Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024

Lin, Chunshui; Huang, Ru-Jin; Duan, Jing; Qu, Jing; Liu, Jiahua; Liu, Yi; Luo, Yan; Huang, Wei; Xu, Wei; Zhan, Yanan; Liu, Zhitao; Liu, Sihan; Zhang, Qingshuang; Liu, Quan; Liu, Zirui; Lou, Shengrong; Yang, Huinan; Huang, Dan Dan; Huang, Cheng; Wang, Hongli

doi:10.5194/egusphere-2025-2521

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https://doi.org/10.5194/egusphere-2025-2521

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25 Jul 2025

| 25 Jul 2025

Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024

Chunshui Lin, Ru-Jin Huang, Jing Duan, Jing Qu, Jiahua Liu, Yi Liu, Yan Luo, Wei Huang, Wei Xu, Yanan Zhan, Zhitao Liu, Sihan Liu, Qingshuang Zhang, Quan Liu, Zirui Liu, Shengrong Lou, Huinan Yang, Dan Dan Huang, Cheng Huang, and Hongli Wang

Abstract. Since the Clean Air Act was implemented in 2013, China has witnessed a reduction of over 50 % in the annual average concentration of fine particulate matter (PM_2.5). Despite these emission cuts, the formation mechanism of secondary organic aerosols (SOA), a crucial constituent of PM_2.5, remains inadequately understood. In this study, we performed a model-assisted analysis of field sampling data collected in Shijiazhuang. The results show that, compared to 2014, the contribution of SOA to the total organics (from 27 % in 2014 to 87 % to in 2024) exceeded that of primary organic aerosol (POA) during the winter haze in 2024. Although the model underestimated the measured SOA levels, incorporating the transformation of transported POA into SOA under high relative humidity (RH) conditions helped bridge the gap between model predictions and on-site measurements. The increase in SOA contribution occurred amidst large emission reductions, which accounted for 70 % of the decline in POA levels, while meteorological factors contributed an additional 10 %. Increased contribution of SOA was also found in other North China Plains areas, which underscores the pressing necessity for coordinated regional initiatives to effectively mitigate SOA levels across the NCP, thereby tackling the transboundary nature of air pollution.

Received: 29 May 2025 – Discussion started: 25 Jul 2025

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Journal article(s) based on this preprint

19 Feb 2026

Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024

Atmos. Chem. Phys., 26, 2635–2647, https://doi.org/10.5194/acp-26-2635-2026,https://doi.org/10.5194/acp-26-2635-2026, 2026

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Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2521', Anonymous Referee #3, 11 Aug 2025
Lin et al. investigated the growing role of secondary organic aerosol in PM2.5 pollution in Shijiazhuang by both field observations and modelling. They found that the contribution of SOA to the total OA exceeded that of POA during the winter haze in 2024 compared to 2014, and aqueous oxidation of POA might explain the increased SOA. The response of OA to emission reduction was also discussed. The results are interesting, and the manuscript is well-represented and organized. It is publishable after the following questions have been well addressed.
In lines 105-106, the 2024 emission data is constructed based on the MECI 2020 and the 2024 measurement, but it is not clear for this scale.

In line 160, the PM1 components in 2014, but PM2.5 in 2024, will cause uncertainties when comparing the growth role of SOA. A reasonable discussion should be added.

In Figures 1a and b, the composition of PM2.5 has changed greatly in 2024 compared to 2014. In particular, the fraction of nitrate exceeded that of sulfate. This means aerosol acidity and aerosol liquid water content (ALWC) might have greatly changed at different pollution levels, which should influence aerosol formation, including both inorganic and organic aerosols. Those factors should be accounted for when you discuss SOA formation.

In Figure 2, although the correlation between SOA and RH is meaningful to understand the role of aqueous reaction in SOA formation, it should be more reasonable to discuss it using the fraction of ALWC in PM2.5 because ALWC should be related to RH, temperature, and chemical composition of aerosol.

OA was reduced by 79% in 2024 compared to 2014, with a 78% reduction during the most polluted period. It is meaningful to separate the reduction of POA and SOA.

In lines 185-187, aqueous oxidation might promote SOA formation, while oxidation of SVOC and IVOCs might also be enhanced in 2024 because of increased oxidation capacity.

In Figure 3, the mean values of observed ncPOA and SOA concentrations should be shown for a better understanding of the performance of the model.

In lines 205-206, the data in Figure 3b can not support this contribution (80) of SOA to OA. Please clarify. And it can give the detailed data (such as a table) for the cities in Figures 3c and 3d to make it easier to understand.

