Dear Authors,

Thank you for the careful revisions and replies to the reviewers’ comments. I have read the revised manuscript with great interest and also got an opportunity to read reviewer’s views on the revisions. I consider that the manuscript is almost ready for publication. I have two minor comments, as listed below, that I would like you to consider:
1) On pages 3-4, line starting at 33 the statement “At the same time the study of Judd et al. (2020) on the Long Island Sound Tropospheric Ozone Study (LISTOS) campaign in the New York City/Long Island Sound region has been submitted”. I find the statement little odd in a publication. Please note that the manuscript by Judd et al is already published in AMT. Maybe you could have a statement like “A similar study was carried out by Judd et al. (2020) using observations from the Long Island Sound Tropospheric Ozone Study (LISTOS) campaign in the New York City/Long Island Sound region in the United States”. In my opinion, both of the studies (Tack et al and Judd et al) are very helpful in accessing the quality of TROPOMI NO2 retrievals and unraveling complexities associated with validation of satellite NO2 retrievals in different environments and possibly different seasons. I encourage you to cite Judd et al and make those connections as appropriate.
2) On page 12, line 13: I think the reference “Henk Eskes, personal communication, 15 March, 2020)” is unnecessary as Henk Eskes is also a co-author. Please check.

In addition, please address the following comments from the reviewer. Most of these comments are minor and can mostly be addressed by revising the statements.

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Technical comments:
Line 5 page 2: Technically the temporal frequency of TROPOMI is not unprecedented, so edit 'spatio-temporal' to just 'spatial'. It is true that the quality of data allows easier use of daily data than its predecessors that require averaging of many overpasses, but this isn’t stated and as written it could be confusing to a reader thinking that TROPOMI measures more frequently.

Page 17 Line 22: The airborne data picks up fine spatial structure, but when sampled to the TROPOMI pixel, this should be taken care of through the spatial averaging and not a source of low bias, correct? Therefore, this statement should be taken out here. It would be more appropriate to state something like this when discussing the right side of the figure where the impact of undersampling is clearly shown as an impact.

Page 20 Line 17: Technically Pandora can operate in MAX-DOAS mode so maybe clarify as Pandora direct-sun measurements.

There is a definition late in the paper about what urban background is: ‘Note that in this work urban background is defined as an area in a polluted environment, which is not directly affected by pollution plumes’. However, it is not clear how data is ‘classified’ as urban background. Can this be clarified? Especially with the discussion in Sect 6.

Other clarifications and minor corrections:
Page 4 Line 10 and Figure 1: I see 6 red markers rather than five. One is in France so perhaps this can be mentioned in the caption and text.

Figure 1: The arrows are 180 deg off from the wind direction (table says the wind is from 36 deg), which I later put together that they were just pointing out emission sources and not the wind vector coming from the emission source. Considering flipping these arrows or make it more obvious that it is not a wind direction.

Line 19 page 2: Please clarify what it means by having potential. Potential for what?

Page 6 line 18: Should (near)infrared be (shortwave)infrared?

Page 3 line 30: It says that a TROPOMI pixel comprises of 4000 APEX pixels but throughout the rest of the paper it states 2700. Is this an error or is there a caveat to this that should be clarified when these numbers are stated.

Page 10 line 8: It is unclear what the 50% larger than is referring to. Is it that APEX AMF is 50% larger than TROPOMI with this box profile?

Page 10 line 27: The phrase ‘When considering the entire APEX scenes instead of single pixels,’ then going on about APEX pixels resampled to TROPOMI, I am confused what entire apex scene means for this sentence? I expect other readers would be too. Would the point still be clear if the phrase was removed?

Page 16 line 12: remove ‘the latter’

Bottom of page 19: Does this mean that the LOS sampling of aircraft data is 10km too? Can the authors state explicitly how far out they integrate data out in the LOS?

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I look forward to receiving the revised manuscript.

Best Regards,

Lok Lamsal
AMT Associate Editor