Review of “Long-term trends and drivers of aerosol pH in eastern China” by M. Zhou et al.

This study presents the results of nine years of hourly gas and particle composition measurements in Shanghai, China.  Trends in aerosol pH over the measurement period are examined, as are the factors driving variability in aerosol pH.  The measurement period includes a major reduction in some emissions due to a nationwide regulation, so the resulting effects on aerosol pH can be examined.  Further, aerosol pH over the next 30 years is modeled under three future emissions scenarios.  Overall, the results are novel and add important insight to the growing body of work characterizing aerosol pH, including drivers and trends.  The writing is mostly clear, though specific areas for improvement are noted below.  I recommend the manuscript for publication after the following issues are addressed:  

Major Comments

• The estimates ALWC_o seem unreasonably small (lines 120 - 127)? How was organic aerosol measured?  Was it PM_2.5 as well, or was it PM₁?
• I found the convention used in Figures 1b, 3, and 5 very confusing. The pie charts below each figure are useful and seem straightforward to interpret, but the bar charts need substantial editing.  For example, in Figure 1b, the effect of NVCs on the pH trends changes signs with time.  Ultimately, using Fig. S6, I was able to deduce that the positive value associated with NVCs for 2011-2013 meant that NVCs had gone up, and the negative value associated with NVCs for 2013 – 2015 meant that NVCs had gone down.  However, it took far too much time to interpret and is still not easily understandable even after spending much time on it.  The convention used by Tao and Murphy (2021) is much clearer – I suggest edits to follow their approach.  
• Discussion about the limited effects of future emissions control measures on haze pollution (e.g., line 35-36, 298-299) is just wrong. Although the partitioning of NH₃ and HNO₃ may shift towards the particulate phase in the future, it does not mean their total PM concentration has increased.  If the total concentration (i.e., NH₃ + NH₄⁺) decreased enough, then a shift in partitioning towards the particle phase could still occur with a decrease in the aerosol NH₄⁺  This discussion would be much better with associated predictions of the PM_2.5, NH₄⁺, SO₄^2-, and NO₃^- aerosol concentrations.
• The Conclusions section needs substantial revision. A brief recap is ok, but Section 4 is mostly redundant with the prior section.  Rather than just reiterating what has already been said, more discussion of the significance of the work is warranted. 
• Finally, the entire manuscript needs to be edited for language consistency – specifically, verb tenses change within and between paragraphs. There are too many instances to list here.   

Technical/Minor Comments

• Line 25: define all acronyms on their first use (e.g., NVCs, YRD)
• Line 28-29: sentence needs grammatical editing.
• Line 39: suggest deleting “studies”
• Line 44-45: cite also Tilgner et al. (2021) in this group.
• Line 73: cite also Vasilakos et al. (2018) and Nenes et al. (2020)
• Line 77: “composition” should be singular
• Line 78: suggest changing “characterizing” to “characterize”
• Line 96: change “to be” to “were”
• Line 96: “calibration” is not the right term here – LiBr is used as an internal standard
• Line 102: due to unmeasured species (organic acid ions, carbonate) – it is quite possible to measure the aerosol inorganic composition accurately and not achieve an ion balance. Given what we know about organic acid concentrations, it is actually surprising that such a balance is observed.
• Line 104: cite also Stieger et al. (2018)
• Line 106: suggest deleting “techniques”
• Line 107: give the instrument(s) used to measure T and RH
• Line 128: cite also Battaglia Jr., et al. (2019)
• Line 237-238: the diurnal behavior of aerosol pH is not just consistent with Beijing, but is far more consistent (qualitatively) with many other locations like the SE USA (Guo et al., 2015), eastern US (Battaglia et al., 2017), Chicago (Battaglia et al., 2017), which shows the important influences of T and RH on aerosol pH.
• Line 274: “active actions” should be changed
• Line 284: comma needed before “respectively”

References

Battaglia Jr., M. A., Weber, R. J., Nenes, A., and Hennigan, C. J.: Effects of water-soluble organic carbon on aerosol pH, Atmos. Chem. Phys., 19, 14607–14620, https://doi.org/10.5194/acp-19-14607-2019, 2019.

