the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2025
Growing role of secondary organic aerosol in the North China Plain from 2014 to 2024
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 (PM2.5). Despite these emission cuts, the formation mechanism of secondary organic aerosols (SOA), a crucial constituent of PM2.5, 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2521', Anonymous Referee #3, 11 Aug 2025
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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
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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.