Comment on “The shifting of secondary inorganic aerosols formation mechanism during haze aggravation: The decisive role of aerosol liquid water”

The aerosol liquid water plays a profound role in secondary inorganic aerosols. reported. Fully understanding ALW and its roles are fundamentally important in atmospheric physicochemical processes, especially the liquid chemical transformation of SO2 and NOx. The manuscript gave measurement results of the secondary inorganic aerosol properties under different aerosol liquid water content. The results are interesting and the manuscript can be published after denoting the following comments.

Major Comments:

1 This manuscript is submitted as a full research paper to the ACP. The data acquisition and analysis method should be placed in the main manuscript but not in the supplement. At the same time, some calculating methods and definitions were not described in the manuscript.

2 The subfigures in figure 3, figure 5, and some others in the supplementary materials should be in parallel with the main one. I recommend these figures be reorganized.

3 Section 3.1 noted that 2019). The calculated results (Supplement, S2.2) showed the “predominant chemical species of ammonium gradually varied from the coexistence of ammonium sulfate ((NH4)2SO4) and ammonium nitrate (NH4NO3) to the coexistence of ((NH4)2SO4), NH4NO3 and ammonium chloride (NH4Cl) with haze aggravation”. I’m not convinced by this conclusion. The author should provide the SIA ratios (including Cl^-) under different pollution levels. As shown in Fig.S3, the Cl^- also exists during the clean periods. The variation of ph under different pollution levels should also be given.

4 The author concluded that the process in lines 216 to 218 from the fact that HONO and PANS elevated with the haze aggravation. However, I think the variation of the ratio of HONO/PM2.5 and PANS/PM2.5 under different pollution levels should be the proxy of their conclusion. The increase of HONO and PANS with the pollution levels can be attributed to the accumulation of pollution precursors and this pollution cannot be diluted.

Minor Comments:

1 Line 24, the NOR, SOR, and NTR were not defined in the abstract.

2 Line 30-32, I got what the author means, but I think it should be noted that the NH3 should also be concerned during the severe haze stage.

3 Line 58, it is not clear why “Therefore, it is urgent to fully understand the

chemical regimes and behavior of reactive gases during different pollution stages and propose reasonable strategies.” Some explanations should be given.

4 Line 76, the heating season at this place is not appropriate, as it is not mentioned before in the introduction.

5 The S1.1.1, S1.1.2, and S1.1.4 should be placed in the main manuscript.

6 Line 129, the definition of different classified pollution levels should be placed in the main manuscript as the following parts referred to the definition many times.

7 Line 154, how was the conclusion get from figure 2?

8 Line 196-197, please explain why the ratios (PM1.0/PM2.5 and PM2.5/PM10) can be used as the proxy of the hygroscopic growth of particulate matter.

9 line 208, the method of calculating the ε (NO^T₃) should be given.

10 Line 235, the number of points should be given.

11 Figure 7, the 3D plot is not necessary as the fill color can give the results.

Some minor comments on the supplements:

1 I’m not smart enough to get the meaning of the caption in Figure S9(a).

2 S2.2, the first two lines were not clear.

Comment on “The shifting of secondary inorganic aerosols formation mechanism during haze aggravation: The decisive role of aerosol liquid water”

General comments：

Aerosol liquid water is an important constituent in PM_2.5, which has a significant impact on the secondary aerosols. This manuscript reported a long-term observation of PM_2.5 and estimated ALWC and pH. And a detailed analysis on their relationships in different pollution conditions was performed. It is fully within the journal scope, but the authors should resolve the following questions before it can be published on ACP.

