|This paper has improved, but still needs work, as detailed in the following. Thus I do not recommend publication in this form, but encourage the authors to further improve this manuscript.|
While I don’t disagree with the overall conclusions of the paper, I still struggle to find support for the authors’ claims of nitrification apparent along isentropes in Figures 3, and 4, as detailed further below. While I remain skeptical of their interpretation of the figures, their claims are eminently testable. Just pick several isentropes in the vortex air sampled and plot normalized ozone and hno3 along each isentrope in another time panel at the base of Figures 2, 3, 4. In a sense this is what the RNFDs are showing, but those do not show variations along isentropes, which the authors repeatedly claim show, qualitatively, nitrification.
Figures 2-4 could be improved, as detailed further below, by clearly marking in the GLORIA panels the regions considered vortex air, and limiting the flight track altitude displays in the a) panel to those regions in the vortex corresponding to the data shown in the subsequent panels.
2.5-7 “While …” Break this confusing sentence into two, or otherwise modify it to be clearer and so it doesn’t have to be read twice/three times.
3.3-4 This is a pretty definitive statement on NAT nucleation with no reference. Since this is one of the remaining unknown questions concerning PSCs, the authors should temper this statement. Change “can” to “may”, and then qualify it by indicating that evidence is still lacking to confirm this.
5.13 There is no reference for Tritscher et al., 2018.
5.22 It is unusual to begin referencing figures 2 and more when figure 1 has not been introduced. Is the reader now to skip ahead to these figures?
7.5 In the reference Eckstein et al. (2018) for the RNFD those authors use isolines of 0.9, 0.4, 0.05. Here the authors use a single isoline of 0.02, so if I understand correctly 98% of the correlation points are included. Would the results change if this rather generous isoline were increased to 0.05 or even 0.2?
7.8 “In order” is used way too much. Here to even start a sentence, and these two words never improve a sentence. Just leave them out here, and elsewhere. The sentences will be clearer.
Figure 2. I don’t understand the reason to show the entire flight track in Fig 2a) for the day. Why not just show the portion of the flight track which relates to Figs 2b, c, d, e)? Particularly since the flight doubles back on itself, so it is almost impossible to identify the leg corresponding to the data after way point D. This would also eliminate the need for waypoint A. Or one could show the flight track, but not the tangent point altitudes except in the places where the observations are accepted as cloud free. That is the information desired by the reader.
9.4-5 It would be of interest for the reader to know that the authors are discussing particle enhancement efficiency and which size of particles may be enhanced, rather than that Fig. 2c) is “showing the measurements on 20 January 2016.” The reader already knows that since the discussion is about Figure 2, which has already been introduced as presenting the measurements on 20 January 2016.
9.7 Please specify where the local HNO3 maximum detected by Gloria is. It is not obvious to this reader that the particles are only associated with this.
9.26 Suggest to add to this sentence the following. “… by CLAMS, and CLAMS completely misses the vertical structure in the HNO3 measurements.”
9.33-34 Readers would appreciate it if the non-vortex regions were clearly marked on Figs 2b, c, d).
Figure 3 and 10.2-5 “Compared to the ozone values only varying slightly along an isentrope, the HNO3 volume mixing ratios show larger variations at levels of constant potential temperature …” This statement is not supported by Fig. 3c). Once the regions between 8:40-9:20 and 11:30-11:50 are eliminated, the majority of the HNO3 measurements are relatively flat along the 340 K isentrope (12:50-14:00), the only isentrope shown within the data regions. In fact the ozone and hno3 distributions are not that dissimilar. Thus I have to respectfully disagree with the authors’ conclusions that the data show local enhancements of hno3. Test this by plotting o3 and hno3 along the time line on an isentrope.
11.4-6 and Figure 4 Again readers would appreciate it if the edge/non-vortex regions were clearly marked on the b, c, d) panels. Given the scale of panel a) it is hardly a useful graphic since the flight tracks are so overlain.
11.8-12 Aside from the hno3 maximum at 370 K, point A, which is barely in the vortex air, the hno3 distribution appears pretty constant along the isentropes, and certainly not more variable than ozone, although the color scales are so different perhaps that is what is misleading? Still I find the following statements to be less than obvious from figures 4b, c). “Ozone values along the isentropes vary only slightly. The measured HNO3 distribution (Fig. 4c) shows a higher variability along the isentropes with local maxima for altitudes higher than 9 km reaching maximum values of up to 6 ppbv at flight altitude embedded in background values of 2 to 3 ppbv.” The reader does not see the authors’ claims about this flight further tested until Figure 6d), where the RNFD for this flight is shown. And indeed the flight is not showing the slopes of hno3 vs o3 that are apparent from the January flights, which lends further question as to the authors statements above. Is this why the March flight was not included in Figure 5?
12.10-14 Is flight 5 sampling vortex air or non-vortex air and at what altitudes? In which flights is the non-vortex air included? The authors do not say. What range of latitudes and altitudes is covered in flight 6? Does it include vortex and sub-vortex air, or non-vortex air? Since flights 5 and 6 are going to be the reference and were not discussed in detail earlier, additional information characterizing the air masses sampled is required here. Parenthetical clauses longer than a few words should be avoided. If it is important, use a full sentence. In this case the sentence forced this reader to re-read the sentence in which the clause is embedded, to remember how the sentence began.
Figure 5. Why isn’t the 18 March 2016 flight, where the authors just claim to see nitrification of the LMS, shown on this figure?
12.16 Didn’t the authors just say the December flight was used as an early winter reference? There is no need to repeat.
14.1-2 To be clear change this to. “The model cross-sections compared to measurements for flights…”
Figure 6. legend, what is meant by “reference” is this the CLAMS model reference with no perturbations, as shown in Figure 5? If so this should be stated either in the legend, “model reference” or the figure caption.