Dear Dr Alexander

Thank you for submitting your revised manuscript to Atmospheric Chemistry and Physics.

Both reviewers have suggested minor revisions, but actually raised quite substantial points requiring modifications of several sections and figures, as well as inclusion of previously published data for model validation.

In particular, the uncertainties associated with both lab measurements of the 17O excess of O3 (Morton et al. 1990) and potential methodological biases in the NO2- oxidation method of Vicars et al. may have been underestimated.

Please address the points raised by the reviewers, as well as my complementary comments below, when you submit your revised manuscript and reply to reviewers' comments.

Sincerely
Jan Kaiser
Editor ACP



Introduction (p. 5/l. 9) and conclusions (19/12):
Please add a caveat that previous modelling efforts have made different assumptions about the preferential transfer of central and terminal O atoms to NO2 and NO3, and the 17O enrichment of different ozone isotopomers. This is still not clear enough.

5/2: Remove tilde sign and adjust interval to encompass full range of observations (6 to 54 ‰ based on Krankowsky et al. 1995; 19 to 41 ‰ based on Johnston & Thiemens 1997).

5/4: Likewise, the range shown here is too narrow. It's 30 to 46 ‰ for Morton et al. (1990). Please also add "et al." to the reference.

17/26 & 20/1: Replace tilde sign by actual range value with uncertainty. All measurement results should be rounded according to their uncertainty and stated with an estimate of their measurement uncertainty. Approximation symbols should therefore not be used (unless you are approximating a mathematically exact number, e.g. π ≈ 3.14). In any case, the correct approximation symbol has two wavy lines (≈). It is not the tilde sign (~), a symbol which has perhaps made it into the literature due to limitations of early typewriters.

Figure S1: More than half of the plot appears with the color corresponding to the colorbar maximum. Please include a variant of the plot with an increased maximum value so that variations in τ >= 2 d can be distinguished, or perhaps add contour lines for values higher than 2 days.

Figure S3: Please explain the meaning of the dashed lines in the figure caption.

Figure S6: The caption should refer to Fig. S3.

I am happy to accept the revised manuscript for publication subject to a minor technical correction (sorry for not spotting this before): ACP requires the use of SI units, so the few occurrences of "ppb" and "ppbv" in the main text as well as "pptv" and "ppbv" in Figures 7 and 8 should be replaced by the corresponding SI units (pmol mol^{–1} for pptv and nmol mol^{–1} for ppb/ppbv).

Jan Kaiser
Editor Atmospheric Chemistry and Physics