Major comments:

1. The English should be improved. Some of the sentences are too long to follow. Also the format of units should be revised. L52 in the abstract “μg/m³”. L105, the format of μg/m³… Please unify the units in the manuscript.
2. Throughout the whole text, you are saying the aqueous phase reactions and secondary formation in the local area. I am doubting whether there is long-range transport or primary emission resulting in the accumulation of secondary inorganic aerosols? And also the boundary layer change can also affect the concentration. Did you consider these factors other than secondary formation?
3. Section 2.2 I doubt in the estimation of Rp and Sp. You cited the previous studies which conducted in different locations? Is it representative for your site?
4. L189-191. I don’t know how to get the conclusion about the homogenous gas-phase reaction from Fig. 2. Please explain it.
5. L194-195. You said the ammonia has an important role in the atmospheric oxidative modification. Please listed the supporting information here.
6. L209-213. Is there any possibility that the homogenously formed NH₄NO₃, (NH₄)₂SO₄ partitioned into the aqueous phase?
7. L250-254. I'm confused by this sentence. Are you sure the oxidation was taken place in the aqueous phase initiated by the protons or it is just a thermal equilibrium from the gas phase to the aqueous phase?
8. L265-270. Did you try to find any data from China? Since your data was from a Chinese site, I think it would be better to compare the other sites from China. Additionally, it is better to cite some foreign sites, like USA.
9. Why did you choose the wavelength at 380 nm? Is it efficient for photochemistry?
10. Section 3.2.4. A major concern here is that how can I distinguish the secondary formation and transport of NO3- just from ΔNO3-？ It seems that you use ΔNO3- to represent the secondary formation of NO3-. I can’t agree with you about this.

Minor comments:

Main text:

1. Some of the abbreviations in the main text are confused to me. Please list their full names when they appeared for the first time. And the figures in the manuscript and SI should be reorganized since some of them are too small to see them clearly, like Fig. 5b and Figs. S3, S8 and S9.
2. Did you measure the transition metal ions during your campaign? If yes, it is better to show the heavy metal data than just cite the literatures. Or I don't think it is necessary to talk about heavy metals.
3. Please show the exact pH value in summer.
4. This figure is quite difficult for me to follow. It is better to simplify it and become easier to understand.
5. The Z axis is repetitive with the color bar.

Supplementary:

1. Most of the abbreviations are defined in SI. I think you should move them into the main text.
2. Fig. S8b didn't show the NO3 concentration. It is SIA.
3. As a main part of your manuscript, Table S1 should contain ALWC in different stages.

The manuscript offered a fully discussion on atmospheric interest, the formation mechanisms of secondary inorganic aerosols during a month-lasting haze periods by occupying the scientific stoichiometry methods. The manuscript is nicely constructed and the result presented in this work is relevant because, unlike the previous works, this work not only highlights the importance of both reaction medium and its corresponding gaseous precursors on secondary inorganic aerosol generations, but also identifies the chemical behavior transformation of gaseous precursors during haze aggravation. The findings of this manuscript could provide a new insight on secondary inorganic formation mechanisms. Nevertheless, some details must be addressed before it is accepted for publication at ACP.

1. Many μg/m³ were mistakenly written as ug/m³, pls. carefully checked.
2. There were many abbreviations in this manuscript, pls. give the full-length of the words at their first appearance.
3. Some important effects of haze pollution should be mentioned in the introduction part as the motivation and urgency, such as climate and health.
4. Line 235, the data points in summer mostly lied in HNO3 sensitive region. How is this defined? I can hardly find the sensitive domain boundaries during summer time.
5. The authors considered that ALW equals to 75 is the determining parameter for haze generations in studied area, however, for which step in the haze processes is equivalent to ALW=75. The definition of the haze processes is far easier than the definition of ALWC.
6. Line 244, it should not be the pH of secondary inorganic aerosol.
7. Line 287, I can hardly find Fig.S9a corresponded to the S-curve proposed by Guo et al. (2017). Pls. check carefully.
8. Line 362-365, grammar check.
9. The details of solar spectrophotometer did not mention in the manuscript. pls. provide it. In addition, Line 314-316 was poorly expressed, serious pollution stages were much higher than 1/4.
10. Detailed calculation method of aqueous NO3- concentrations should place in this manuscript to make easier reading for readers.
11. Compared with the sulfate heterogeneous rate calculation, NOR, SOR and NTR used more frequently in this manuscript and were the key parameters on mechanism presentation, thus, I recommend authors a fully rewritten on method part after a fully consideration.