Nenes, A., Pandis, S. N., Weber, R. J., and Russell, A.: Aerosol pH and liquid water content determine when particulate matter is sensitive to ammonia and nitrate availability, Atmos. Chem. Phys., 20, 3249–3258, https://doi.org/10.5194/acp-20-3249-2020, 2020.

Stieger, B., et al., Measurement of PM10 ions and trace gases with the online system MARGA at the research station Melpitz in Germany – A five-year study, J. Atmos. Chem.,  75, 33-70, 2018.

Tao, Y., Murphy, J. G., Simple Framework to Quantify the Contributions from Different Factors Influencing Aerosol pH Based on NHx Phase-Partitioning Equilibrium, Environ. Sci. & Technol., 55, 15, 10310–10319, 2021.

Tilgner, A., Schaefer, T., Alexander, B., Barth, M., Collett Jr., J. L., Fahey, K. M., Nenes, A., Pye, H. O. T., Herrmann, H., and McNeill, V. F.: Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds,  Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-58, accepted for publication, 2021.

Vasilakos, P., Russell, A., Weber, R., and Nenes, A.: Understanding nitrate formation in a world with less sulfate, Atmos. Chem. Phys., 18, 12765–12775, https://doi.org/10.5194/acp-18-12765-2018, 2018.

This manuscript uses a long-term dataset of hourly observations of inorganic aerosol constituents and water-soluble gases from Shanghai to quantify the drivers of aerosol pH variation over a decade during which substantial air quality regulations were introduced in China. The authors examine long-term trends from 2011-2019, which allows them to assess the impacts of the Air Pollution Prevention and Control Action Plan introduced in 2013. They also project how future emissions reductions may change aerosol mass loadings and pH. This is a very novel and interesting analysis.

Overall, my main concern with the manuscript is that the methods section is much too brief. The authors need to provide a lot more explanation of how they generated the data that are presented in Figures 1, 3, 5, and 6. Since each of there figures represents perturbations or trends to some previous averaging period, the way in which the data are averaged (and perturbed) needs to be explained more clearly. For the long-term trend in Figure 1, the approach seems fairly obvious, but the way in which the seasonal and diel cycles are formulated in Figure 3 and 5 is quite confusing.

Specific comments:

P1 L33-36 It is hard to understand the meaning of this sentence. Are the authors suggesting that NH3 and NOx emission controls are not going to be effective in the time period leading up to 2050, or that they won’t be effective after that point? Further – is it appropriate to examine the values just on their own – what would happen if only SO2 reductions were implemented? Presumably NH4+ and NO3- would increase much more.

The data in Figure S2 look much more tightly correlated in the later years. Can the authors comment on whether this reflects improvements in the accuracy/precision of the measurements or whether the relative importance on the measured ions to the overall ion balance may have changed?

The sensitivity tests mentioned in Section 2.3 (lines 143-152) and in Figure S4 are not sufficiently well-described. What did the authors alter and what did they hold constant in each test? The results appear to span a different range of ALWC for each variable. In general, I did not find this section added much to the manuscript, and only made me confused about the method. I suggest removing this section unless it can be much more clearly explained.

The language in Section 3.4 is a little confusing because the authors describe the changes in absolute amounts of particle and gas phase ammonium and nitrate somewhat interchangeably with their partitioning ratios and it becomes hard to keep track of what metric is being described. It might be more clear to focus first on the absolute particle phase amounts and then explain the changes in the context of the partitioning. The sharp increase in the particle phase partitioning of nitrate between 2040 and 2050 is quite difficult to understand in the BHE scenario – what explains it?

Figure S6 – The caption should explain why are the rates of change only calculated starting in 2013 when the data record starts in 2011. And why a separate slope is calculated for the latter part of the record.