|The authors have addressed most of my comments. I have a few further comments that would like the authors to address.|
Firstly, as the manuscript currently stands, it appears that the organic nitrates being discussed are solely organic nitrates formed from OH+a-pinene. However, as the authors acknowledged and agreed, that some of the a-pinene is expected to react with ozone. The organic nitrates formed from different oxidations (OH, ozone, NO3) can have different hydrolysis lifetimes and fates. With this, I think it is imperative that the authors be more upfront and make it clear throughout the manuscript that the organic nitrates being discussed can be from OH+a-pinene and ozone+a-pinene.
Secondly, as the authors also acknowledged, that there are likely further reactions ongoing after the lights are turned off (figure 3). With this, the authors need to further discuss what this implies regarding the calculated gas-particle partitioning coefficients of organic nitrates. If there are further reactions (and that such reactions contribute fairly substantially to the observed organic nitrates), this would introduce large uncertainties in the concentration of the particle-phase ON and affect the values of the calculated partitioning coefficient.
Please see specific comments below.
1. Assumption that only five forms of nitrogen are present: What about HO2NO? The use of H2O2 as OH precursor leads to formation of HO2. In the presence of NO2, does this result in the formation of large amounts of HO2NO2?
2. Uncertainties in H2O2 and [OH]: The authors need to include the discussions in the uncertainties in the revised manuscript (i.e., as noted in their response, injection method, and chamber UV spectrum). It is reasonable that if the H2O2 is introduced via bubbling, there will be larger uncertainties in H2O2 and [OH]. However, I’d be quite surprised if the second method (withdrawing a known amount of H2O2 and injecting that into the chamber with air) also introduced large uncertainties. This method has been used widely in many recent laboratory chamber work, and I am not aware that this method would lead to large discrepancies in estimated [OH] based on models (from H2O2 concentration and cross section) vs. based on VOC decay. The authors noted that the model matched data for NO, NO2, and ozone at H2O2 levels lower than those injected. How much lower? Please include this info in revised manuscript. The level of H2O2 has direct implications on [OH] and fate of a-pinene (i.e., OH vs. ozonolysis) and possibly the types of organic nitrates formed and related discussions.
3. The authors seem to argue that it does not really matter whether a-pinene is reacting with ozone or OH, and noted that “the interest of these experiments is the organic nitrates formed in the oxidation of α-pinene through NO reacting with the peroxy-radical” and “ozonolysis of α-pinene is not expected to result directly in ON”. I do not agree. Ozonolysis of a-pinene can also lead to the formation of peroxy radicals, which can also react with NO to form organic nitrates. A recent work by Draper et al. (2015, ACP) showed the formation of N-containing species in ozonolysis of monoterpenes in the presence of NOx. As the reviewer pointed out earlier and the authors agreed, the relative amount of different types of organic nitrate (e.g., primary, secondary, tertiary) can be quite different in NO3 and OH oxidations. If the organic nitrates discussed in this manuscript have contributions from ozonolysis (and this seems very likely from the data presented in the manuscript), the authors need to discuss this clearly in the revised manuscript and make the readers aware that the organic nitrates being discussed are not just organic nitrates from OH+a-pinene.
4. The authors responded to my question about the uncertainty in m/z 30 (organic interference). They need to include a sentence in the revised manuscript to make the readers aware of this uncertainty.
5. Continued reaction after the lights off but before temperature increases (figure 3): The authors agreed that there is continued reaction but noted that “Despite these limitations it is clear that both the Org/SO4 and ONaer/SO4 ratios decrease with heating, consistent with semi-volatile organics and organic nitrates.” I agree with this, but I also think that this can introduce large uncertainties in the calculated ON partitioning coefficient and this must be made clear in the revised manuscript. In this work, the partitioning coefficient is calculated from particle-ON data from ACSM and modeled gas-phase ON. If the particle-ON shown in the figure has interference from other processes (i.e., continued reaction), this would directly translate to large uncertainties in the partitioning coefficient. Obviously this depends on the extent to which such continued reactions are occurring, but judging from the continued increase in org/SO4 after the lights are turned off (I won’t be able to tell when the lights are turned off based on the data), it is clear that the contributions of these continued reactions (to org, and to ON) are not small.