The two reviewers both found that the work had scientific merit and was suitable for publication in ACP. They both, however, had some important comments related to (i) the tracer-tracer plot Figure 9, (ii) the use of dPAN/dPV as both tracers co-vary, (iii) Figure 5 - choice of O3 cut-off value. I think that the authors have responded to these (and other minor) scientific points well and that the paper can now be accepted.

However, before publication there are still a number of typos and presentational things to tidy up. I assume that the ACP editing office which catch all that remain. A few examples:

I) Page 8 Line 15. The new test has created a 1-sentence paragraph. That is not good and the text should be integrated with other text.

II) Figure 1 caption: 'Example' not 'exemplarily' (which would fail a spell check).