the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Jul 2026
Molecular evolution of oxygenated organic molecules in a cloud-influenced forested mountain environment
Abstract. Atmospheric oxygenated organic molecules (OOMs) link the oxidation of volatile organic compound to secondary organic aerosol formation, yet their molecular evolution in cloud-influenced forested mountain environments remains poorly constrained. Here, gas-phase OOMs were measured using an iodide-adduct chemical ionization mass spectrometer at a high-altitude forested station in southeastern China during autumn 2023. Among the 1503 identified OOMs, isoprene-derived OOMs were primarily controlled by daytime OH-initiated oxidation, whereas monoterpene-derived OOMs were driven by coupled OH and NO3 chemistry. Positive matrix factorization resolved nine distinct OOM factors, revealing that biogenic oxidation dominated OOM production during warm periods (mean temperature ~20 °C), accounting for 68–79 % of the total OOMs, whereas anthropogenic and regional transport factors became prominent during cooler periods (below 15 °C), accounting for up to 63 % of the total budget. Cloud processes exerted a pronounced influence on OOM composition through competitive wet scavenging and multiphase chemistry. During cloud events, isoprene-derived and monoterpene-derived organic nitrates (MT-ONs) decreased by 75 % and 46 %, respectively, while the relative contribution of MT-ONs increased from 23 % to 40 %, indicating distinct cloud-processing mechanisms and hydrolysis pathways. Furthermore, the contribution of sulfur-containing OOMs increased substantially, highlighting the importance of aqueous-phase formation pathways. These results demonstrate that cloud processing influences the composition and sources of OOMs in forested mountain atmospheres and should be considered when evaluating the atmospheric fate of biogenic oxidation products.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3643', Anonymous Referee #1, 05 Aug 2026
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RC2: 'Comment on egusphere-2026-3643', Anonymous Referee #2, 14 Sep 2026
This study presents a comprehensive molecular-level characterization of gas-phase oxygenated organic molecules (OOMs) at a cloud-influenced forested mountain site in southeastern China. By combining iodide-CIMS measurements with kinetic calculations, positive matrix factorization, tracer correlations, wind-regression analysis, and cloud-event comparisons, the authors investigate the roles of biogenic oxidation, anthropogenic influence, regional transport, and cloud processing in regulating OOM composition. The contrasting oxidation behavior of isoprene- and monoterpene-derived OOMs and the responses of organic nitrates and sulfur-containing OOMs to cloud processing are particularly interesting. The manuscript contains a valuable dataset and is generally well organized. However, several aspects of the mechanistic interpretation and methodological description require further clarification to strengthen the conclusions. The following comments should be addressed.
- Section 3.3.1: Following the discussion of the diurnal variations of non-nitrate biogenic OOMs in Fig. 4, kinetic calculations are further used to evaluate the relative roles of OH- and O3-initiated oxidation. Given that OH concentrations were estimated rather than directly measured, the atmospheric meaning of the calculated reaction-rate ratios could be further clarified. For example, do these ratios primarily represent the relative reaction rates of the precursor VOCs with different oxidants, or can they be further interpreted in terms of the relative importance of different oxidation pathways? In addition, please discuss whether the observed diurnal patterns of the OOMs in Fig. 4 provide additional observational support for the proposed oxidation pathways.
- Section 3.3.1: P1 and P3 were both influenced by clouds but showed distinctly different responses of biogenic OOMs. Although the effects of temperature, solar radiation, and precursor availability have been well discussed, it remains less clear how meteorologically regulated OOM production and cloud-related removal jointly determine the observed OOM responses. Please further discuss how the interplay between these processes leads to the contrasting responses of biogenic OOMs during P1 and P3.
- Section 3.4.4: The enhancement of the sulfur-containing factor during cloud events, together with its relationship with RH, provides useful evidence for aqueous/multiphase processing. However, organosulfur compounds may also be affected by precursor availability and transport. I suggest slightly moderating the relevant wording and describing these observations as being “consistent with” or “supporting” aqueous/multiphase formation, rather than implying a unique formation pathway.
