Overall, I found the paper to be very well written and found that the novel isotopic tagging chemical mechanism developed in this study could serve as a very useful tool in the future to evaluate and understand NOy chemistry and is of great interest for the readership of GMD. However, I did have several major concerns that should be addressed before final publication: 

(1) I could not find the mechanism files on GitHub and at minimum a link of where to find these files should be included in the text. They should be indexed with a DOI for permanence /reference and should be accessible for reviewers to see before publication. Currently, the unavailability of these files means the work is not reproduceable and is not in compliance with GMD's data policy. 

(2) The mechanism developed adds 55 new species and 727 replicate reactions to RACM2. In order for these improvements to more easily be incorporated into other box model mechanisms available in F0AM (that may treat BVOC reactions with NOy differently than RACM2), or for these improvements to more easily make their way into CTMs as the authors suggest, I firmly believe the authors need to include as a supplement a table with a complete listing of the 55 species added including their SMILES or InChI codes like is done in the Bates et al., 2022 ACPD supplement (https://doi.org/10.5194/acp-22-1467-2022-supplement). Including this information will allow anyone wanting to take these reactions w/ new species and map them to chemical mechanisms other than RACM2 to their mechanism in an automated way that may lump compounds in different ways (e.g. using python's RDKit library). Pye et al., 2023 (https://acp.copernicus.org/articles/23/5043/2023/) makes an incredibly strong case that when SMILES or InChI codes available with new chemical mechanisms, end users who may want to take what's developed and incorporate it into other mechanisms is much easier to do in an automated way using RDKit. This is *especially* true for large mechanisms where doing this by hand is extraordinarily tedious. I specifically recommend the authors include the InChI codes rather than SMILES codes because they are better at taking into account isomerization and isotopes in a way that SMILES codes do not. Using RD-Kit, end users can transform the new species into molecules when InChI codes or SMILES codes are provided, thus enabling automated comparisons to other mechanisms (e.g. like MCM, which also provides these codes), CMAQ (Pye et al., 2023), and  GEOS-Chem, which is moving to also incorporate these codes currently in their mechanism metadata. Thus, for the more widespread adoption of this tool, and for it to truly have the impact the authors are hoping, I really think its extremely critical for the authors to provide these with the paper in a supplement. I know this may seem tedious especially for small inorganic molecules, but the value add for future users by providing this is quite critical for mechanisms of this size. 

(3) The other reviewer raises a really critical point that the average tropospheric d¹⁷O(O₃) is 26%, but the authors currently don't take into account known variations from this average.  To address this point, I think there should at least be some discussion of the known variations away from this in the main text. I agree this assumption certainly limits the extension of the mechanism for 3D-CTMS and suggest that if that it would be very valuable for future users of the mechanism if there was some built in way to account for this variation within the mechanism (perhaps by including other +/-% values with suggestions of when to use as a comment in the mechanism for the relevant rxns?). At minimum some discussion is needed about this in the main text. 

For the Editors/Authors: I didn't find the lack of comparison to observational data as problematic as the other reviewer. I don't think the purpose of this paper was to actually interpret observations, but merely provide a tool for people to do so in the future. To that end, I think the choice of submitting to GMD was entirely appropriate and that it is not necessary for its publication to include that analysis here. I believe that is an entirely different paper for an entirely different journal, but that this work simply provides the tools necessary to enable others to do that sort of work.