Source-Resolved Volatility and Oxidation State Decoupling in Wintertime Organic Aerosols in Seoul

Kim, Hwajin; Jeong, Jiwoo; Moon, Jihye; Kang, Hyun Gu

doi:10.5194/egusphere-2025-3738

Preprints

https://doi.org/10.5194/egusphere-2025-3738

Preprints

22 Aug 2025

| 22 Aug 2025

Source-Resolved Volatility and Oxidation State Decoupling in Wintertime Organic Aerosols in Seoul

Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

Abstract. Organic aerosols (OA) are key components of wintertime urban haze, but the relationship between their oxidation state and volatility – critical for understanding aerosol evolution and improving model predictions – remains poorly constrained. While oxidation – volatility decoupling has been observed in laboratory studies, field-based evidence under real-world conditions is scarce, particularly during severe haze episodes. This study presents a field-based investigation of OA sources and their volatility characteristics in Seoul during a winter haze period, using a thermodenuder coupled with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS).

Positive matrix factorization resolved six OA factors: hydrocarbon-like OA, cooking, biomass burning, nitrogen-containing OA (NOA), less-oxidized oxygenated OA (LO-OOA), and more-oxidized OOA (MO-OOA). Despite having the highest oxygen-to-carbon ratio (~1.15), MO-OOA exhibited unexpectedly high volatility, indicating a decoupling between oxidation state and volatility. We attribute this to fragmentation-driven aging and autoxidation under stagnant conditions with limited OH exposure. In contrast, LO-OOA showed lower volatility and more typical oxidative behavior.

Additionally, NOA – a rarely resolved factor in wintertime field studies – was prominent during cold, humid, and stagnant conditions and exhibited chemical and volatility features similar to biomass burning OA, suggesting a shared combustion origin and meteorological sensitivity.

These findings provide one of the few field-based demonstrations of oxidation–volatility decoupling in ambient OA and highlight how source-specific properties and meteorology influence OA evolution. The results underscore the need to refine OA representation in chemical transport models, especially under haze conditions.

Received: 01 Aug 2025 – Discussion started: 22 Aug 2025

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.

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Journal article(s) based on this preprint

23 Jan 2026

Source-resolved volatility and oxidation state decoupling in wintertime organic aerosols in Seoul

Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

Atmos. Chem. Phys., 26, 1145–1161, https://doi.org/10.5194/acp-26-1145-2026,https://doi.org/10.5194/acp-26-1145-2026, 2026

Short summary

Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3738', Anonymous Referee #1, 05 Sep 2025
Kim et al. investigated the characteristics of organic aerosol (OA) in Seoul during wintertime haze events. In contrast to previous studies conducted in the region, this work successfully identified a nitrogen-containing OA (NOA) factor, likely associated with biomass burning, in addition to five other OA factors using positive matrix factorization (PMF). A notable finding was that the oxidation state of oxygenated OA (OOA) did not align with volatility: the less-oxidized OOA (LO-OOA) was found to be less volatile than the more-oxidized OOA (MO-OOA). This observation challenges the conventional classification of OOA as semi-volatile or low-volatile and supports the use of LO-OOA and MO-OOA terminology.
The study provides insights relevant to understanding air quality in Seoul, one of the world’s largest metropolitan areas, where haze events are increasingly linked to nitrogen-related pollutants. Although NOA contributed only a minor fraction of total OA, its enhancement offers clues about particle-phase chemistry that may worsen air quality and could inform the design of mitigation strategies. Furthermore, this study represents the first attempt in Seoul to connect the oxidation state of OA with volatility, adding useful information to the literature.
However, many of the findings presented are not conceptually new within the broader atmospheric chemistry field. In addition, several interpretations remain speculative, with limited supporting evidence. In particular, Section 3.3 does not convincingly explain why MO-OOA in Seoul appears more volatile than LO-OOA. For these reasons, the work may be more appropriate for publication as a Measurement Report in its current form, unless the discussion and interpretation are substantially strengthened.
Additional comments are provided below.
Major comments:
Section 3.2: This study is presented as a characterization of wintertime aerosol, yet comparisons are made with fall data. Please clarify the source of the fall dataset—was it obtained from your previous work or from another published study? If it is adopted, this needs to be explicitly stated earlier in the section and consistently noted throughout the manuscript, including in figure captions.

Section 3.3: The discussion in this section could be substantially strengthened. For instance, the relatively high volatility of MO-OOA may represent a distinct feature of Seoul compared to other megacities. A more thorough comparison with results from other TD-AMS studies would help contextualize your findings. Given that one of the stated objectives of this work is to “improve understanding of wintertime OA in Seoul,” highlighting how Seoul’s OOA differs from or aligns with other urban environments would significantly enhance the discussion.

Line 163: The claim that enhanced MO-OOA and NOA formation during haze is due to stagnation is not convincing without supporting meteorological data. Please provide wind speed/direction or back-trajectory analyses.

Line 172: Could factor resolution differ between V- and W-mode? Please show whether NOA is detected in both modes and perform PMF sensitivity tests to mode/resolution.

Line 194: Since amines can also originate from solvents, might some portion of NOA be linked to solvent emissions? Given Seoul’s solvent usage, this possibility should be addressed.