- Section 3.4.2: The Afternoon ISO oxidation factor shows a moderate correlation with IEPOX-SOA (r = 0.59), which is interpreted as evidence for a linkage between gas-phase OOMs and particle-phase SOA formation. However, both gas-phase isoprene oxidation products and IEPOX-SOA are also expected to respond to common controlling factors, such as isoprene precursor abundance and photochemical activity. Please briefly discuss whether these common influences may partly contribute to the observed correlation, in addition to direct gas-to-particle processing.
- Texts S5–S6: Several equations describing the oxidation-rate calculations appear to contain inconsistent reactant concentrations or rate coefficients. For example, [ISO] appears in the monoterpene oxidation equations. Please carefully verify these equations, clarify whether these inconsistencies are typographical errors, and explicitly provide the reaction-rate coefficients used in the calculations so that the reported rate ratios can be readily followed.
Citation: https://doi.org/10.5194/egusphere-2026-3643-RC2 -
AC1: 'Comment on egusphere-2026-3643', Yele Sun, 21 Sep 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3643/egusphere-2026-3643-AC1-supplement.pdf
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General comments
This manuscript presents a valuable and high-quality molecular-level characterization of oxygenated organic molecules (OOMs) in a cloud-influenced mountain environment using I-CIMS measurements. The dataset is unique, and the PMF analysis provides important insights into the sources, evolution, and atmospheric processing of OOMs under cloud-influenced conditions. The study therefore has the potential to make a meaningful contribution to our understanding of atmospheric multiphase chemistry. I recommend it for publication after addressing the following comments:
Major comments:
While the overall conclusions are generally well supported, several key mechanistic interpretations rely primarily on observational correlations. For example, attributing the observed differences among the four periods mainly to cloud processing is inherently challenging because cloud occurrence is accompanied by substantial changes in temperature, solar radiation, precursor concentrations, and air-mass characteristics. Although these factors are discussed individually, the manuscript would benefit from a more balanced assessment of their combined influences and the associated uncertainties. In particular, it would strengthen the paper to more clearly distinguish conclusions that are directly supported by the observations from those that represent reasonable but less constrained interpretations.
In addition, the manuscript attributes the formation pathways of several OOM factors primarily to OH- and NO3-initiated oxidation based on molecular fingerprints, diurnal variations, and estimated oxidation rates. These interpretations are generally convincing and well motivated. However, they would be further strengthened by a more explicit discussion of the uncertainties associated with the PMF factor assignments and the potential overlap among different oxidation pathways. Since several characteristic compounds can be produced through multiple oxidation mechanisms, acknowledging this limitation more explicitly would provide a more balanced interpretation and enhance the robustness of the conclusions.
Specific comments:
L49–51. The authors summarize previous I-CIMS observations in forest environments very well. However, since the present study focuses on a mountain site, it would be beneficial to briefly mention previous molecular-level OOM observations conducted at similar atmospheric conditions or high-altitude stations to better highlight the novelty of the present work.
L118–126. The semi-quantitative calibration approach is only briefly described in the main text. Although detailed information is provided in the Supplement, one or two additional sentences summarizing how the relative sensitivity and transmission efficiency were determined would help readers unfamiliar with this method.
L200. When discussing the lower volatility of CHON-OOMs, the authors may also cite laboratory studies demonstrating that organic nitrates preferentially partition into the particle phase due to their relatively low volatility.
L235-236. In Sect. 3.3, the authors estimated OH and NO3 radical concentrations using the steady-state approach. Is the steady-state assumption still valid under cloud conditions?
L272–291. The discussion of cloud impacts on organic nitrates focuses mainly on wet scavenging and hydrolysis. The authors may consider mentioning that differences in aqueous solubility among different classes of organic nitrates could also contribute to the observed compositional changes during cloud events.
L347–348. The manuscript attributes the nighttime enhancement of the Nighttime ISO+MT oxidation factor primarily to NO3 chemistry. Since gas-particle partitioning can also influence the abundance of condensable oxidation products, it would be worthwhile to briefly discuss whether partitioning changes between daytime and nighttime could contribute to the observed variations.
L447-448. The terms "warm periods" and "cool periods" are used throughout the manuscript. For clarity, I suggest indicating the approximate mean temperatures when these periods are first introduced.