|The authors have done a thorough job with the revisions and the manuscript has greatly improved. I only have two more comments:|
1. Role of vapor wall loss:
The authors added a section on “Wall loss of organic vapors”, Figure S4, and some other related discussions. I was not aware that the particle surface areas in these studies are so much lower than those in previous studies. In this case, I agree with the authors’ comment that SOA yields from a-pinene ozonolysis or photooxidation can depend on particle surface area, when surface area is this low.
However, my original question remains: what is the role of vapor wall loss in the interpretation of the data (differences in yields under different NOx and SO2 conditions) and conclusions presented in this work? The authors noted in the revised manuscript “For the aerosol surface area range in most of the experiments in this study, the influence of vapor wall loss on SOA yield was relatively small (<~40% for particle surface area larger than 3×10-6 cm2 cm-3, Fig. S4). Yet, for the experiments at high NOx and low SO2 for α-pinene and limonene, the influence of vapor wall loss on SOA can be high due to the low particle surface area, especially at lower αp.”.
Based on Figure S4, the value of fcorr is roughly 12 and 4 for limonene/highNOx/lowSO2 and a-pinene/highNOx/lowSO2, respectively. Please also indicate the values of fcorr for other experiments (other combinations of NOx and SO2 for limonene and a-pinene experiments) on Figure S4. Based on the above sentence in the revised manuscript, is it correct that for all other experiments, the fcorr is lower than 1.4?
If so, how shall one interpret the differences in SOA yields shown in Figure 3, in the context of the effect of vapor wall loss on SOA yields? The high NOx / low SO2 data have the lowest yields, and these are the experiments with lower particle surface areas where yields can be drastically underestimated due to vapor wall loss. In this case, the higher SOA yields obtained in high SO2 experiments could potentially be explained as a result of a smaller extent of vapor wall loss in these experiments? And, if one can correct for the effect of vapor wall loss, the yields under different NOx and SO2 experimental conditions might be similar?
Overall, more clarification is needed regarding the role of vapor wall loss on the data interpretation and conclusions in this work.
2. Organic nitrate volatility:
In line 310 the authors noted that the volatility of organic nitrates is not low enough to nucleate; in line 364 it was noted that the organic nitrates are highly oxygenated and expected to have low volatility. Putting these together, are the authors suggesting that the organic nitrates are likely of low volatility but not low enough to nucleate? Please clarify this in the revised manuscript.