In this article, the authors analyze 12 years of field observation data from Zhengzhou, China, to investigate trends in aerosol composition concentrations and its acidity. This approach is well-aligned with the scope and objectives of the “Measurement Report”, making the study highly relevant. However, there are contentious aspects in the discussion and final conclusions of this article. Specifically, whether crustal materials play a dominant role in driving the changes in aerosol acidity remains uncertain. This issue affects several parts of the article, including its main argument and even the title. I believe the authors are capable of making substantial revisions to the article.

Here are my specific recommendations:

Lines 15-17: Compared to the reduction of ammonia emissions, the reductions in acidic precursors such as SO₂ and NO_x have been more significant, leading to an overall increase in atmospheric acidity (including aerosols, clouds, and precipitation). Studying the trends in aerosol acidity is highly meaningful, but it is essential to consider the combined impact of the reduction in both acidic and basic precursors.

Lines 19-21: The 12-year observation period and the corresponding years for the PM concentration results do not align. While I understand that the authors began observations in 2011, the abstract should be rephrased to ensure clarity and consistency in the presentation of the time frame.

Lines 28-29: According to mainstream forecasts regarding the need for further PM reduction under China’s carbon peaking and carbon neutrality policy, future emission reduction strategies will primarily focus on acidic precursors. The expression here needs to be more precise and cautious.

Lines 63-65: There is a lack of logical flow from the previous discussion of acidity changes to the conclusion/summary in this sentence. Additionally, aerosol acidity is unlikely to be neutral or even approach neutrality by nature in general.

Lines 66-67: This may not be entirely accurate. The relative contribution of sulfate and nitrate does not directly determine aerosol acidity. The authors need to identify the true driving factors behind the pH trend. In my opinion, the main drivers are the NH₃/NH₄⁺ multiphase buffering, ALWC, and non-volatile cations, rather than other components or temperature. This is because, over the long term, the impact of temperature variations on pH is minimal within the same season.

Lines 85-86: Given the limitations, under what circumstances does it go from 4 to 7?

Lines 93-94: After reviewing the research progress, I suggest concluding with a summary that introduces the focus of this article. Specifically, what makes Zhengzhou and other cities different, what issue this study aims to address based on previous research, and what contributions this study makes. This is essential in scientific writing.

Line 123: Provide the version of ISORROPIA and the time resolution of the input components.

Line 160: The spring of 2021 saw multiple rare dust storms in the North China Plain, but why was the CM not high in 2021? Additionally, the resolution of the figures in both the main text and SI needs to be improved to at least 300 dpi, as many figures in the SI are unclear.

Line 170 (Figure S3): It is recommended to use the same y-axis range for consistency. Regarding the drivers of seasonal variations (increase or decrease), I believe the discussion here lacks sufficient rigor, with an incomplete chain of evidence. I suggest that the authors consider separately discussing natural and anthropogenic sources, especially the impact of spring dust storm events on dust levels. In section 3.3, the source-related discussion is also mentioned, and therefore, this part needs to be strengthened.

Lines 239-241: I believe ALWC is an important factor influencing pH, and it should not be solely attributed to sulfate. The authors need to carefully consider this point. Specifically, it should be evaluated whether the continuous decrease in ALWC affects pH, and whether this impact might even outweigh the influence of changes in aerosol composition.

Lines 280-282: Where is the data for this section sourced from?

Lines 283-286: Although many researchers have done significant work on the impact of ammonia reduction on PM levels, I believe that under China’s current policies, future emission reductions will not primarily focus on ammonia. Therefore, the authors need to reconsider whether the focus of this "Measurement Report" should be placed on ammonia. I suggest that the authors revise all statements related to ammonia, including the title.

Zhang et al. analyzed the particle pH collected in a Chinese inland city. This analysis is based on 4228 filter samples collected from four seasons in 2011 – 2022. As a measurement report, the study fits into the scope of Atmospheric Chemistry and Physics.  The authors claim that the evolving particle pH was driven by the interplay of declines in SO₄^2-, increases in TNH_x, and rises in Ca²⁺ concentrations over time. The manuscript can be considered for publication once the comments below are addressed.

Major Comment

1. Lines 111 -112 and 120 – 121: It is unclear whether Ti was measured or not.
2. Lines 135 – 136: The linear regression proposed by Wei et al. (2023) is based on the dataset collected in the wintertime but not from 2013 to 2020. This needs clarification. Since the regression was based on wintertime data, it is questionable if the relationship is still valid for data in seasons other than winter.
3. Section 2.2.3: Why was 1000 m chosen as the height in the HYSPLIT simulations? If 1000 m was already well above the boundary layer height, how do the simulated back trajectory simulations represent the ground-level measurements? In addition, how was the optimum number of clusters chosen by the authors? What clustering technique was used in the analysis? What data was used as the meteorological input?
4. Sections 3.1 and 3.2: How did the author estimate the decreases in the mass concentrations? Which year was chosen as the reference year? A proper analysis using Mann-Kendall and Sen’s slope should be carried out here and in other places associated with trend analysis.
5. Please briefly describe the Air Pollution Prevention and Control Action Plan and Three-Year Action Plan in the main text. In general, readers outside China have no idea about these policies.
6. Please explain why Zhengzhou is representative of a typical inland city in North China in the main text. There is no background information about aerosol research conducted in Zhengzhou and how they compare with those findings in other typical Chinese megacities, such as Beijing or Shanghai.

Minor Comment

1. Figure S1: Please label the highways, coal-fired power plant, and gas-fired power plant on the map.
2. Section 2.1: What are the uncertainties in the measured concentrations for individual components?
3. Lines 114 – 115: Apart from just citing references here, please provide the details about analytical methods and quality control in the supplement.
4. Section 2.2.2: What was the set activity coefficient of H⁺ in the model?
5. Line 172: There is no data showing that the WS was higher in spring and summer compared to autumn and winter.
6. Figure S4: Is there any data for Ca²⁺ in 2015?
7. Lines 229 – 231: Please provide a discussion about why the pH trend is similar to those in Beijing but different from those in Shanghai and Hong Kong.
8. Figure 5: How did the authors estimate the contribution of a component to the changes in pH?
9. Where is the figure or table associated with Lines 256 – 266?

Technical Comment

1. Line 17: It is unclear what is "particle pH response".
2. Line 19: It should be “12-year”
3. Graphical abstract: Please use a different color for the pH. Apparently, the color light blue is too bright for visualization.
4. Line 57: Please use a word different than “research hotspot”.
5. Line 64: Please use a different term instead of “fostered a persistent belief”.
6. Eq 4: It should be “NH₄⁺”
7. Figures in main text and SI: the labels for x- and y- axis and annotations are too small to read. All figures need to be modified for better visualization.
8. Lines 173 and 174: more effective than what?
9. Line 246: NO₃^- can be used to replace nitrate ions.
10. Line 252: SO₄^2-/NO₃^- can means the ratio between SO₄^2- and NO₃^-. I assume the authors use “/” to represent “or” here. If so, please use the word “or”.
11. The format of “TNH_x” should be consistent throughout the manuscript.
12. Some color choices in figures need to be reconsidered. Certain annotations in color are not color-friendly for visualization.