 production in the model (dif_1). However, for dif_2 and dif_3, they highlight different competing processes, but they do not mention even once that an erroneous modeled NO2 abundance could also be affecting the model-observation disagreement. I think this should be explicitly mentioned for all cases. In particular, for dif_2, Barrera et al., (2020) highlighted that VSL bromine impacts in the mid-latitudes depend on the recycling efficiency of ClONO2 and HCl in the lowermost stratosphere. Could modeled ClONO2 recycling also be affecting NO2 abundance in the lower stratosphere and indirectly affecting the modeled BrONO2 sinks?

Minor comments:

P4, L109: “the related a-priori profile for the target species BrONO2 was set to zero”: I’m not an expert in this field, but I thought that for satellite retrievals it was necessary to include a non-zero a-priori profile.

P5, L138: “Around that, the blue shading indicates the variability of the estimated errors between all latitude bands”. Please rephrase. Does “that” points at the mean total error? Is it “between” of “for” all latitudes?

Fig. 1: Rapid eye-reading the figure, it is evident that below 20 km, Tot parameter errors and spectral noise are the dominant contribution to the total error, while at higher heights it is mostly dominated by spectral noise. A simple sentence on the text highlighting this would be useful.

P8, L167: “During the MIPAS measurement periods, from the model output first all data within one hour around 10 LT and 22 LT were selected. Depending on their latitude, longitude and altitude, they were then assigned to day and night-time conditions and averaged over the observational bins of 10 latitude and three-day periods”. Do you mean that model output was filtered for specific hours to match MIPAS observations? And also, did you consider any additional condition to filter day/night time values very close to twilight conditions where the radiation intensity is reduced (mostly at high latitudes). Please make it clear.

P9, L93: Do you mean a “seasonal” signal instead of annual? What do you mean by outstanding?. Finally, it would be useful to provide a couple of sentences summarizing the Maximum and Minimum values observed for different heights and latitudes before getting into the MIPAS-Modelling comparison.

P16,L198: What is the Averaging Kernel Matrix? A kernel including the 10 LT and 22 LT hours? If that is the case, please make it consistent to the description in Section 2.2.

P16, L220: “Such differences are also present during day, albeit to a smaller extent (up to about 4 pptv, see Fig. 8).” … due to the smaller BrONO2 abundance. Clearly, as BrONO2 levels are higher during the night, then the absolute differences will be higher during the night than during the day, where BrONO2 vmr are smaller. I wonder if the relative/percentage difference between model and observations are similar during the day than during the night? Independently of the answer, this could be explicitly mentioned in the text.

Fig. 9: The “diff night” and “diff day” panels show a very similar profile, though the absolute variation is considerably different. Have you estimated if the relative/percentage difference between day/night are similar?

P24, Eq.1: You have only applied the expression to night-time modelled values. Have you performed the same analysis with daytime values? If so, did you find any difference worth to be mentioned?

P25, L345-346: “Since the model adjustment of Bry from BrONO2 is much larger in the tropical stratosphere (about 2.5 pptv) than at mid-latitudes (about 0.5 pptv), the second explanation would affect more strongly our estimation of Bry in the tropics.” What do you mean here? That the estimated Bry will be more realistic for high BrONO2/Bry ratios? Wouldn’t it be worth to compare the Bry estimation for each latitudinal bin of 10° at different seasons, and to determine if the estimated Bry stratospheric abundance for each band is consistent?

Fig. 12: I found a bit confusing that the colored symbols with measurements (other than MIPAS) at different locations are shown in all 3 panels. Wouldn’t it be better to include in each panel only those observation corresponding to the latitudinal band where it was measured?

Fig. B1 and B2: It could be useful to show the modelled NO2 difference in a 3^rd column on the right, as it has been shown for BrONO2 in the main text.

Would it be worth to include the estimated AOA in the published dataset?

Language editing comments:

P1,L10-11: Rephrase item (1) in the abstract.

P2, L33: ist produced à is produced

P2, L35: coupled strongly à strongly coupled

P3, L69: VSLS should be defined after it first usage in L64.

P4,5: ESA, HITRAN, ECMWF, etc. acronyms are not defined.

P16, L209: “values of less than” à “values smaller than”

P17, L259: Define SAD as it first usage in L244.

P22, L299: Tab. D1 à Table D1

P24, L327: Dependent à Depending

P24, L337: “vary from 21.0 +/- 1.4 pptv and 21.4 +/- 1.4 pptv for the northern and southern mid-latitude regions”, respectively.

P25, L350: Replace Obviously by Notably or other word … as it is not obvious that measurements performed with different instrument will provide equivalent results.

References

Barrera, J. A., Fernandez, R. P., Iglesias-Suarez, F., Cuevas, C. A., Lamarque, J.-F., and Saiz-Lopez, A.: Seasonal impact of biogenic very short-lived bromocarbons on lowermost stratospheric ozone between 60° N and 60° S during the 21st century, Atmos. Chem. Phys., 20, 8083–8102, https://doi.org/10.5194/acp-20-8083-2020, 2020.