In lines 206-210 and Figure 3, another concern is that the contributions of biogenic sources (such as isoprene and pinenes) and aliphatics to SOA are not considered, which will introduce some uncertainties to the aging of POA.

Why does the contribution of local emissions of ncPOA in SJZ increase greatly in 2024 compared to 2014?

In lines 227-235, Figure S4 can not support this expression and conclusion. And the scenario setting in Section 3.3 is confusing, and it should be clarified. And the legend in Figure 3c should be checked. I think the difference shows the emission rather than meteorology.

In Figures S3 and S4, the performance of SOA modelling is terrible, particularly in 2014. The diurnal curve can not be correctly captured by the model. So, how are you sure about the SOA formation from aqueous oxidation of POA?

Figure S10 is not cited.

The formatting problems should be checked and revised, such as the upper and lower marks (lines 84, 130-134, etc.), the fonts (lines 37-45, 62-70), and reference information, like journal, volume, pages, and doi (lines 343-345, 366-369, etc).
Citation: https://doi.org/10.5194/egusphere-2025-2521-RC1
RC2: 'Comment on egusphere-2025-2521', Anonymous Referee #1, 18 Aug 2025

This manuscript investigated the differences in aerosol mass concentration and chemical composition between 2014 and 2024 for a large city in the North China Plain. The two campaigns were performed using a Q-ACSM and SP-AMS, respectively. The major finding was that with large reductions in primary emissions, the contribution of SOA to OA increased significantly, from 27% in 2014 to 87% in 2024. The results contribute to the understanding of long-term variations in ambient aerosols in China. I think this manuscript could be considered for publication if the following concerns could be properly addressed.
First, the 2014 and 2024 measurements were conducted at different sites (i.e., urban and suburban, respectively), indicating that results from the two campaigns were not directly comparable. For example, it is not surprising that the SOA contribution was higher for the suburban site (i.e., the 2024 campaign), even if the anthropogenic emissions were unchanged. To explain the observed variations of SOA, the influences of different factors (e.g., primary emissions and the types of measurement site) should be properly distinguished. This is essential for an ACP paper.
Second, the CMAQ performance was questionable. As indicated by Figure S3, the observed and simulated SOA results in fact had little correlation. I don’t think this level of agreement could properly support the related discussions in the main manuscript.
Third, the comparison between CMAQ and AMS (or ACSM) results must also be performed for POA. A reasonable agreement is the precondition for the related discussions in section 3.2.

Citation: https://doi.org/10.5194/egusphere-2025-2521-RC2
AC1: 'Comment on egusphere-2025-2521', Ru-Jin Huang, 18 Oct 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2521/egusphere-2025-2521-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2521-AC1
AC2: 'Comment on egusphere-2025-2521', Ru-Jin Huang, 18 Oct 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2521/egusphere-2025-2521-AC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2521-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2521', Anonymous Referee #3, 11 Aug 2025
Lin et al. investigated the growing role of secondary organic aerosol in PM2.5 pollution in Shijiazhuang by both field observations and modelling. They found that the contribution of SOA to the total OA exceeded that of POA during the winter haze in 2024 compared to 2014, and aqueous oxidation of POA might explain the increased SOA. The response of OA to emission reduction was also discussed. The results are interesting, and the manuscript is well-represented and organized. It is publishable after the following questions have been well addressed.
In lines 105-106, the 2024 emission data is constructed based on the MECI 2020 and the 2024 measurement, but it is not clear for this scale.

In line 160, the PM1 components in 2014, but PM2.5 in 2024, will cause uncertainties when comparing the growth role of SOA. A reasonable discussion should be added.

In Figures 1a and b, the composition of PM2.5 has changed greatly in 2024 compared to 2014. In particular, the fraction of nitrate exceeded that of sulfate. This means aerosol acidity and aerosol liquid water content (ALWC) might have greatly changed at different pollution levels, which should influence aerosol formation, including both inorganic and organic aerosols. Those factors should be accounted for when you discuss SOA formation.

In Figure 2, although the correlation between SOA and RH is meaningful to understand the role of aqueous reaction in SOA formation, it should be more reasonable to discuss it using the fraction of ALWC in PM2.5 because ALWC should be related to RH, temperature, and chemical composition of aerosol.

OA was reduced by 79% in 2024 compared to 2014, with a 78% reduction during the most polluted period. It is meaningful to separate the reduction of POA and SOA.

