General comments:

Li et al. present a model analysis of secondary inorganic aerosol (SIA) in major cities of Eastern China.  They show data of PM_2.5 from a network across China from 2012 to 2016 and speciated PM_2.5 from major supersites in Eastern China.  They analyze the response of sulfate and nitrate to the reductions in NO_x that have occurred in the 4 year period, and to a theoretical series of reductions spanning 10 to 80%.  Due to the response of oxidants to NO_x, the changes in SIA are approximately linear in summer but highly non-linear in winter, indicating NO_x saturation of oxidant production in winter.

The results are relevant to the understanding of air pollution and its recent trends in China and is of interest to the readership of ACP.  Publication is recommended following attention to the specific comments below.

Specific comments:

Line 71: Neither of the stated assumptions, stagnant atmosphere or a fixed NO₂/NO_y ratio, is a condition for a box model.  Suggest eliminating this statement.

Line 77-78: Is the NO_x sensitivity of Chinese ozone a well accepted result?  Increasing O₃ trends have been attributed, for example, to effects associated with decreasing PM_2.5, e.g.,

Li, K., D.J. Jacob, H. Liao, L. Shen, Q. Zhang, and K.H. Bates, Anthropogenic drivers of 2013–2017 trends in summer surface ozone in China. Proceedings of the National Academy of Sciences, 2019. 116(2): p. 422.

Line 248: It appears that the oxidation ratio of sulfur decreases, but nitrogen increases, between the warm and cold seasons.  Is that correct?  In this case, there is more going on than just enhanced secondary oxidant formation.  There is also a change in chemical mechanism between S and N.

Line 249: Here and above, specify more clearly what is meant by the two pathways – presumably this is the difference between OH + NO₂ (gas phase) and N₂O₅ hydrolysis (heterogeneous), but it is not clear from this text or figure 6 what this specifically means.

Line 258-259: Two comments.  First, the global burden of nitrate production pathways may, or may not, be relevant at a regional level.  Second, there are more updated papers indicating that the two pathways considered here are more equal at the global scale.

Alexander, B., T. Sherwen, C.D. Holmes, J.A. Fisher, Q. Chen, M.J. Evans, and P. Kasibhatla, Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations. Atmos. Chem. Phys., 2020. 20(6): p. 3859-3877.

Line 275: The responses do not appear “bell shaped”.  Is there a better descriptor?

Line 301-319: The changes in sulfate in Figures 8 and 9 is quite difficult to discern, so it is not easy to match this text to the changes in the figure.  Can sulfate be plotted on its own scale to illustrate these changes?  Also, the small dependence of sulfate on NO_x indicates that OH is also not a strong function of NO_x in this model, but later the OH changes are quoted as being large compared to the changes in sulfate.  If OH is the most important factor for sulfate changes, why are the modeled sulfate changes so much smaller than the changes in OH?

Line 329-330: Same comment for sulfate.  Changes are very difficult to see.

This paper investigates the secondary inorganic aerosol formation in cities in China focusing on nitrate aerosol, using observed data from a Chinese network of station and a WRF-Chem mesoscale model to analyze the response of PM2.5 to NOx emission changes on a seasonal basis. The main findings are the almost linear response of secondary inorganic nitrogen to NOx emissions changes during summer while in winter the system is highly non-linear. Sensitivities to ammonia and sulfur dioxide emissions, which have been shown to be important, are discussed only based on existing literature. The paper is within the scope of Atmos. Chem. Physics and could be suitable for publication in the journal after a number of improvements suggested below.

My major comment is that the authors miss to show a clear view of the interplay between O3, SO2, NOx, NH3 and PM2.5. The authors provide some information in this respect in the conclusion but the manuscript can be largely improved in this respect. Therefore, I would suggest that the authors check on relevant references and integrate some of them in the introduction or discussion session. Some are listed below but more might be found by a literature search.

For instance, they could take notice of the following relevant publications for NO2, NH3, SO2 interactions:

Yu, G. et al. Nat. Geosci. https://doi.org/10.1038/s41561-019-0352-4 (2019) and relevant News and Views comment: https://doi.org/10.1038/s41561-019-0358-y

Li, K. et al. PNAS, 2019, 116(2), 422.

For O3 in China the paper by Liu and Wang, ACP 2020 https://doi.org/10.5194/acp-20-6305-2020

The paper by Nenes et al ACP in ACP on aerosol’s sensitivity

Nenes et al., ACP, 2021 https://doi.org/10.5194/acp-21-6023-2021 on the impact of pH on the partitioning and therefore the lifetime of nitrate and ammonia aerosols

Nenes et al., ACP, 2020; https://doi.org/10.5194/acp-20-3249-2020, 2020 on the sensitivity of PM mass to ammonia and nitrate availability

The papers on HNO3 formation from other pathways than the NO2+OH reaction (there might be more)

Brown, S. S., et al. 2004, Geophys. Res. Lett., 31, L07108, doi:10.1029/2004GL019412.

Vrekoussis et al 2005, Geophys. Res. Lett., 33, L05811, doi:10.1029/2005GL025069

Karydis et al., ACP, 2015, www.atmos-chem-phys.net/16/1491/2016/

Specific  comments:

Line 189: capture

Line 194: MB

Line 211-212: that the consideration of the optimized… reduces…

Lines 228: was more than …

Line 232: (because OM has more than C, O and H) I suggest adding ‘mainly’ for the hydrogen and oxygen

Line 235: could you provide standard deviation for percent fractions ?

Lines 236: SIA are the dominant components of PM2.5 – 48.6% is not the majority – it is below 50%

Lines 237-242: please explain the divers, are temperature, liquid water content of the aerosol impacting these ratios

Lines 258-259: discussion could profit from additional literature – also heterogeneous reactions on dust can produce nitrate aerosol (see references above)

Line 267: nonlinear behaviour in winter

Section 3.3.1 & 3.3.3, I miss here the discussion on the involvement of SO2 emission changes to PM2.5 aerosols and thereby to O3 and nitrate aerosol

Line 310: have minor changes

Lines 312, 313,345-348: How significant are these correlations?

Line 320: do not forget acid replacement reactions (e.g. reactions on dust or sea-salt aerosol)

Lines 324-325: do not forget here the reduction of NO + O3 titration reaction

Line 337: reactions

Line 391 move ‘during summer-autumn’ to the end of line 392

Lines 408-415 and 419-427 would fit better in introduction of discussion. Please move appropriately.