This paper by Tomsche et al. presents new SO₂ and SO₄^2- (along with other trace gas species) measurements obtained during the BLUESKY aircraft research campaign in May/June 2020, during the European Covid-lockdown which offered an unprecedent opportunity to measure air masses characterised by drastically decreased air pollution. Interestingly, the authors find strongly enhanced SO₂ and SO₄^2- concentrations in the upper troposphere and lower stratosphere, respectively, despite overall reduced air pollutant emissions. While these observations are of great value and offer a benchmark against which the chemistry in air quality and other chemistry transport models can be tested, I find that the conclusions provided in the abstract not to be supported by enough evidence within the paper. The explanations for the enhanced sulfur-species concentrations comprise a range of different possibilities, which the authors fail to meaningfully constrain by their evaluations. I therefore cannot recommend this paper for publication in ACP in its current form.

Specific comments:

L72, Section 2.1 BLUESKY mission:

1. It would be interesting for the reader to have a short summary of the mission goals added here. It seems obvious but it should be made more explicit as you have done nicely in the abstract.
2. Figure 1. Could the paths of the different flights be represented in different linestyles or shades of blue/red to help emphasize that they were carried out on different days? The current figure may show the coverage, is otherwise though not very informative.
3. Also, you talk about coordination of the flights between Falcon and HALO, but the coverage is rather different. What was the main aim of the coordination?

L96 Sensitivity of measurement to moisture: It seems somewhat arbitrary to use a specific altitude as cut-off since you could find samples with high/low moisture content even below/above 8 km depending on the meteorological situation you’re flying in. What is the range of H₂O mixing ratios you can/cannot easily perform this correction for? Did you measure H₂O and if yes, with which instruments?

L138 Could be written more clearly. I suggest to replace ‘along the same longitudes and vice versa’ with ‘and vice versa along the longitudes outside this range’

L142 Related comment. I suggest to explicitly say that O3 and HNO3 are positively correlated as expected and repeat the longitude range here, since ‘in the mentioned longitude range’ may not be clear to readers given that you talk about two in L138.

L148 I would rewrite this sentence here to point towards the more in-depth analysis and discussion in Section 5 and without claiming it is ‘just’ from long-range transport. As it currently stands here, I cannot judge from Figure 5 whether the evidence you provide is good enough to underpin this result. For example, I see one trajectory rising from rather low altitude starting around the Eastern coast of North America ending at the measurement location.

Figure 3: I suggest adding the tropopause height onto the figure.

L163 It would seem important to indicate the average altitude of the dynamical and/or thermal tropopauses over the region during this time period as well, not to give the impression of choosing what fits best your lower bound of the mixing layer. I would expect the 2PVU tropopause being close to the 330K isentrope, so it would confirm your choice of ozone value for defining the chemical tropopause.

L203 Is this an expected concentration range for the mixing layer to be found in at these latitude bands and season?

Figure 4 and discussion section 3.3: It is really hard to follow the discussion of this figure. The only really outstanding feature I can detect when looking at these panels is that there are some high concentrations in SO₄^2- in the troposphere (at ozone values below 100 ppbv and CO values between 100 and 125 ppbv). It would be nice to have this feature explained by the backward trajectories. Otherwise, it is expected that SO2 decreases and SO₄^2-  increases as one goes into the stratosphere due to the aging of air, which is reflected both in the very strong anticorrelation between the two sulfur species and also in the SO₄^2- / (SO₄^2-  + SO2) ratio visible in Figure 2 and 3. Maybe you could circle the points you are referring to in the figure in case I have missed what you are really referring to?

L215/Figure 5. This evaluation would be more effective and convincing if you would identify the exact times/altitude/geographical location of when unusual trace gas observations are made and then calculate your backward trajectories from locations on a finer grid point around these. I then would suggest not only to show lat-lon plots, but also time-altitude evolution of the trajectories and also PV along with it, so that the reader can see where/when tropospheric influence or strong uplift within the Asian monsoon may have happened.

