the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Nov 2025
Characteristics and processing of aqueous secondary organic aerosols during autumn in suburban Eastern China: role of aerosol liquid water, aerosol acidity, and photochemistry
Abstract. Aqueous-phase secondary organic aerosols (aqSOA) constitute a large fraction of SOA, thereby exerting significant influence on air quality, climate, and human health. However, its formation mechanisms remain unclear due to limited observational evidence. We conducted field measurements of particulate matter (PM) composition by deploying high-resolution aerosol mass spectrometry in a suburban environment during autumn in Nanjing, China. The characteristics and formation pathways of aqSOA are comprehensively investigated by using Positive Matrix Factorization (PMF) method. Our results show that aqSOA accounted for 27.6 % of oxidized organic aerosols, exhibiting elevated O:C ratios (0.78) and strong correlations with nitrate and aerosol liquid water (ALW). The important role of acid-catalyzed reactions is also revealed by the enhanced production of aqSOA at lower aerosol pH conditions. Under elevated nitrate and ALW levels, a pronounced morning aqSOA peak was frequently observed; whereas a noon-time aqSOA peak was also observed on several days, likely governed by photochemistry and aqueous-phase reactions. These findings highlight the critical roles of nitrate, ALW, acidity, and photochemistry in driving aqSOA production in polluted urban environments. This study advances the mechanistic understanding of aqSOA formation and provides insights into the mitigation of SOA in Eastern China.
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RC1: 'Comment on egusphere-2025-5610', Anonymous Referee #1, 09 Dec 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5610/egusphere-2025-5610-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2025-5610-RC1 -
RC2: 'Comment on egusphere-2025-5610', Anonymous Referee #2, 15 Dec 2025
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- General Evaluation
This manuscript presents an autumn field campaign in suburban Nanjing and combines HR-ToF-AMS measurements with PMF and E-AIM modeling to discuss aqSOA and its coupling with nitrate/ALW/acidity and photochemical processes, and further proposes Type I/II diurnal patterns. Overall, the research question is clear, and the dataset and analytical framework are relatively complete with regional relevance. However, several key criteria and parameter assumptions (e.g., Type classification, E-AIM inputs/uncertainties, CE) still need to be made more transparent and reproducible. In addition, parts of the mechanistic discussion should adopt more cautious causal language and clarify relevant boundary conditions. Therefore, I recommend minor revision.
- Major Comments
To minimize potential confusion with MO-OOA and other highly oxidized factors, please consider adding the following in the Supplementary Information (SI): (i) a side-by-side comparison of the mass spectral profiles of aqSOA and MO-OOA; and (ii) a comparison of their correlations with nitrate and ALW (optionally, if offline dicarboxylic acid/oxalate data are available, these may be included as additional constraints). Please also briefly summarize the key interpretation points for Figs. S1–S3 and the major sources of uncertainty.
Please clearly specify the classification criteria (e.g., peak timing window, whether double peaks are allowed, whether smoothing is applied), the definition of “rainy days,” and provide a “date–type” list (can be placed in the SI) to facilitate independent verification.
Without introducing additional complex quantitative analyses, please add a short discussion acknowledging that boundary-layer evolution and physical mixing/dilution in the morning may modulate the observed peak shape around 09:00–10:00. Please clarify that the main conclusions are primarily inferred under chemically constrained conditions.
Please provide the key E-AIM inputs (e.g., whether gas-phase NH₃ is included) and the main uncertainty sources. Please also explain the basis for adopting the pH = 3 threshold and include a simple sensitivity test (SI is sufficient). In terms of wording, please prioritize “supports/indicates/is consistent with” over overly strong causal statements.
III. Specific / Line-by-Line Comments
L49–52: Please add 1–2 recent regional studies and/or review papers to support the statement that “OA decreases while the SOA fraction increases.”
L120–122: Please add one sentence noting the potential bias/uncertainty of assuming CE = 0.5 during nitrate-rich periods.
L249–253: Please provide the criteria for defining rainy days (threshold, data source, and whether post-rain hours are excluded) and list the excluded dates in the SI.
L258–260: Please clarify how the ~1 h lag was estimated (e.g., peak-time difference, cross-correlation) and add one sentence on its possible implications for kinetics and/or phase-state transitions.
L279–281: Please correct the grammar (“This suggest” → “This suggests”) and soften the causal chain phrasing (e.g., “photochemistry → nitrate → mediates aqSOA”).
Figure 4: Please report the sample size N, and provide either the p-value or the 95% confidence interval (either one is sufficient).
Citation: https://doi.org/10.5194/egusphere-2025-5610-RC2
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