In lines 185-187, aqueous oxidation might promote SOA formation, while oxidation of SVOC and IVOCs might also be enhanced in 2024 because of increased oxidation capacity.

In Figure 3, the mean values of observed ncPOA and SOA concentrations should be shown for a better understanding of the performance of the model.

In lines 205-206, the data in Figure 3b can not support this contribution (80) of SOA to OA. Please clarify. And it can give the detailed data (such as a table) for the cities in Figures 3c and 3d to make it easier to understand.

In lines 206-210 and Figure 3, another concern is that the contributions of biogenic sources (such as isoprene and pinenes) and aliphatics to SOA are not considered, which will introduce some uncertainties to the aging of POA.

Why does the contribution of local emissions of ncPOA in SJZ increase greatly in 2024 compared to 2014?

In lines 227-235, Figure S4 can not support this expression and conclusion. And the scenario setting in Section 3.3 is confusing, and it should be clarified. And the legend in Figure 3c should be checked. I think the difference shows the emission rather than meteorology.

In Figures S3 and S4, the performance of SOA modelling is terrible, particularly in 2014. The diurnal curve can not be correctly captured by the model. So, how are you sure about the SOA formation from aqueous oxidation of POA?

Figure S10 is not cited.

The formatting problems should be checked and revised, such as the upper and lower marks (lines 84, 130-134, etc.), the fonts (lines 37-45, 62-70), and reference information, like journal, volume, pages, and doi (lines 343-345, 366-369, etc).
Citation: https://doi.org/10.5194/egusphere-2025-2521-RC1
RC2: 'Comment on egusphere-2025-2521', Anonymous Referee #1, 18 Aug 2025

This manuscript investigated the differences in aerosol mass concentration and chemical composition between 2014 and 2024 for a large city in the North China Plain. The two campaigns were performed using a Q-ACSM and SP-AMS, respectively. The major finding was that with large reductions in primary emissions, the contribution of SOA to OA increased significantly, from 27% in 2014 to 87% in 2024. The results contribute to the understanding of long-term variations in ambient aerosols in China. I think this manuscript could be considered for publication if the following concerns could be properly addressed.
First, the 2014 and 2024 measurements were conducted at different sites (i.e., urban and suburban, respectively), indicating that results from the two campaigns were not directly comparable. For example, it is not surprising that the SOA contribution was higher for the suburban site (i.e., the 2024 campaign), even if the anthropogenic emissions were unchanged. To explain the observed variations of SOA, the influences of different factors (e.g., primary emissions and the types of measurement site) should be properly distinguished. This is essential for an ACP paper.
Second, the CMAQ performance was questionable. As indicated by Figure S3, the observed and simulated SOA results in fact had little correlation. I don’t think this level of agreement could properly support the related discussions in the main manuscript.
Third, the comparison between CMAQ and AMS (or ACSM) results must also be performed for POA. A reasonable agreement is the precondition for the related discussions in section 3.2.

Citation: https://doi.org/10.5194/egusphere-2025-2521-RC2
AC1: 'Comment on egusphere-2025-2521', Ru-Jin Huang, 18 Oct 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2521/egusphere-2025-2521-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2521-AC1
AC2: 'Comment on egusphere-2025-2521', Ru-Jin Huang, 18 Oct 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2521/egusphere-2025-2521-AC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2521-AC2

Peer review completion

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

AR by Ru-Jin Huang on behalf of the Authors (18 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Nov 2025) by Guangjie Zheng

RR by Anonymous Referee #3 (18 Nov 2025)

Suggestions for revision or reasons for rejection

Lin et al. did a good job in addressing some of my comments and the concerns raised by the reviewers. The manuscript has been improved, but I still have some comments and concerns on the replies and revised manuscript. It is recommended to publish after the following questions have been well addressed.

For the response to comment 1 of reviewer 1: It is surprising that the author just gives a scaling factor of 0.5 for 2020 MEIC data to obtain MIEC 2024 to enable more accurate result. It means that the emission intensity was cutoff a half in 2024 compared to 2020? It sounds not reasonable and unrealistic. As shown in Figure S2, the modeled and observed data did not match well. These indicated that random scaling factor was selected. The author should give more reasonable, available and convinced analysis and discussion.

For the response to comment 2 of reviewer 1: The author just mentioned it existed limitation and uncertainty when directly comparing PM1 and PM2.5. What is the detailed uncertainty of SOA concentration or percentages in PM1 and PM2.5? How is the growth role of this uncertainty? It should give specific discussion.

