Authors should address the following comments:

Page numbers are for the response from the authors – main text starts on p. 16.

One general comment: With the addition of new text, a number of paragraphs become quite large. Consider breaking them up into smaller paragraphs.

P. 17

L. 5: Style issue – I suggest you do not start a sentence with an acronym. Perhaps use: “The IFS…”. Same elsewhere in the paper and in the supplement.

L. 12: Introduce acronyms when first used, in the abstract, in the main manuscript, and in the supplement. For example, do so for MOPITT and OMI.

P. 22

L. 20: I suggest you use “…without degrading the…”.

P. 23

L. 3: Include subscript in O3.

L. 14: Do you need “practically”?

P. 33

L. 24: It is Lefever et al. (2015). The paper is out in ACP.

P. 35

L. 15: Do you need “clearly”? Same P. 40, L. 15, P. 42. L. 2.

P. 63

Fig. 1 caption: Identify the information provided above the plots (eg., mean, std). Same for Fig. 8, 17.

P. 74

Fig. 14 caption: I suggest you indicate the number of sites – I now you mention it in the legend, but this would make the information clearer. Same for Fig. 15, and Fig. 23 (in this case, number of measurements).

Supplement

P. 2

L. 17: “…the Rodgers…”.

L. 23-24: Omit extra “de”.

P. 4

L. 26: “Brewer-Mast”.

P. 6

Table S1: Have you defined NOx?

P. 9

Table S3: If the authors wish, they could use the original spelling of some of the stations: Ny Ålesund; Sodankylä. There may be others.