In this work the authors suggest an alternative tracer-tracer metric for exploring the ‘Extra-tropical Transition Layer’ using the correlation between sulfate aerosol and ozone.   The work is based on aircraft measurements of aerosol sulfate using an Aerosol Mass Spectrometer aboard the DLR HALO aircraft and trace gas measurements from the HALO and DLR-Falcon from 2020 over central Europe.   In part 1 the authors show a robust relationship between sulfate and ozone near the tropopause, and seek to use the variability in this relationship to identify atmospheric processes, of particular interest is the persistent minima in sulfate mixing ratio near the tropopause.   In part 2, the authors identify a specific case of enhanced sulfate from one ascent in one flight and then perform an analysis to conclude that this enhancement is the result of mixing through the ExTL followed by gas to sulphate aerosol conversion from a distant volcanic eruption. 

Major comments:

1. The correlation between aerosol sulfate and ozone is interesting, and establishing a background ratio is useful to identify perturbations in sulfate aerosol on top of the steeply sloped vertical aerosol gradient.   However, the analysis of aerosol sulfate seems to be performed in isolation from the other aerosol properties that must surely have been made on the same measurement platform, or even by the same instrument (the C-ToF-AMS).   In terms of the analysis in part 2 off the enhanced sulfate plume observed in RF01, a discussion of the aerosol size distribution and the variation of other chemical constituents (particularly organics and nitrates) would provide both context and a much more convincing argument that this particular enhancement is volcanic in origin.  Furthermore, including the size distribution of the observed sulfate would reassure the reader the analysis of the C-Tof-AMS is actually capturing the bulk of the sulfate aerosol within the somewhat limited size range.
2. The authors make a convincing argument that the enhancement observed on RF01 is in fact anomalous and possibly of tropospheric origin (figures 5, 6 and 8). However, the argument that this enhancement is of volcanic origin and is due to gas to particle conversion in the ExTL is less convincing.   The authors seem to start with the conclusion that this is in fact a volcanic event, then search for a candidate volcano and use a combination of forward and backward trajectory models to try and link the observation to the event, which is convoluted and unconvincing.   A more convincing argument to show this event is volcanic in origin (and not anthropogenic for example) would be to present the total aerosol composition and size distribution, as discussed above.   It is also interesting that RF01 only observed this enhancement on one or maybe two climbs of the flight, despite reaching higher altitude and theta on the subsequent climb.  Is this enhancement limited in geographic scope?   Without a broader analysis and a more convincing identification of the source of gas phase sulfur, part 2 of the paper is of less scientific relevance than part 1.

Minor Comments:

The figures are not well aligned with the text that references them, making it difficult to reference while reading the text.

Line 28:  I am not sure what you mean by ‘photochemical dissolving’?

Line 71 / Section 2.1:  It may be important to note the limitation of the HALO aircraft measurement with respect to altitude – it appears to be limited to a peak altitude of around 12km, which may not really reach the top of the ExTL in May at latitudes below about 45N leading to a bias to higher latitude measurements.

Line 163 and Figure 3: The persistent minima in sulfate at the ‘ozone tropopause’ of 90 – 120 ppb is surprisingly consistent and robust.  This may be worth more than a passing mention.

Line 186: It is not clear what you mean by ‘not connected to the stratosphere’ when the airmass meets both the PV > 2 and ozone > 120 ppb criteria by a large margin.

Line 219:  Won’t dilution equally impact all tracers and not just CO?

Line 225:  The stratospheric water vapor background is closer to 5 ppm, why would you expect it to be 10 – 15 ppm, and doesn’t Fig 8e show that the water vapor us is in the range of 10 – 20 ppm?

Figure 7: Is there an easy way to show latitude on this figure?  It may help understand the geographic extent of the anomalous layer and why it is only observed on some of the climbs?

Figure 8: The caption does not agree with the figure labels, and the color bar labels for theta are incorrect.

Figure A1: The y-axis label is incorrect.

Any field observations and measurements are valuable for model development. Global aerosol models predict that certain amounts of sulfate will accumulate at high altitudes above 12 kilometers, and this observation confirms this. This is an observational paper reporting aircraft field measurements during the CAFE-EU/BLUESKY mission. The authors try to find out the reasons behind these observations. Therefore, the objectivity of the analysis is very important for this article. The authors further analyzed the observations using a variety of methods and combined all analyzes in an attempt to trace the origin of the entire observation. Overall, the material in this manuscript is well organized and well written, and therefore deserves acceptance and publication.

