|In general, the authors have done a superb job of revising their manuscript in response to the comments of the referees. I very much appreciated their detailed responses to the reviews. I have only a few remaining points of clarification for the authors to consider before their paper is published.|
*p5, L20: homogenize --> become homogenized
*p5, L24-34: I have some concerns about the discussion that has been added in these lines. First, I do not believe that it is true that "ozone concentrations return rapidly to usual values almost as soon as PSCs disappear". PSCs formation typically ceases in mid-September, but ozone values remain low well past that date, even in the collar region (and certainly in the vortex core, where the ozone "hole" is in full force through October). Second, it is not clear what the word "it" is referring to in L29. It may be that the authors mean that only a few PSCs have not sedimented out of the lower stratosphere and thus remain available, in which case "such that only a few PSCs remain available in the lower stratosphere" would be better than "yielding only small amounts of it". Finally, the action of confined diabatic descent as the vortex spins up at the start of the winter also plays a major role in the increase of HNO3 in March.
*p6, L17: I think it would be fairer to say that Arctic lower stratospheric temperatures "frequently" dip below 195 K, rather than that they "sometimes" do. Also, "on broad areas" --> "over broad areas".
*p7, L10: Contrary to --> In contrast to
*p13, L5-6: I had missed the significance of these lines in reading the previous manuscript. Just to make sure that the point is totally clear to all readers, the authors might change the wording here to "Regions of low clouds or those characterized by emissivity features that are sharp (e.g., deserts) or seasonally varying (e.g., ice shelves) are known to cause problems for the retrieval of HNO3 using the IASI spectra". Then on p14, L10, they could add "the ice shelves of" between "above" and "western Antarctica".
*p14, L5-7: I found the text added here confusing. First the influence of SF on HNO3 is described. Then it is stated that previous studies have shown that SF contributions to O3 variability are largest in the lower stratosphere. This is followed by the sentence: "Our results for the mid to high latitudes suggest opposite behaviour for HNO3 (as was also reported for O3 by Wespes et al. (2017))." It is not clear what "opposite" means here; since the immediately preceding sentence talks about the O3 response being largest in the lower stratosphere, as written this seems to imply that that is not the case for HNO3, but I think the authors meant instead that the geographic distribution of sensitivity to SF of HNO3 is not the same as that of O3. Moreover, the parenthetical saying that similar results were reported for O3 by Wespes et al. (2017) appears to contradict the first part of the sentence mentioning "opposite" behavior. This may be because the latter paper focuses on tropospheric, not stratospheric, ozone. In any case, this part of the discussion needs to be clarified.
*p15, L2: It would be clearer to say "Arctic" instead of "polar" here.
*Figure 2: I do think that it was a good idea to remove from this plot the period in September to December 2010 when the amount of data distributed by EUMETSAT was abnormally small. However, I noticed that the results in all four EqL bands changed substantially from those in the original manuscript throughout all of 2010 (even in the January to April 2010 timeframe, well before the data gap), particularly in the south. I was not expecting such a large change to the curves outside of the interval directed affected by the data shortage when those dates were removed from the analysis.