Chemical composition, source and formation mechanism of urban PM<sub>2.5</sub> in Southwest China

Zhang, Junke; Su, Yunfei; Chen, Chunying; Guo, Wenkai; Feng, Miao; Song, Danlin; Jiang, Tao; Chen, Qiang; Li, Yuan; Li, Wei; Wang, Yizhi; Tan, Qinwen; Wu, Ruohan; Pu, Ruiyan; Lu, Minhui; Shen, Xuhui; Huang, Xiaojuan

doi:https://doi.org/10.5194/egusphere-2023-1593

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https://doi.org/10.5194/egusphere-2023-1593

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28 Jul 2023

| 28 Jul 2023

Chemical composition, source and formation mechanism of urban PM_2.5 in Southwest China

Junke Zhang, Yunfei Su, Chunying Chen, Wenkai Guo, Miao Feng, Danlin Song, Tao Jiang, Qiang Chen, Yuan Li, Wei Li, Yizhi Wang, Qinwen Tan, Ruohan Wu, Ruiyan Pu, Minhui Lu, Xuhui Shen, and Xiaojuan Huang

Abstract. Despite significant improvements in air quality in recent years, the Sichuan Basin (SCB) is still facing frequent haze pollution in winter. In this study, the chemical components of PM_2.5 in a typical pollution period at the beginning of 2023 in Chengdu, a megacity in the SCB, were characterized by bulk-chemical and single-particle analysis, and the PM_2.5 sources and formation mechanism of pollution were analyzed. The average mass concentration of PM_2.5 during the study period was 95.4±29.7 μg m^-3. Organic matter (OM) was the most abundant component (35.3 %), followed by nitrate (22.0 %), sulfate (9.2 %) and ammonium (7.8 %). The aerosol particles were classified into five categories: mineral, OM, S-rich, soot and fly ash/metal particles, and most of them were in the state of internal mixing. The entire observation period could be divided into two non-pollution periods (NP-1 and NP-2) and two haze periods (Haze-1 and Haze-2). With the evolution of pollution, the bulk-chemical and single-particle analysis exhibited similar characteristics, suggesting that Haze-1 was mainly caused by pollutants related to fossil fuel combustion, especially mobile sources, while Haze-2 was triggered by the rapidly increasing secondary pollutants. The PM_2.5 sources included dust (8.5 %), biomass burning (3.5 %), coal combustion (15.5 %), industrial processes (6.5 %), vehicular emissions (25.6 %) and secondary sources (40.5 %). Analysis of WRF-Chem model results showed that the average contributions of local sources and regional transmission to pollution in Chengdu were the same (50 % vs 50 %). In addition, the source composition and WRF-Chem simulation results in different periods confirmed our analysis of the formation mechanisms of the two haze events.

This study confirms that, despite the significant reduction in pollution experienced by Chengdu in the past decade, further significant reductions in PM_2.5 are still needed, with particular emphasis on vehicular emissions and secondary sources. High intensity local emissions or large amounts of regional transmission may cause serious haze events, and more effective policies for local emissions reduction or joint prevention and control of regional air pollution are necessary in the future.

Received: 12 Jul 2023 – Discussion started: 28 Jul 2023

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04 Mar 2024

Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: a case study at the beginning of 2023

Junke Zhang, Yunfei Su, Chunying Chen, Wenkai Guo, Qinwen Tan, Miao Feng, Danlin Song, Tao Jiang, Qiang Chen, Yuan Li, Wei Li, Yizhi Wang, Xiaojuan Huang, Lin Han, Wanqing Wu, and Gehui Wang

Atmos. Chem. Phys., 24, 2803–2820, https://doi.org/10.5194/acp-24-2803-2024,https://doi.org/10.5194/acp-24-2803-2024, 2024

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RC1:
'Comment on egusphere-2023-1593', Anonymous Referee #1, 15 Oct 2023

Please see my comments attached.

Citation: https://doi.org/10.5194/egusphere-2023-1593-RC1
- AC1: 'Reply on RC1', Junke Zhang, 05 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1593/egusphere-2023-1593-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC1
RC2:
'Comment on egusphere-2023-1593', Anonymous Referee #2, 23 Oct 2023

Please see attached PDF.

Citation: https://doi.org/10.5194/egusphere-2023-1593-RC2
- AC2:
  'Reply on RC2', Junke Zhang, 05 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1593/egusphere-2023-1593-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC2
  - AC3: 'Reply on AC2', Junke Zhang, 05 Dec 2023
    
    A correction: The title of the revised manuscript mentioned in the response file should be: Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: A case study at the beginning of 2023
    
    Citation: https://doi.org/10.5194/egusphere-2023-1593-AC3
- AC4: 'Reply on RC2', Junke Zhang, 05 Dec 2023
  
  A correction: The title of the revised manuscript mentioned in the response file should be: Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: A case study at the beginning of 2023
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC4

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1593', Anonymous Referee #1, 15 Oct 2023

Please see my comments attached.

Citation: https://doi.org/10.5194/egusphere-2023-1593-RC1
- AC1: 'Reply on RC1', Junke Zhang, 05 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1593/egusphere-2023-1593-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC1
RC2:
'Comment on egusphere-2023-1593', Anonymous Referee #2, 23 Oct 2023

Please see attached PDF.

