|The authors have made reasonable efforts to address the concerns of the reviewers, while largely restricting themselves to modifying the paper text rather than performing new analysis. One of their key figures (Fig. 5) is still quite confusing. They have, however, made substantial and valuable additions to the supplementary materials, with a great deal of additional explanation. |
The authors’ restricting themselves to only publicly accessible datasets is a somewhat artificial justification for limiting the scope of the study, considering that the author list includes the PIs responsible for the collection of a high fraction of all AMS datasets currently in existence. Of course, the close collaboration of experimental and modeling PIs is necessary even when only publicly available data is used, but it was a little disappointing the authors did not at least include the results of WINTER 2015. Nevertheless, I appreciate that significant effort has gone into the study already and I think it is reasonable for the authors to follow this up in subsequent papers.
Overall I consider the paper a valuable and interesting contribution to the literature and I recommend accepting it subject to the minor comments below.
Figure 4 shows very large differences between complex and simple schemes in China, and I did not find a dedicated discussion of the reasons for this. Yet the complex and simple schemes are well correlated, and both overestimate OA, in the outflow region (if we assume Korea is the outflow region). At line 626 you might expand on your existing comments, perhaps referring specifically to this and emphasising the large uncertainties associated with IVOCs which some authors suggest could be responsible for more SOA than the Pye and Seinfeld (2010) treatment would suggest – for example, see Zhao et al, Sci Rep 2016.
Figure S1, S4 please say in the caption what the colors represent, or remove them.
Fig 6,10,S2 I still think the authors should use red for sulfate and other colors which do not form part of the AMS scheme, such as purple, for their various organic schemes.
Initially I found the sentence “ISOA and Org-Nit are generated exclusively through the aqueous uptake pathway and do not include any ‘non-aqueous’ OA” confusing, because I read ‘aqueous uptake’ to mean ‘uptake by cloud water, then cloud processing’. And Fisher et al use the term ‘uptake’ but not ‘aqueous uptake’. Of course there is no cloud processing here– the pH of the droplets considered by Marais et al more or less precludes them being activated. But it would be good to make this clearer in the text.
The lab study of Brégonzio-Rozier et al (ACP 2016) cited by Marais et al as a justification for not considering aqueous formation of SOA from isoprene via cloud processing found a very small mass yield of in-cloud isoprene SOA. However, if I interpret their Figure 1 correctly, they did observe a strong enhancement compared to conditions that were first very dry, and then at 80% RH, suggesting that cloud processing could be important compared to ‘aqueous uptake’ in some conditions -I think it’s too early to tell. I realise cloud-processing of SOA is still hugely uncertain and hard to parameterize, but perhaps the authors might briefly mention and discuss the body of research on in-cloud SOA? In-cloud SOA would presumably correlate with aqueous uptake, so I guess it is possible that when the authors find improved performance by replacing VBS with aqueous uptake for isoprene, they are in fact compensating, to some extent, for in-cloud production?
In the SI, it would be good to clarify that the C5-LVOC lumped product, despite its name, still doesn’t partition via the VBS, instead via aqueous uptake. This is clear from the text further down, as C5-LVOC features in the long list of stuff that forms non-volatile aerosols irreversibly, but nevertheless, retaining the name C5-LVOC is potentially confusing.
Would it be possible to include a version of Figure 5 which excludes the upper troposphere as a supplementary figure, so that the surprising classification of the southeastern US as remote can be further clarified, and the different altitudes can be compared?