Figure 5 Why is there an empty panel (j)?

L253-262 I cannot fully follow your argumentation here. Don’t you miss to account for the 72% reduction in air traffic due to Covid when you calculate your 50% increase in aviation SO₂ emission increases?

L291-295 This conclusion is not very satisfying. Much more should have been done to better disentangle all the different possible explanations of the enhanced SO2 throughout the upper troposphere. The reader knows in principle as much as before reading your paper about the potential sources of the enhanced sulfur species. If you could do the backward trajectories in a more targeted way, that is identifying the origin of high-SO2 events for different flights more meaningfully, this could be a much better step to answer the question you wish to answer.   

Section 5: It would be interesting to see whether the clear anti-correlation in SO₄^2-  with ozone seen for the flight in Figure 2 is also found for other flights. This could then be used to identify truly anomalous correlations for which again more targeted backward trajectories could potentially yield much more insight than is provided here. As the discussion is presented here, multiple explanations from existing literature but no additional insights are provided, leaving the reader once more unsatisified about having learned anything new.

L360 I do not think that you have provided real evidence that you have measured enhanced SO₄^2- concentrations in the stratosphere, since we do not know what the ‘normal’ background would be. The difference seen in Figure 6 could simply be due to measuring within the troposphere at altitudes 11-15 km for the CAFE-Africa campaign data, which to my understanding was performed within the tropics.

Minor corrections/typos:

L31 geo engineering --> geoengineering

L90 in flight --> in-flight

L146 delete ‘to’ in front of ‘266 ppb…’

L179 write either ‘with stratospheric sulfate aerosol’ or ‘with the stratospheric sulfate aerosol layer’

This manuscript is overall an excellent overview of the BLUESKY mission, and presents some rather interesting measurements of the UTLS in a peculiar case (the beginning of the COVID-driven decrease in emission). It has the potential to be a rather important reference going forward, but clearly the manuscript needs more work, and to be cleaned up a bit.

I generally agree with all the comments already laid out by Reviewer 1, so I find it pointless to repeat them one by one: but in particular, I found the possible causes of enhanced SO₂ to be a bit too much handwaved in section 4.

For instance, at L. 283, the paragraph starts with "Beside the conversion to H2SO4..." but then continues by mentioning that most of the SO₂ would be reduced by its conversion to sulfates through OH oxidation. Something you clearly go back to at the end of the paragraph, by simply repeating the same thing. But no convincing proofs are given. Even just the addition of a vertical profile of humidity from ERA5 reanalysis (which is what van Heerwaarden et al. uses) would go a long way into proving that, indeed, SO₂ lifetime has been increased by lower OH concentrations.

Something similar can be said for Section 5.

A rather long list is given, but I don't find a compelling critical assessment of possible causes. Starting from the bottom, sure, we know COS contributes to the stratospheric aerosol layer, but what causes would have produced an increase significant enough to be observed in this campaign (noting that photolysis of OCS happens much higher up, so its eventual conversion to aerosols and signal at the lower altitudes would not be as instantaneous as for SO₂). So in all of this, Figure 6 is really not that helpful (also, the legend in panel b) seems to be hiding the 2017 data...). Are we sure those different measurements are performed over similar regions? The Andrés Hernández (2022) reference seems to be targeted at lower altitudes, and I can't find SO4 measurements there, so it's hard to judge if the comparison is correct or not - and it's the only point offered for comparison! This hand-waviness is reproduced as is in the Conclusions, where the phrase " The enhanced stratospheric sulfate aerosol, which was observed, was likely impacted by the volcano Raikoke, and smaller sources" doesn't really say anything at all.

In a future revision, both Section 4 and 5 need to be strengthened and mroe analyses need to be performed to make sure this really reads like a Research Article. Otherwise, the editor and the authors could consider shortening those parts and limiting the scope of the work to a "Measurement report" type of manuscript for ACP.

One more typo: "One major source of SO4 2- in the stratosphere are volcanic eruptions" should be "is" and not "are"