This manuscript still leaves some room for improvement. Due to the limitations of observation, the conclusion drawn from the current limited observations is only a possibility or a reasonable explanation. In addition, the analysis in this manuscript focuses on the sulfate anomaly discovered on Flight 01. There are approximately 45 observation points, and its proportion in all observations is relatively small. Other observations with similar meteorological conditions did not show similar characteristics. This often means that reality is more complex, and there may be something behind it that we currently don’t know about. Therefore, I hope that the authors will be aware of this in order to provide more explanations and treat the conclusions with more caution in a revised version of this manuscript.

The following are specific comments:

1. Line 8, "non-refractory aerosol --- to 800nm", Can I assume that this category of sulfate roughly represents the total amount of it in the stratosphere?
2. Line 10, "background sulfate", Please add discussion about this part
3. Line 14, "volcanic SO2", Judging by the text, the analysis supporting this argument is insufficient.
4. Line 90, "the accuracy of the AMS is about 30%", Perhaps a discussion of how 30% accuracy affects the results of the analysis could be added to the text.
5. Line 96 to 99, "O3 and HALO --- ,respectively", Is CO measured on the HALO?
6. Line 102, "on gaseous SO2", Is SO2 only measured on DLR-Falcon ?
7. Line 105, "the total uncertainty --- for HNO3", uncertainty is a bit high
8. Line 109 to 110, "there hours --- in the horizontal", 3 hours compared to 30 seconds; 50 kilometers compared to 6 kilometers.
9. Line 111, "potential vorticity and equivalent latitude", Please briefly explain how equivalent latitude is calculated; Is it based on model predictions or measurements? If it is based on forecasts, how does the resolution of the ERA5 data affect the analysis?
10. Line 117, "231", why 231 ? please explain 
11. Line 137, "two modes", what are they?
12. Line 138, "can be --- air masses", reference
13. Line 146, "observations", Does it contain DLR-Falcon data?
14. Line 147, "Fig. 3", If my observations are correct, the lowest sulfate concentration observed in each flight measurement in the stratosphere appears to be different. If true, does the author have any explanation?
15. Line 150, "900 to --- measurement flights", Curious to know what the author has to say about why each flight observation has its own unique ozone to sulfate ratio
16. Line 163 to 164, "Figure 3 --- chemical tropopause", What is the background concentration of sulfate in the chemical tropopause?
17. Line 175 to 176, "The presence --- mixing processes", why is stratospheric CO mixing line set to less than 20ppbv ? 
18. Line 179 to 180, "However,  --- see also Fig. C1)", It seems that the entire RF01 flight data is abnormal, not just the part where the sulfate concentration is above 0.3 ppbv. Because the linear regressions for ozone and sulfate appear to match very well across observations.
19. Line 185, "Fig. 5", Are the potential temperatures shown in Figure 5 measured? if so, have you compared the potential temperature of the measurements with the potential temperature of the ERA5 data set? Since the equivalent latitude is from ERA5.
20. Line 197, "around 1",  >0.25
21. Line 198, "the static --- stratospheric values", In addition to the region focused on in this study, many other regions also show the potential for air masses to transition from the troposphere to the stratosphere
22. Line 211, "five ppmv", approximately 10 ppmv in Figure 8
23. Line 212 to 213, "all of --- mixing lines", please elaborate
24. Line 234, "0.01 ppbv", 0.1 ppbv ?
25. Line 234, "gas-to-particle conversion", It could also be due to the removal process
26. Line 272 to 273, "this is ---the stratosphere", This is just a possibility, the link between observations and eruptions is very weak
27. Line 275, "seven weeks after the eruption", lack of evidence for this
28. Line 289 t o 290, "During this --- the stratosphere", In Figures 4, 8, and 9, there seems to be such a transmission line for air masses from the upper troposphere to the stratosphere. But the line's constituent points come from measurements at different locations, elevations and times, meaning they could come from completely different source areas.
29. Line 291, "quasi co-located DLR-Falcon", please elaborate
30. Figure 2, "subset", please define subset
31. Figure 8, It would be better to add a discussion of why higher potential temperatures occur when ozone concentrations are below 200 ppbv.