|This paper describes results of modeling particle measurements in stirred-flow reactor chamber experiments consisting of dark and light-induced reactions of organic compounds emitted from plants (BVOCs) in the presence of ozone, SO2, and NOx. The model used a relatively detailed atmospheric mechanism to represent the gas-phase reactions with additional lumped organic reactions to represent particle formation and with a nucleation and particle growth model. Effects of different assumptions regarding the relative importance of H2SO4 and condensable organic products in the nucleation model were examined. Fair fits were obtained to the particle number and volume data, though parameters had to be adjusted to fit these data. The subject matter may be appropriate for publication in this journal, though as discussed below the irradiation experiments are not good representations of atmospheric conditions.|
This is the third version of a manuscript that I previously reviewed. This second version incorporates several improvements in the model employed, gave much better discussions of the methods and results, and had much more modest and appropriate conclusions. The main conclusion is now that the type of experiment being modeled is not ideal for evaluating mechanisms for new particle formation, though they can fit their data with several reasonable models. The results are suggestive but not conclusive, and overall the tone of the conclusion is appropriate given this. Even though the results are not conclusive the experimental and modeling results shown are of interest and do indicate effects of varying assumptions that can be made for the conditions that are studied. Because of the interest in particle formation and the complexities in modeling it, this paper makes a sufficient contribution for it to be appropriate for this journal.
Although the 2nd version was a significant improvement over the previous version, I still had some problems with the discussion and presentation of the methods and results in some cases that I thought the authors should consider. I believe the current (3rd) version and the authors' response to my previous comments addressed my major concerns, and I believe it is acceptable for publication after addressing only a few minor issues, discussed below.
In Section 3.1, Page 6, lines 15-20, they say that they represented only the initial reactions of OH, O3, and NO3 with some compounds whose subsequent reactions were not represented in MCM "considered as a sink of OH, O3, and NO3 without any other influence on ... gas-phase chemistry. However, at least the OH reaction are not really sinks for OH, they regenerate most of the OH back after some NO to NO2 conversions. The NO to NO2 conversions (which affect ozone formation and, indirectly, radical levels) are also ignored in the O3 and NO3 reactions that are neglected. This isn't important in this application because the error in ignoring the regenerated OH doesn't matter, since they are using OH calculated from consumption rates in the model, and their main interest is calculation of rates of formation of particle precursors, not O3. However, it would have been less of an approximation to use reactions of the most similar MCM species, as they did with the other compounds considered more important. They don't need to re-do the calculation, but just point out that this approach affects accuracy in the model in predicting radicals and ozone. Note that the discrepancies shown on Figure 3 cannot be explained by this omission in the model.
It might be useful to include in Table S1 a footnote indicating how the subsequent reactions were represented, which could be (1) based on MCM for this compound, (2) based on MCM for another compound (state which), or (3) only the initial reaction is represented, not the reactions of radicals or species formed. This is stated in the text, but having the footnotes here is a good reference and may allow shortening the text if desired by the editor.
The curves with the yellow dashed lines on Figure 4, which seem to be exactly on top of the solid black line, should included in the legend on the figure as well as the figure caption.
I didn't notice this in my previous review, but it looks like Table 2 has three columns. It might be helpful to have column headers in this case.
In Section 4.2.1, page 22, lines 4-6 they state that their model simulations shown on Figure 6a with FEP parameters was about 2-3 times higher than observed. They should say this is for Day 1. For Days 2-4 the overprediction wasn't near as much (about 50% higher).
It would have been helpful to me if Table 3 showed which simulations used "R1" and "R2" so the reader doesn't have to refer back to elsewhere in the text to see what this means. Either that use terms that correspond to the captions on Table 3 when discussing Table 3 and Figure 11 in the text. My understanding is that R1 corresponds to "SQT ox by O3" and R2 corresponds to "SQT ox by OH". If I am not understanding this correctly, then the discussion has to be clarified.