|This is a revised version of a paper accompanying the community release of version 4.0 of the CAABA/MECCA atmospheric chemistry box model, with the model code included as an electronic supplement. The authors have made a number of changes in response to reviewer comments, and the paper is substantially improved compared with the version that was originally submitted. In particular, the information provided about the treatment of the OH-initiated VOC chemistry is much clearer and more informative. The paper is therefore much closer to being publishable in GMD.|
I still have one major comment, which relates to the presented mechanism comparison. As indicated before, it is important that some results are presented, and I am pleased that the mechanism performance comparison provided by the authors is now more detailed, and with additional mechanisms included. However, despite the improvement in presentation clarity, the reader is left wondering what the main causes of the performance differences are. Are they because of the applied terpene speciation and the number of different terpenes treated in the different mechanisms? Are they due to differences in the treatment of isoprene chemistry? In relation to the former point, it is not clear how the terpene speciation was assigned to the different mechanisms (and referring to MIM1/MIM2, you cannot initialize with 500 pmol/mol of nothing!). I think a little more explanation would be useful in the manuscript, along with some more detailed supporting information in the supplement so that the reader can actually understand what was done and what the results actually mean. Perhaps do three comparisons (i) isoprene alone; (ii) isoprene + a-pinene + b-pinene for MOM, MCM and JAM002; (iii) the existing comparison for all mechanisms except MIM1/MIM2, with an explanation of how the extra terpenes were assigned in the MCM and JAM002 simulations. This sequence of comparisons would help the reader to see the origin of the performance differences. The current illustration alone is of limited value and is insufficient.
I also have some minor and typographical comments, as follows:
Page 1, line 1: Insert space between “4.0” and “of”.
Page 2, line 10: The authors seem to have missed the point concerning my previous comment about the use of the terms “VOCs” and “NMHCs” – perhaps I was not as clear as I should have been. As shown in Fig. 1, the emitted VOCs are not only hydrocarbons (HCs), but oxygen- and nitrogen-containing organic compounds too. Therefore, “CH4 + NMHCs = HCs” or “CH4 + NMVOCs = VOCs”, but CH4 + NMHCs ≠ VOCs”. My original query was really why not use the term VOCs (or NMVOCs) throughout?
Page 2, line 18: A very minor point, but I think “primarily” should be “primary” when referring to the emitted species (or could be omitted without changing the meaning). "Primary" is an adjective qualifying the species as distinct from “secondary” species. “Primarily” is an adverb (e.g. as used on page 12, line 12) describing the primary purpose of something, i.e. meaning “mainly” or “for the most part”.
Page 4, line 35: “It is” rather than “It’s”?
Page 7, Fig. 3 caption: Is it “MIM1” or “MIM2”?