Dear Ms Albertin

many thanks for submitting your revised manuscript to ACP. I am happy to accept it for publication subject to the minor revisions listed below.

Kind regards
Jan Kaiser
Editor ACP



Fig. 1 axis labels: Please add "amount of ..." before x- and y-axis labels.

l. 129: Δ(17Ο) uncertainty is most likely an overestimate because the uncertainties in δ(17O) and δ(18O) are correlated. This is confirmed by the Appendix B1. Please provide a more realistic estimate of the Δ(17Ο) uncertainty.

You should remove the trademark (™) and registered (®) signs (e.g. used after Finnigan, Corning, 2B Technologies). They are marketing symbols and unnecessary in academic writing.

Times of day should reported using a 24 hour-clock (https://www.atmospheric-chemistry-and-physics.net/submission.html#math)

l. 190: "keeps", "reaches", "maximum" (singular), not maxima (plural)

l. 261: 30 pmol mol–1 would be a very low NOx concentration - please check this statement.

l. 299: The last term should be (6.6±1.5) ‰.

l. 368 & l.681 : Please add unit K after 2470, 1310 and 360 and remove "molecule–1" ("molecule" is not a unit).

At various places (e.g. l. 167 to 187) you use the term "concentration", but refer to amount fractions. In Table 1 and Fig. 2, l. 159 and 162 you call these amount fractions "mixing ratios". The term "mixing ratio" is ambiguous because it can refer to amount fractions, mass fractions and so-called reduced concentrations (at a specific temperature and pressure, eg. 273.15 K and 1 bar).

Instead of "amount fraction", the deprecated term "mole fraction" is often used although this would be equivalent to referring to a mass fraction as "kilogram fraction".

You should preferably use the term "amount fraction", but provided you define the term appropriately, I can accept "mixing ratio" or "mole fraction" as well.

Dear Ms Albertin

many thanks for submitting your revised manuscript to ACP.

The Δ(17Ο) uncertainty cannot be be calculated from simple error propagation of the uncertainties in δ(17O) and δ(18O) because the latter are correlated. The combined error has to take the covariation between δ(17O) and δ(18O) into account. This is confirmed by the Appendix B1 (l. 555 and Fig. B1) where you state that the uncertainties for nitrite standards are 0.8 ‰ δ(17O) and 1.8 ‰ δ(18O). A simple error propagation would give a Δ(17Ο) uncertainty of 1.2 ‰, but the actual uncertainty is only 1/4 of that (0.3 ‰)

0.3 ‰ would therefore be a more realistic estimate of the overall uncertainty.

Also, you stated on l. 555 that "If the deviation is constant, it means that the isotopic signal is not degraded with time and its standard deviation is considered as the uncertainty in our δ17O(NO2) and δ18O(NO2) measurements". This means that the latter uncertainties should be 0.8 ‰ for δ(17O) and 1.8 ‰ for δ(18O). It is unclear how you arrive at the higher values of 1.1 ‰ and 2.5 ‰, respectively, further down (l. 561).

Please revise the uncertainty estimates in the manuscript to more realistic, justifiable values.

Kind regards
Jan Kaiser
Editor ACP