the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Measurement report: Formation and brownness of aqueous secondary organic aerosol from the aged biomass-burning emissions in the Sichuan Basin, China
Abstract. Secondary organic aerosol (SOA) formed via complex chemical mechanisms was the major contributor to atmospheric aerosol pollution and climate forcing worldwide. The aqueous-phase oxidation was an important pathway for SOA formation and the aqueous SOA (aqSOA) exhibited absorption properties across ultraviolet to visible range. Here, we reported the direct ambient observation of SOA formation and absorption properties in the aqueous phase from the Sichuan Basin, China. Considerable aqSOA was originated from the aged biomass-burning emissions via aqueous-phase reactions instead of photo-chemical reactions under high aerosol liquid water content (ALWC) conditions, especially during the polluted period. The substantial impact on brown carbon (BrC) absorption from SOA was observed from 370 nm to 660 nm (27.5 %−43.2 %). This study highlighted the significant contribution of aqSOA formation from aged biomass-burning emissions to the BrC budget and absorption, especially at night. The mean aerosol absorption Ångström exponents from 370 nm to 880 nm (AAE370−880) was 1.95, higher than that observed in fresh and photo-chemically aged biomass-burning emissions. This study revealed the aqSOA formation and brownness from aged biomass-burning emissions and highlighted the importance of aqueous-phase reactions on air quality and climate.
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RC1: 'Comment on egusphere-2025-101', Anonymous Referee #1, 24 Apr 2025
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AC2: 'Reply on RC1', Chao Peng, 16 Jun 2025
Dear Reviewers,
We appreciate the comments of reviewers and have revised the manuscript to address the review comments from all reviewers. We have provided a point-to-point response to the concerns, suggestions, and questions. We hope our work could satisfy both the reviewers and the editor. Our answers to each question have been marked in blue text, which can be seen in the Supplement. The annotated line numbers refer to the revised copy of the manuscript.
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AC2: 'Reply on RC1', Chao Peng, 16 Jun 2025
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RC2: 'Comment on egusphere-2025-101', Anonymous Referee #2, 21 May 2025
This study reported the direct ambient observation of SOA formation and absorption properties in the aqueous phase from the Sichuan Basin, China. The results showed considerable aqSOA was originated from the aged biomass-burning emissions via aqueous-phase reactions instead of photo-chemical reactions under high aerosol liquid water content (ALWC) conditions, especially during the polluted period. This study revealed the aqSOA formation and brownness from aged biomass-burning emissions and highlighted the importance of aqueous-phase reactions on air quality and climate. This is a manuscript with potential. I recommend the manuscript be revised before being accepted for publication.
- Lines 83-87. The authors mention the geographic variability of the North China Plain amount the Sichuan Basin, but there is a study on aqSOA in these two regions is not described in detail, it is recommended to add a comparative analysis to better highlight the regional uniqueness of this study.
- Section 2.1. As a field measurement report, this paper lacks a map of the geographic location of the observatory sites, which does not allow us to visualize the geographic information about the Sichuan Basin, so we suggest that a map of the observatory sites and the surrounding terrain be added.
- Line 151. Further details are required on the basis for setting the CE value of ToF-ACSM to 0.5.
- Section 2.3.2. The ISORROPIA-II model estimates ALWC from these inorganic constituents only, but will the organic constituents affect the estimates, especially at higher OA conditions.
- Lines 220-222. There is a need for further clarification on the judgment of clean-up and contamination periods, the standards are different in each country and it is recommended to add comparisons with other standards.
- Lines 294-296. The authors suggest that aqSOA may also be produced by aqueous phase reactions of fossil fuel-related OA components, but this is only speculative and a more convincing basis is needed.
- Lines 350-352. These results indicated that considerable aqSOA might be formed from BBOA, which was more intensive than OOA at high ALWC levels during PP. What are the authors' judgments about the magnitude of their intensity, and what are the differences in "intensive"?
- Lines 372-373. These results indicated that considerable aqSOA might be formed from BBOA, which was more intensive than OOA at high ALWC levels during PP. Any details on what the mechanism is for this result?
- Lines 406-407. The authors emphasize the importance of aqueous-phase reactions, but how can the contribution of photochemical pathways be judged?
- Lines 497-500. How was this result obtained and please provide further clarification on the mechanism?
- The conclusion section needs to be further condensed; it seems that parts of the conclusion have been emphasized several times, and there is a need for the author to reorganize the structure of the conclusion to give a complete and clear conclusion.
Citation: https://doi.org/10.5194/egusphere-2025-101-RC2 -
AC1: 'Reply on RC2', Chao Peng, 16 Jun 2025
Dear Reviewers,
We appreciate the comments of reviewers and have revised the manuscript to address the review comments from all reviewers. We have provided a point-to-point response to the concerns, suggestions, and questions. We hope our work could satisfy both the reviewers and the editor. Our answers to each question have been marked in blue text, which can be seen in the Supplement. The annotated line numbers refer to the revised copy of the manuscript.
Data sets
Chemical composition of PM2.5 and Abs370,BrC for Yongchuan campaign C. Peng et al. https://doi.org/10.5281/zenodo.14626304
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Review to "Measurement report: Formation and brownness of aqueous secondary organic aerosol from the aged biomass-burning emissions in the Sichuan Basin, China" by Peng et al.
This manuscript by Peng et al. uses ambient measurements of organic aerosols (OA) chemical composition and aerosol light absorption to understand the relation between the sources of OA and their absorption properties. The authors claim, using positive matrix factorization on OA, that a significant fraction of secondary OA during pollution episodes in the Sichuan Basin originate from the aqueous processing of biomass burning OA in aerosol water. These secondary OA formed in the aqueous phase are further argued to contribute largely to the light absorption by secondary brown carbon.
While this study makes great use of different datasets and statistical methods to understand the relation between OA sources and aerosol light absorption during clean and polluted periods, the text contains a lot of repetitive information that are often not clearly expressed. I also do not understand why this manuscript has been submitted as a measurement report. Indeed, the scientific conclusions from this manuscript are not limited to a description of observations made during a measurement campaign, but are based on parameters calculated/inferred from these observations (e.g., using positive matrix factorization, or a multiple linear regression method).
Below is a list of major and minor comments on the manuscript that I believe should be considered by the authors to facilitate publication into ACP. A number of technical corrections, mostly regarding language, are also suggested for improved clarity.
Major comments:
Minor comments and technical corrections:
References:
Paatero, P. and Hopke, P. K.: Discarding or downweighting high-noise variables in factor analytic models, Analytica Chimica Acta, 490, 277–289, https://doi.org/10.1016/S0003-2670(02)01643-4, 2003.