Dear authors,

I have received further comments from the reviewers. Reviewer 1 has some minor comments that I copied below. Reviewer 2 comments are attached as pdf. Please address the critical comments from Reviewer 2 in detail and revise the manuscript accordingly.

Best,
Sally

--------------------------
Reviewer 1 comments
I thank the authors for addressing my comments. The manuscript is largely improved. I recommend publication after the authors address the following comments.
1. In Figure 4(a), please add the variation of NO+/NO2+ ratio as a function of diameter.
2. In Eqn. (9), the organic nitrate yield should be included to calculate “production potential of organic nitrates”. 𝐾𝑖∙[𝑉𝑂𝐶𝑖]∙[𝑁𝑂3] only represents the loss rate of NO3 radical.
3. When using Eqn. (11), I suggest to use “potential SOA formation” in the discussion.

Dear authors,

Thank you for your response to the reviewer's comments. I think the manuscript has been further improved. I have received additional comments from the reviewer #3 (copied below).

Further, I have two comments:

1. SOA yield is dependent on organic aerosol mass. When using the SOA yields in literature to estimate SOA from VOC+NO3 (Table 3), please ensure that the yields used are from laboratory experiments that covered the relevant range of aerosol mass in the field campaigns. Please clarify this in the revised manuscript.

2. There are a number of grammatical mistakes / typos, e.g., line 303 should be "associated with", etc. Please correct them.

Once the reviewer's and my comments are addressed, the manuscript can be published in ACP.

Best,
Sally

------
The authors have continued to improve the manuscript with their revisions, including additional evidence and framing the discussions more appropriately. I recommend publication after one further consideration.

Regarding the discussions of the size distributions of NO+ and NO2+ ions. It is good the authors have clarified that this is in fact UMR m/z 30 and m/z 46. Consequently, all denotations of those should be changed to m/z 30 and m/z 46 as NOx+ ions should only be used to indicate HR-resolved ions.

However, the discussions/conclusions regarding those size distributions are still a bit under-supported/overstated. While it is stated that <10% of those ions were from CH2Ox+ ions, on average for the HR-MS data, there are 3 reasons that does not suffice along to then go on and make a strong argument that the size-dependent trends in m/z30 / m/z46 demonstrated organic nitrates dominate at small sizes and are not present at large sizes:

1) The bulk MS data may be dominated by concentrations at larger sizes, thus the CH2O+ ions may in fact comprise much larger fractions than 10% compared to either NOx+ ion at the smaller sizes. In fact, for biogenic SOA, it would be surprising if that were not the case. See published spectra of biogenic SOA. For example, in a comparison of UMR and HR nitrate quantification, Fry et al. [2018] showed that on average for a biogenically-influenced region, CH2O+ was half of the signal at m/z 30. The author could estimate some rough upper limits that the presence of interfering ions could have on the actual NOx ratio at lower sizes. At minimum, this aspect should be acknowledged.

2) The average contributions shown in Fig. S10 as HR spectra, are season averages. It is not clear to a reader if they are representative of the nighttime periods of interest. An analysis using the time-dependent relative contributions could have been more useful.

3) Just because the contribution to nitrate at lower sizes appears to be more from organic nitrates, does not mean they are not present at larger sizes. There seems to be some faulty logic there. This was the reason I suggested in the past review to calculate size distributions of organic and inorganic rather than only framing the discussions in terms of the inorganic/organic relative apportionment which is less relevant to the scientific arguments being made in this paper about organic nitrate production. Maybe the inorganic nitrate is just less present at smaller sizes.

Fry, J. L., Brown, S. S., Middlebrook, A. M., Edwards, P. M., Campuzano-Jost, P., Day, D. A., Jimenez, J. L., Allen, H. M., Ryerson, T. B., Pollack, I., Graus, M., Warneke, C., de Gouw, J. A., Brock, C. A., Gilman, J., Lerner, B. M., Dubé, W. P., Liao, J., and Welti, A.: Secondary organic aerosol (SOA) yields from NO3 radical + isoprene based on nighttime aircraft power plant plume transects, Atmos. Chem. Phys., 18, 11663-11682, https://doi.org/10.5194/acp-18-11663-2018, 2018.