Citation: https://doi.org/10.5194/egusphere-2023-1593-RC2
- AC2:
  'Reply on RC2', Junke Zhang, 05 Dec 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1593/egusphere-2023-1593-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC2
  - AC3: 'Reply on AC2', Junke Zhang, 05 Dec 2023
    
    A correction: The title of the revised manuscript mentioned in the response file should be: Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: A case study at the beginning of 2023
    
    Citation: https://doi.org/10.5194/egusphere-2023-1593-AC3
- AC4: 'Reply on RC2', Junke Zhang, 05 Dec 2023
  
  A correction: The title of the revised manuscript mentioned in the response file should be: Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: A case study at the beginning of 2023
  
  Citation: https://doi.org/10.5194/egusphere-2023-1593-AC4

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Junke Zhang on behalf of the Authors (05 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (05 Dec 2023) by Dara Salcedo

RR by Anonymous Referee #3 (05 Jan 2024)

RR by Anonymous Referee #4 (08 Jan 2024)

Suggestions for revision or reasons for rejection

The Sichuan Basin is one of the regions severely affected by haze in China. This study focused on Chengdu, the capital of Sichuan Province, and conducted in-depth analysis of the haze processes that occurred in early 2023 through field observations and model. By reviewing the review process of this manuscript, I found that the quality of the revised manuscript has significantly improved compared to the original version. Meanwhile, the results obtained in this study are interesting and can further enhance our understanding of the formation mechanism of air pollution in the city. However, there are still some scientific issues that need to be clarified or improved in the current version. Therefore, a revision is necessary before considering acceptance.

Line 50-55, some references about haze and non-haze study as well as control policy evaluation should be cited. Such as: Variations in PM2.5, TSP, BC, and trace gases (NO2, SO2, and O3) between haze and non-haze episodes in winter over Xi’an, China. Atmospheric Environment, 2015, 112, 64-71. Saccharides in summer and winter PM2.5 over Xi'an, Northwestern China: Sources, and yearly variations of biomass burning contribution to PM2.5. Atmospheric Research, 2018, 214, 410-417. Inter-annual variability of wintertime PM2.5 chemical composition in Xi'an, China: Evidences of changing source emissions. Sci. Total Environ. 545, 546–555.

Line 60-62. I suggest the authors provide more comprehensive evidence and data on the improvement of air quality in China in recent years. It is insufficient to provide an example based solely on the study results from Beijing. In fact, there are already many comprehensive reports or literature on the implementation effectiveness of emission reduction policies.

Line 100-102. “Despite numerous studies having used multiple methods to investigate the physical……” Please provide necessary references.

According to section 3.1, a haze event usually lasts for several days, while the collection time for each single particle sample was only 30 s to 3 min. I want to know how the authors ensure that the single particles they analyze are representative.

Line 263-264. “In particular, at the beginning of 2023, Chengdu has become the city with the highest number of motor vehicles in China” Please provide the data source.

Line 294-295. “although there was a significant increase in pollution at night, the pollution level in the daytime was lower than that on 3 February.” Why? If the emission reduction policies during the haze alarm period have had a positive effect, what are the reasons for the high values at night? What are the main sources of pollution?

Line 369. “S-rich particles”? Is this a new type of particulate matter? If so, please provide necessary introduction.

Line 535-539. “This is mainly related to the various pollution reduction measures implemented in Chengdu, and even the broader SCB, in the past decade, such as……” I think some references are necessary here.

Line 577-579. “This means that if Chengdu’s air quality is to achieve further improvement after reaching the current CNAAQS, it may need more attention paid to the contribution of local sources”. I think this statement is unreasonable. In fact, the contribution of regional transmission reached 46%. Such a high contribution cannot be ignored in the future pollution control process. Therefore, I suggest that the authors revise it.

Although the chemical composition results obtained based on bulk-chemical and single-particle analysis exhibited strong consistency, there is a significant difference in their magnitude of change. For example, from Haze-1 to NP-2, the contributions of SNA and SIA particles decreased by 15.6% and 27.1%, , respectively; from NP-2 to Haze-2, the contributions of SNA and SIA particles increased by 24.2% and 42.4%, respectively. What factors are causing this difference?

Line 626-628. “……with the reduction of pollution and the enhancement of atmospheric oxidation in recent years (Zhao et al., 2020; Wang et al., 2023), the formation mechanism of haze is also undergoing dynamic changes……” What changes in pollution mechanisms have been found in this study compared to previous results? I think some necessary discussions need to be added to the main text.

In section 3.4 source apportionments, the sources profiles of each factors should be given, and related references should be cited to support the source apportionment results.

Figures: (1) The marking of “fly ash/metal particles” in Figure 9 is not clear enough, please modify it. (2) Figure 11 is not clear enough, and its quality needs to be improved. (3) Figure 12. It is "relative humidity" rather than "humidity".

