|Overall, I am very pleased with how thoroughly the authors addressed my first comments, and I am ready to support the paper for publication, as long as one major comment is addressed. As an aside, I congratulate the authors in that I have overheard sincere community interest in the ACPD paper from my colleagues.|
Major comment based on Specific comment 4b: Given that 62 different clouds were examined, it was not clear to me why a select few were chosen for more detailed analysis. Please provide this information.
Response – There were 62 cloud “samples”. The numbers of individual clouds were fewer, since more than one profile was conducted through a cloud layer. We have added a few words to clarify this and the reason for the selections on lines 281-290.
New comment from the reviewer: Ah, I clearly misunderstood that there were 62 independent, separate clouds sampled (to me, the terms “cloud averaged data point” or “Forty-five cloud samples” (e.g., line 650) imply independent samples, taken from separate clouds). Now that I know that the points are not averages from single clouds, but rather averages of penetrations within clouds, I suggest replacing the terminology of “cloud-averaged data points” and “cloud samples” in the paper. Despite the new text added on lines 281-290, these terms may mislead other readers (particularly, but not only, if they are skimming the abstract and conclusions), and there are more accurate terms that can be used. At minimum, I suggest rephrasing to something like “the average of individual cloud penetrations”.
However, I strongly believe more analysis may be required here. Individual cloud penetrations within the same cloud (e.g., Fig. 5), should not be treated as independent samples, as they are in the text. Failing to do so will bias the results (e.g., Tables 1 and 2) toward the characteristics of clouds with the most profiles sampled.
As it was not stated how many independent clouds were sampled, particularly for the HA cases, it is hard to fully evaluate the impact of this issue on the results. It would be helpful if the authors could provide as much information as they can about how many independent clouds they actually sampled. Something along the lines of what the authors show with the new supplemental Figure 7 is helpful, but alone it is not sufficient. I also strongly recommend that the authors redo the statistics with independent clouds separately. If the findings are not still the same, further discussion in the text will be needed.
• Regarding the following newly added sentence: “Variations in particle chemistry
will induce some variance in these results, but because activation diameters are estimated starting with larger particles and moving to smaller sizes, changes in chemistry only offer the possibility of activation of particles still smaller than
estimated here.” My understanding is that size and hygroscopicity are two opposing constraints on activation, and thus depending on the situation, activation
could depend on either. I suggest the authors rephrase this sentence.
• Why add in the Koellner reference at all? It is fairly long and not as descriptive
as simply saying, as the authors did in their response, that: 1) back trajectories suggested that the air arrived from Canadian forest fires, 2) there was an [~20 ppbv?] increase in CO concentrations compared to other days sampled that month, and 3) the size distribution transitioned to larger sizes, which suggests some BB influence?
• Line 83: “Lohmann and Leck (2005) hypothesized the need for highly surface- active particles to explain CCN [activity?] at S less than 0.3%.”