For the response to comment 3 of reviewer 1: The author did not address this question. In Figures 1a and b, the composition of PM2.5 has changed greatly in 2024 compared to 2014. In particular, the fraction of nitrate exceeded that of sulfate. This means aerosol acidity and aerosol liquid water content (ALWC) might have greatly changed at different pollution levels, which should influence aerosol formation, including both inorganic and organic aerosols. Those factors should be accounted for when you discuss SOA formation. However, the author just mentioned that it may alter the formation of SOA. What is the specific influence? How to affect the SOA formation.

For the response to comment 4 of reviewer 1: I agree with the author that POA was not
necessarily internally mixed with the inorganic components that play key roles in regulating ALWC, but the SOA formation would be affected according to aqueous chemistry. It would be reasonable replacing RH with ALWC.

For the response to comment 12 of reviewer 1: I think the author did not address this question well. The author just mentioned that the daytime SOA formation was more important in 2014 than in 2024. First, this conclusion was not reasonable due to the terrible model performance. Second, the author should present the contribution of different process in CMAQ model in 2014 and in 2024 to check the model performance. Meanwhile, any process contributing to SOA formation lack should be added to the model rather than using the bad result. This model validation was the fundamental of the further analysis and conclusion. The more analysis and discussion are necessary to get a good model performance, then differentiate the SOA formation from aqueous oxidation of POA.

Hide

ED: Reconsider after major revisions (01 Dec 2025) by Guangjie Zheng

Please consider the remaining concerns from the reviewers:

Lin et al. did a good job in addressing some of my comments and the concerns raised by the reviewers. The manuscript has been improved, but I still have some comments and concerns on the replies and revised manuscript. It is recommended to publish after the following questions have been well addressed.

For the response to comment 1 of reviewer 1: It is surprising that the author just gives a scaling factor of 0.5 for 2020 MEIC data to obtain MIEC 2024 to enable more accurate result. It means that the emission intensity was cutoff a half in 2024 compared to 2020? It sounds not reasonable and unrealistic. As shown in Figure S2, the modeled and observed data did not match well. These indicated that random scaling factor was selected. The author should give more reasonable, available and convinced analysis and discussion.

For the response to comment 2 of reviewer 1: The author just mentioned it existed limitation and uncertainty when directly comparing PM1 and PM2.5. What is the detailed uncertainty of SOA concentration or percentages in PM1 and PM2.5? How is the growth role of this uncertainty? It should give specific discussion.

For the response to comment 3 of reviewer 1: The author did not address this question. In Figures 1a and b, the composition of PM2.5 has changed greatly in 2024 compared to 2014. In particular, the fraction of nitrate exceeded that of sulfate. This means aerosol acidity and aerosol liquid water content (ALWC) might have greatly changed at different pollution levels, which should influence aerosol formation, including both inorganic and organic aerosols. Those factors should be accounted for when you discuss SOA formation. However, the author just mentioned that it may alter the formation of SOA. What is the specific influence? How to affect the SOA formation.

For the response to comment 4 of reviewer 1: I agree with the author that POA was not
necessarily internally mixed with the inorganic components that play key roles in regulating ALWC, but the SOA formation would be affected according to aqueous chemistry. It would be reasonable replacing RH with ALWC.

For the response to comment 12 of reviewer 1: I think the author did not address this question well. The author just mentioned that the daytime SOA formation was more important in 2014 than in 2024. First, this conclusion was not reasonable due to the terrible model performance. Second, the author should present the contribution of different process in CMAQ model in 2014 and in 2024 to check the model performance. Meanwhile, any process contributing to SOA formation lack should be added to the model rather than using the bad result. This model validation was the fundamental of the further analysis and conclusion. The more analysis and discussion are necessary to get a good model performance, then differentiate the SOA formation from aqueous oxidation of POA.

Hide

AR by Ru-Jin Huang on behalf of the Authors (11 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Jan 2026) by Guangjie Zheng

AR by Ru-Jin Huang on behalf of the Authors (21 Jan 2026) Manuscript

Journal article(s) based on this preprint

19 Feb 2026

Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024

Atmos. Chem. Phys., 26, 2635–2647, https://doi.org/10.5194/acp-26-2635-2026,https://doi.org/10.5194/acp-26-2635-2026, 2026

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Since China's 2013 Clean Air Act cut PM_2.5 by over half, winter haze in the North China Plain persists due to secondary organic aerosols now dominating primary pollutants, requiring urgent regional cooperation to address model-underestimated chemical transformations and cross-border pollution.


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