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ED: Publish subject to minor revisions (review by editor) (09 Jan 2024) by Dara Salcedo

AR by Junke Zhang on behalf of the Authors (14 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (18 Jan 2024) by Dara Salcedo

Dear Authors, I greatly appreciate the effort you made to improve the manuscript. Please consider the following additional comments:

Response to Comment 5, from referee #4: “In fact, the first comment on the number of motor vehicles in Chengdu appeared in the introduction section, and we provided necessary references, i.e. https://www.mps.gov.cn/n2254098/n4904352/c9244719/content.html (Line 94)”.
The given reference is not in English. Please provide a reference that is understandable to all readers of ACP.

Comments 6 and 10, from referee #4.
Please include the response to these comments in the text in order to clarify the issue for other readers.

Comment 12, from referee #4: “In section 3.4 source apportionments, the sources profiles of each factor should be given, and related references should be cited to support the source apportionment results.”
I agree that the source profiles can be described in the supplemental material document. However, the references to support the association of a source to each profile are still missing.

In addition, just after submitting my decision regarding the previous version of the manuscript, I received an additional report from an anonymous referee, which I am copying below. Although their comments are minor, I think that they can further improve the manuscript. Hence, I ask you to consider them too.

**************************************************************************

In this study, the authors synthesized multiple methods to study the characteristics, sources and formation mechanisms of atmospheric PM2.5 pollution in Chengdu. They captured two typical pollution processes and conducted in-depth analysis of their formation mechanisms, and the results can be used as a reference for local pollution reduction. Compared to previous studies on this city, the research methods and results in this study have a certain degree of innovation. In addition, after carefully reading the first round of review comments and the authors' responses, I believe that the authors responded to the comments with high quality and made necessary modifications, which significantly improved the quality of the manuscript. However, there are still some issues that need to be further modified and improved before being considered for acceptance. The detailed comments are as follows.

1. The authors conducted a detailed analysis of the formation mechanism of two haze processes during the observation period. I would like to know if there is any difference in the formation mechanism of winter haze in Chengdu compared to previous studies? Therefore, it is very necessary to compare with previous study results and add relevant discussions.

2. Line 107-108. The authors mentioned that “At the beginning of 2023, Chengdu experienced several severe haze events, during which the observed PM2.5 mass concentration frequently exceeded the CNAAQS.” Please provide more quantitative descriptions about the severity of this pollution, which will help readers understand the necessity of this study.

3. Line 172. The numbers of single particles analyzed during the four periods were 1325, 1159, 995 and 1870, respectively. I want to know if these analyzed particles are representative? Especially in the third period, the number of particles is less than 1000. This is crucial for the feasibility of the study results.

4. Line 252-253. “This inevitably leads to a the difference in the formation mechanism of air pollution in these two regions in China (Wang et al., 2021)” Why?

5. Line 396-399. The authors provided a detailed explanation for the synchronous increase of T and RH during the haze period in this study. They believe that the elevated RH caused an increase in T. I am curious why a similar phenomenon did not occur in Beijing, where RH increased but the T decreased.

6. Fig. 9/Line 500-503. What are the potential effects of changes in the mixed structure of soot particles. I suggest the authors supplement necessary discussions.

7. Line 524. Please provide a definition of “primary sources”, which types of sources does it include? The current expression is unclear.

8. Line 551-553. “During the NP-2 period, with the removal by precipitation and strong easterly winds, the contributions of secondary sources and vehicular emissions decreased by 14.3% and 9.6%, respectively”. The significant reduction in secondary sources is easy to understand as they are hygroscopic. Why has the contribution of vehicular emissions also shown a significant decrease? The authors mentioned earlier that the main emission species of vehicular emissions are carbonaceous components, which are weakly hygroscopic or hydrophobic.

9. In addition, some comments lack necessary references, such as Line 95-97. The format of references needs to be unified and standardized.

Hide

AR by Junke Zhang on behalf of the Authors (21 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (22 Jan 2024) by Dara Salcedo

AR by Junke Zhang on behalf of the Authors (23 Jan 2024) Manuscript

Journal article(s) based on this preprint

04 Mar 2024

Chemical composition, sources and formation mechanism of urban PM_2.5 in Southwest China: a case study at the beginning of 2023

Junke Zhang, Yunfei Su, Chunying Chen, Wenkai Guo, Qinwen Tan, Miao Feng, Danlin Song, Tao Jiang, Qiang Chen, Yuan Li, Wei Li, Yizhi Wang, Xiaojuan Huang, Lin Han, Wanqing Wu, and Gehui Wang

Atmos. Chem. Phys., 24, 2803–2820, https://doi.org/10.5194/acp-24-2803-2024,https://doi.org/10.5194/acp-24-2803-2024, 2024

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Despite significant improvements in air quality in recent years, Chengdu is still facing frequent haze pollution in winter and its formation mechanism is still not clear enough. We analyzed the typical pollution process that occurred in early 2023 by combining two different perspectives of chemical component determination results, source analysis results and model simulation results. We found that that the joint prevention and control of regional air pollution is still necessary for Chengdu.


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Chemical composition, source and formation mechanism of urban PM2.5 in Southwest China

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Chemical composition, source and formation mechanism of urban PM_2.5 in Southwest China