Molecular composition and processing of aqueous secondary organic aerosol in cloud at a mountain site in southeastern China

Jin, Yali; Luo, Hao; Tang, Siqi; Xue, Shuhui; Nie, Chengyu; Peng, Xiaocong; Zheng, Yan; Xu, Weiqi; Zhang, Guohua; Pan, Xiaole; Sun, Yele; Chen, Qi; Liu, Lanzhong; Zhao, Defeng

doi:https://doi.org/10.5194/egusphere-2025-4322

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https://doi.org/10.5194/egusphere-2025-4322

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17 Sep 2025

| 17 Sep 2025

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Molecular composition and processing of aqueous secondary organic aerosol in cloud at a mountain site in southeastern China

Yali Jin, Hao Luo, Siqi Tang, Shuhui Xue, Chengyu Nie, Xiaocong Peng, Yan Zheng, Weiqi Xu, Guohua Zhang, Xiaole Pan, Yele Sun, Qi Chen, Lanzhong Liu, and Defeng Zhao

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.

Received: 04 Sep 2025 – Discussion started: 17 Sep 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.

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Yali Jin, Hao Luo, Siqi Tang, Shuhui Xue, Chengyu Nie, Xiaocong Peng, Yan Zheng, Weiqi Xu, Guohua Zhang, Xiaole Pan, Yele Sun, Qi Chen, Lanzhong Liu, and Defeng Zhao

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RC1:
'Comment on egusphere-2025-4322', Anonymous Referee #1, 02 Oct 2025 reply
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:
Introduction: the current motivation for a cloud‐droplet study is underdeveloped. The Introduction should more clearly articulate (i) why in-cloud processing matters for OA burdens and properties, (ii) what key uncertainties remain despite prior fog/cloud and online/offline molecular studies, and (iii) why an online, molecular-level approach (EESI-TOF) is uniquely positioned to address those gaps. Please sharpen the problem statement, specify the hypotheses, and state the concrete research questions and expected contributions of this dataset.

Line 246-249 and AqSOA tracers about siloxanes. The current discussion and explanation is not accurate. In particular, the concepts of “tracers” and “enhanced compounds” should be clearly distinguished. Please clarify the likely sources and implications of the siloxanes, or restructure this section accordingly. In fact, siloxanes are commonly used in personal care products and industrial chemical products.The lifetimes of the siloxanes are relatively long and will undergo atmospheric oxidation process to generate SOA, as confirmed in both laboratory and ambient studies [1, 2]. But in the aerosol phase, the high-molecular-weight siloxanes have also been detected from vehicle and jet emissions [3, 4]. The siloxanes could also be attributed to the use of silicone tubing in the sampling line [5]. Could the authors confirm that the silicone tubing was not used in the sampling system? It would also be helpful to show the time series of these compounds over the entire campaign. Was there an AMS or PTR-MS available to provide supporting information? Were any siloxane SOA tracers detected (e.g., Si- containing compounds where one or more methyl groups are replaced by hydroxyl groups)? In summary, the discussion could be better connected the focus of this study, as the siloxanes are good surfactants and could influence cloud droplet activation.

Section 3.3, considering the AqSOA tracers: the mass resolution and capbility of EESI-TOF are not enough to identify the structure of compounds, especially the N-containing species. Many of the proposed tracers could also arise from other sources (e.g., biomass burning, anthropogenic emissions, or in-spray artifacts) rather than exclusively as the tracer of AqSOA. For example, C9H18N2 and C12H23N are more deteched by EESI-ToF from traffic, microplastic, and agriculture, rather than the AqSOA. So it’s hard to treat proposed tracers as candidates unless supported by high-resolution MS/MS, authentic standards, or orthogonal constraints (e.g., gas-phase precursors, isotopic patterns). Again, the concept of the tracers cannot be used in this kind of discussion.

The manuscript relies heavily on earlier and in-campaign citations (71 % before 2020) and does not sufficiently reflect recent literature (29% from the most recent 5 years, e.g. Line 48-57, line 219-222, line 246-294). In particular, line 269 lists tracer studies from 2017 and 2019, but more up-to-date work is available from 2019-2025. Note that EESI-TOF was developed around 2017, first field measurements appeared in 2018–2019 (Switzerland), and the first report of aqSOA measured by EESI was published in 2021. The manuscript should be revised to incorporate these and subsequent studies, ensuring comprehensive and current coverage beyond the authors’ own campaigns.

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.

Chen, Y., et al., Chemical characterization and formation of secondary organosiloxane aerosol (SOSiA) from OH oxidation of decamethylcyclopentasiloxane. Environmental Science: Atmospheres, 2023. 3(4): p. 662-671.

Meepage, J.N., et al., Advances in the Separation and Detection of Secondary Organic Aerosol Produced by Decamethylcyclopentasiloxane (D5) in Laboratory-Generated and Ambient Aerosol. ACS ES&T Air, 2024. 1(5): p. 365-375.

Yao, P., et al., Methylsiloxanes from Vehicle Emissions Detected in Aerosol Particles. Environmental Science & Technology, 2023. 57(38): p. 14269-14279.

Decker, Z.C.J., et al., Emission and Formation of Aircraft Engine Oil Ultrafine Particles. ACS ES&T Air, 2024. 1(12): p. 1662-1672.

Timko, M.T., et al., Composition and Sources of the Organic Particle Emissions from Aircraft Engines. Aerosol Science and Technology, 2014. 48(1): p. 61-73.

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Citation: https://doi.org/10.5194/egusphere-2025-4322-RC1

Yali Jin, Hao Luo, Siqi Tang, Shuhui Xue, Chengyu Nie, Xiaocong Peng, Yan Zheng, Weiqi Xu, Guohua Zhang, Xiaole Pan, Yele Sun, Qi Chen, Lanzhong Liu, and Defeng Zhao

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https://doi.org/10.5194/egusphere-2025-4322-supplement

Yali Jin, Hao Luo, Siqi Tang, Shuhui Xue, Chengyu Nie, Xiaocong Peng, Yan Zheng, Weiqi Xu, Guohua Zhang, Xiaole Pan, Yele Sun, Qi Chen, Lanzhong Liu, and Defeng Zhao

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Short summary

Cloud substantially changes the compositions organic aerosol. How cloud processing of organics occur on molecular level remains unclear. We found that compared with cloud free particles, organics in cloud contains more large molecules likely due to accretion reactions and has more nitrogen-containing compounds. We identify some new compounds formed in cloud. Such modifications of the organics in cloud can further change its physicochemical properties, and impact on climate and human health.


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