the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Dec 2025
Decadal Transition of Summertime PM2.5-O3 Coupling and Secondary Organic Aerosol Dominance in Northwest China
Abstract. The Yinchuan metropolitan area in northwest China, situated between the Tengger and Ulan Buh Deserts, is influenced by both natural dust and anthropogenic emissions. However, the evolution of fine particulate matter (PM2.5) and its interaction with ozone (O3) under the region’s arid climate remain poorly understood. This study integrates decadal observations (2015–2025) with in-situ measurements using an Aerosol Chemical Speciation Monitor and a Vocus Proton Transfer Reaction Mass Spectrometry during summer 2025 to elucidate the changing PM2.5-O3 relationship and sources of organic aerosols. A pronounced shift was identified: Phase I (2015–2018) featured a rapid decline in PM2.5 accompanied by a sharp O3 increase, while Phase II (2019–2025) exhibited stabilized PM2.5 and plateaued O3, indicating reduced O3 sensitivity to particulate controls. The average non-refractory PM2.5 concentration (16.8 µg m-3) was significantly lower than in eastern Chinese megacities, with organics accounting for ~60 %. Positive matrix factorization resolved three organic aerosol factors, revealing dominant secondary organic aerosols (SOA, ~74 %) derived from prolonged photochemical aging. Volatile organic compound analysis showed that anthropogenic and biogenic precursors, including urban terpenes and aromatic oxidation products jointly contributed to SOA formation. Back-trajectory and potential source analyses indicated that Yinchuan’s summer air masses were mainly locally recirculated, with limited influence from long-range transport. These results demonstrate a regional transition toward SOA-dominated fine particles and decoupled PM2.5-O3 dynamics under cleaner conditions, highlighting the need for integrated VOC and oxidant controls to mitigate co-occurring O3 and PM2.5 pollution in arid northwest China.
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Status: open (until 15 Jan 2026)
- RC1: 'Comment on egusphere-2025-5901', Anonymous Referee #1, 22 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5901', Anonymous Referee #2, 04 Jan 2026
reply
The study of Zhou et al , investigates the changes in PM2.5 and ozone concentrations within a 10 year period in the Yinchuan metropolitan area of Northwest China. The authors combine complementary analytical approaches to characterize refractory and non-refractory fine-mode aerosol composition and relate these observations to long-term trends. This is an important study dealing with trend studies and could highlight the impact of regional policies on the dynamics of PM and O3 concentrations. Overall, the dataset and approach are valuable, and the paper is generally well written. The work is innovative in its combined chemical characterization and trend perspective, and it is suitable for publication after minor revisions on the clarity and deeper explanation of a few aspects.
L135: change …. ‘note’ to ..’ not’
L275: What were the primary sources of the observed Chloride? Please identify the most plausible source for Yinchuan area as this is poorly discussed the manuscripted.
L290-295: Why would higher precipitation reduce NOx significantly, yet appears to coincide with an increase in SIA (as discussed around Line 274).
This seems counterintuitive without additional explanation. For example: Precipitation may reduce NOx via scavenging and decreased photochemistry, but SIA could increase if ammonia availability, aqueous-phase oxidation (for sulfate), or gas-particle partitioning (for nitrate or ammonium) is enhanced under high RH conditions.L297:change “It is note” to “It is noted that”.
L364: revise the sentence to read better: use “exhibited strong correlation” instead of “highly correlated”
L438: The discussion on photochemical processes linking ozone formation and SOA is currently too general. A few SOA compounds were detected, but there are no explained mechanisms that connect their presence to ozone dynamics.
Please expand briefly on which photochemical pathways are most relevant for O₃ formation in this region (e.g., VOC–NOx sensitivity regime, role of ROx chemistry, possible contribution of aromatics or biogenic VOCs...
Are they any relationships of the e identified SOA compounds to those pathways and if yes which ones are most relevant. Are the SOA just indicators of photochemical activity or are they direct drivers of O₃ changes? Please make this clear in the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-5901-RC2
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- 1
Zhou et al. investigated the long-term trends of particulate matter pollution in a typical city in Northwest China by analyzing a decade of observational data, focusing on the source apportionment of SOA. They highlighted the significant role of local aging processes of particulate matter in the synergistic pollution of PM2.5 and O3. Overall, this manuscript is well-written. I recommend its publication in ACP after addressing the following minor comments.
1. This study uses a single field campaign to demonstrate the importance of local particulate matter aging in the PM2.5-O3 pollution. Why then analyze PM2.5 and O3 data from the past ten years? Is the single campaign representative? Could this core viewpoint potentially be invalidated over a decadal timescale?
2. Lines 56-70: The logic connecting this paragraph to the research content of this paper is not well established. It is suggested that the authors revise this paragraph from the following perspectives. For instance, what are the current differences in the sources and formation mechanisms of SOA across different regions of China? What is the necessity of using AMS (or ACSM) for this research? Why focus on the sources and formation mechanisms of SOA in Northwest China?
3. Lines 120-122: More details regarding the PMF analysis based on ACSM spectra are needed to be provided. For example, how were the three source factors determined, and what are their corresponding source profiles?
4. Line 175: What does "second-phase pollution control measures" mean? Is it the same as "Phase II" mentioned later?
5. Lines 182-185: What is the logical relationship between the viewpoint expressed in this sentence and the described PM2.5-O3 relationship in Phase I and Phase II mentioned earlier?
6. It is suggested to move Fig. 3 to the Supplementary Information.
7. What is the purpose of Fig. 5? It seems the text does not discuss Fig. 5 in detail. Consider moving Fig. 5 to the Supplementary Information.
8. Lines 238-254: This paragraph is somewhat redundant. Its removal would not affect the overall logic of the paper. The authors may consider removing it.
9. Lines 275-277: Please provide further explanation. How do meteorological conditions lead to rapid changes in aerosol chemical composition? Is there any relevant mechanism or literature support?
10. The source apportionment results based on ACSM data suggest that coal combustion still contributes to OA in summer. Given the availability of concurrent XRF data, could the authors provide further insights from the perspective of PM2.5 source apportionment?
11. Line 365: The full term for "r" (correlation coefficient) should be introduced here, not at Line 382. Furthermore, what is the difference between "r" and "R2" at Line 247? This needs to be unified throughout the manuscript.