Dear authors,

Thanks for your careful response to the reviewers' comments. Reviewer 1 has further comments (see below). Once these are addressed, the manuscript will be accepted to ACP.

Best,
Sally

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Additional comments from reviewer 1

First, one needs to make sure that one does not get the wrong impression of the importance of daytime vs. nighttime formation of nitric acid leading to aerosol nitrate formation. Doing an eye-ball integration under the curve in figure 5b (now 6b) leads to an estimated HNO3 of about 0.6ppb (or about 1.5 ug m-3). Doing the same for 6c leads to about 0.18 ppb (they might want to do this more precisely). Further, they need to provide further support for their statement “Although most of the HNO3 comes from the daytime reaction of OH with NO2, since the deposition rate of HNO3 is very 20 high, the main pathway leading to the formation of PNO3 is the hydrolysis of N2O5 at night, when the temperature is sufficiently low for partitioning to the particle phase.” They could do this by switching of the N2O5 hydrolysis route, or, better yet, have the product of that reaction to be a marked HNO3. It also appears Fig. 6d (used to be Fig. 5d) changed markedly. What was the reason for this change? The revised plot decreases the apparent difference.

They should still make sure it is clear to the reader that the PSAT result is not the same as the source impact. This is clear in Fig. 7. Given that the nitrate is ammonium nitrate, either the availability of nitrate or ammonium controls formation. In Fig. 7, it shows that most of the NH4 is from SNAP10, while little of the NO3 is from SNAP10 (largest contributor is SNAP7). If one were to remove SNAP10 emissions, much of the PNO3 would be removed. Thus, sentences like “Road transport (SNAP7) was predicted to be the most important source for PNO3 with the largest contribution during the cold season…” should be given context, potentially in the conclusions, as well as further discussion of this when PSAT is originally discussed. For example, the statement “While PSAT quanitifies the source of the the ammonium or nitrate in the PM2.5, this is not the same as the source impact as the system may be limited by another component. For example, removing SNAP10 emissions would not only reduce ammonium, but nitrate as well.”

Dear Authors,

Thank you for your response to the further comments by the reviewer. I think you have addressed majority of the comments adequately. However, to strengthen the manuscript, I suggest providing further support for the statement “Our results suggest that the main pathway leading to the formation of PNO3 is the hydrolysis of N2O5 at night, when the temperature is sufficiently low for partitioning to the particle phase”, or, re-phrasing the discussion to emphasize the uncertainties. There is N2O5 left over from daytime chemistry. As the reviewer noted, this statement can be evaluated by switching off the hydrolysis of N2O5 in the model. This should be fairly straightforward so I encourage you to consider revising this part of the manuscript to make it stronger/more accurate. (If you prefer not to do this, please include more discussions on the uncertainties).

On a different note, I suggest including Xu et al. (PNAS, 2015), Xu et al. (ACP, 2015), and Kiendler-Scharr et al. (GRL, 2016) in the citation for the sentence “Reactions of isoprene lead to the formation of SOA mainly during the daytime while nighttime oxidation of monoterpenes by the nitrate radical is responsible for organic nitrate formation” in the introduction, which are all relevant to the discussions here.

Thank you for your submission to ACP.

Best,
Sally