the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Molecular composition and processing of aqueous secondary organic aerosol in cloud at a mountain site in southeastern China
Abstract. Aqueous secondary organic aerosol (aqSOA) contributes substantially to organic aerosol (OA), affecting air quality, human health, and climate. However, the molecular composition and processing of aqSOA in cloud remain unclear due to limited online field measurements. We measured molecular composition of OA online (time resolution 20 s) and tracked its processing at a mountain site in southeastern China, using an Extractive ElectroSpray Ionization inlet coupled with a Time-of-Flight Mass Spectrometer (EESI-ToF-MS). We identified 2084 molecular formulas and compared OA composition from three sample types with adjacent time (<2 h): cloud droplets (CD), interstitial aerosol (INT), and cloud-free aerosol (CF) in representative cloud episodes. CHO class was the dominant constituent, followed by CHON class. The fraction of CHO was lower in CD than that in INT and CF, while the fraction of CHON was higher, which may result from the uptake of organonitrates or nitration in cloud water. Compounds in CD had more carbon, oxygen, and nitrogen number but lower O/C than INT and CF, which is attributed to accretion reactions in cloud water. We identified aqSOA tracers, including 39 new compounds, which were significantly enriched in CD compared with CF. This study also reveals rapid changes of aqSOA composition, which highlight the necessity for high time resolution measurement to capture the processing of aqSOA in cloud. Overall, this study provides clear information of processing of aqSOA in cloud and highlights the importance of accretion reactions, which has implications on the composition and physicochemical properties of SOA.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 29 Oct 2025)
- RC1: 'Comment on egusphere-2025-4322', Anonymous Referee #1, 02 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4322', Anonymous Referee #2, 12 Oct 2025
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The study characterized the molecular composition of the organic fraction of cloud droplet residue, pre-cloud aerosols, interstitial aerosols, and post-cloud aerosols using EESI-ToF-MS. The approach yielded a high-resolution dataset that captured detailed temporal variations in molecular composition. While the study provided additional evidence on the relative contributions of CHO and CHON species in cloud droplet residues and aerosols, its findings largely corroborated previous results rather than extending current understanding. The authors interpreted their data mostly relying on findings from previous studies.
The study’s main points were: 1. 39 new compounds in cloud droplet residues; and 2. the hypothesis that accretion reactions were promoted during cloud processing. However, it remains unclear why these new 39 compounds were detected. Were they revealed because of the dataset’s high temporal resolution, the unique analytical technique, or chemistry specific to the sampled environment?
The study period experienced four cloud events, effectively providing four data points for comparing pre-cloud, CD, INT and CF samples. Caution should be exercised in drawing general conclusions from such a limited dataset. For instance, CE2 was an exception in which CHON did not comprise the largest fraction of total OA, and CHO was not the lowest among the four sample types. Thus, one quarter of the samples did not align with the generalization, and it would be advisable to moderate some of the claims derived from these comparisons.
Specific comments:
- Line 27: “With adjacent time” is not clear. Adjacent to what?
- Line 44-46: Research on the role of aqueous-phase chemistry in SOAs has been going on for more than four decades. Are the studies cited here seminal ones?
- Line 51-52: What are the main findings from Duan et al. (2021)?
- Lines 151-153: Are the ranges for C, H, O, and N – C:10.01–12.81 vs 8.43–11.10, H: 14.59–20.34 vs. 14.23–16.83, O: 5.08–6.00 vs. 5.06–5.72, and N: 0.34–0.43 vs. 0.16–0.35 – between CD and pre-loud really that different?
- Line 278: “Large fluctuations” in what?
Citation: https://doi.org/10.5194/egusphere-2025-4322-RC2 -
EC1: 'Comment on egusphere-2025-4322', Sara Lance, 20 Oct 2025
reply
Please remember to review and follow the "ACP guidelines for authors" when preparing a revised manuscript:
https://www.atmospheric-chemistry-and-physics.net/policies/guidelines_for_authors.html#conclusion
Citation: https://doi.org/10.5194/egusphere-2025-4322-EC1
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This manuscript reports high–time/mass resolution online molecular measurements of OA using EESI-TOF at a mountain site in SE China, comparing cloud droplets, interstitial, and cloud-free aerosol. It is clearly within ACP’s scope, addressing atmospheric composition and processes with implications for aerosol–cloud interactions and climate. However, the discussion of “tracers” requires more caution: the manuscript should clearly distinguish between true tracers and compounds showing enhanced signals, define the concept explicitly as illustrated below.
Major comments:
Minor comments:
Line 61: “Aerosol Mass Spectrometer (AMS) or Aerodyne Aerosol Chemical Speciation Monitor (ACSM)”.
Line 82: revise to “…at the summit of Damaojian Mountain, located in Jinhua City, Zhejiang Province, China.”
Line 105: delete “Here is a brief introduction.”, too informal.
Line 115: “signal-to-background ration (s/b)”.
Line 268: “To the best of our knowledge, this is the first observation of C16H48O8Si8 …”. As mentioned in major comments 2, the description needs to be considered.
Line 308: hours to a day.