Line 199: Where do you expect biomass burning that could influence Seoul’s air to occur? Since NOA (linked to BBOA) is a central finding, more discussion on plausible sources and transport pathways would be valuable.

Line 225: The phrase “…with inorganic secondary species such as biomass burning” is unclear. Please revise.

Line 226: Earlier you noted that LO-OOA lacks m/z 60, but here you attribute it partly to combustion-related activities. This appears contradictory. Please clarify or rewrite.

Line 257–258: The text seems to imply that NOA could be detected as nitrate. However, most reduced nitrogen species (other than nitro-aromatics, based on Xu et al., 2021, AMT) are unlikely to be detected as nitrate by AMS. Please clarify.

Line 261: If metallic sulfates contribute to PM₁, then defining PM₁ as NR-PM₁ + BC may underestimate total mass. Please comment.

Line 289: Since MO-OOA is described as more volatile, the phrase “consistent with its aged, highly condensed nature” seems contradictory. Consider deleting or rephrasing.

Line 308: What alternative oxidation mechanisms or environmental conditions could explain the observed inverse relationship between O:C ratio and volatility? Please specify and cite relevant studies.

Line 311: The authors attribute reduced OH to suppressed O₃ photolysis under haze. However, in polluted boundary-layer conditions, OH production often depends strongly on HONO photolysis in addition to O3 photolysis. If the authors want to keep this statement, I suggest add discussion and relevant reference on this.

Line 312: Under conditions of reduced OH, how are RO₂ radicals formed and how might they contribute to particle-phase autoxidation? Please elaborate and provide references.

Line 315–316: To support the interpretation that fragmentation contributes to the high volatility of MO-OOA, please cite previous studies that compared OA composition using both online and offline techniques.

Line 317: Please revise “semi-volatile species” to “semi-volatile/intermediate-volatility organics.” Additionally, please clearly comment that functionalized but low-molecular-weight compounds can fall in the SVOC–IVOC range and may contribute to the high volatility of MO-OOA. Please provide supporting references.

Line 327–328: A comparison of average OA mass spectra between haze and non-haze periods would be informative. Do non-haze periods show relatively more high-molecular-weight fragments?

Specific comments:
2 Experimental methods
Line 97: Since you are presenting quantitative results, please specify what collection efficiency (CE) value was applied in the AMS data analysis. In addition, could the use of the thermal denuder (TD) influence CE, for example by altering particle phase state or mixing characteristics? Please clarify whether you assumed the same CE with and without TD and provide justification or relevant references. Finally, if you define PM₁ mass as NR-PM + BC, it would be important to cite studies demonstrating that PM₁ in Seoul contains only limited amounts of metals or other refractory components (e.g., dust, sea salt), otherwise this definition may underestimate the total PM₁ mass.

3 Results and discussion
Line 149: Please clarify why a PM level of ~28 µg m⁻³ is considered “moderate.” A reference or comparison to regional air quality standards would help.

Line 186: A reference regarding the atmospheric lifetime and reactivity of amines would strengthen this discussion.

Line 242: Please italicize all “m/z ##” and “f_##” notations throughout the manuscript for consistency.

Line 246: Instead of describing the correlation between NOA and BBOA as “moderate,” please quantify the correlation coefficient and state whether it is higher relative to other factor pairs.

Line 248: The phrase “such as biomass burning” may be redundant here, since this point is already discussed in the BBOA section. Consider removing it.

Line 300–303: Please provide supporting references for this discussion.
Citation: https://doi.org/10.5194/egusphere-2025-3738-RC1
- AC1: 'Reply on RC1', Hwajin Kim, 11 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3738/egusphere-2025-3738-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC1
- AC2: 'Reply on RC1', Hwajin Kim, 11 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3738/egusphere-2025-3738-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC2
RC2:
'Comment on egusphere-2025-3738', Anonymous Referee #2, 14 Sep 2025

Kim et al. present a study of aerosol chemistry during wintertime in Seoul in 2019. Building upon their earlier publication in 2017, the authors conducted a dedicated field campaign focusing on aerosol volatility and obtained several intriguing results that are highly relevant to understanding and modeling aerosol processes in this megacity. One particularly interesting finding is that highly oxidized organic aerosols were shown to be highly volatile, providing observational evidence for autoxidation and fragmentation processes occurring in the particle phase. The dataset is well analyzed, and the manuscript is clearly written. The study fits well within the scope of ACP, and I consider it suitable for publication as a research article, rather than a measurement.
Section Introduction: The Introduction could be further strengthened by expanding the background on aerosol volatility. Including more context on previous volatility-related studies would help frame the contribution of this work.
Section 2.1: This section currently begins with a citation to the authors’ earlier study, which may lead readers to assume that the dataset is the same. However, the actual description of the 2019 field study only appears several sentences later. To improve clarity and logical flow, I suggest first presenting the details of the current field campaign and then referring back to the earlier study for context.
Line 146-150: This paragraph relies on information from the Supplementary Material, which makes it awkward as an entry point into the main results. I suggest either removing it or integrating the content later in the manuscript, once the main results are introduced.
Section 3.1.1: The identification of nitrogen-containing organic aerosols (NOA) could be better supported. I encourage the authors to provide additional evidence, for instance through mass spectral comparison with previous studies, or by applying the NO/NO₂ ratio approach to assess NOA, and then comparing the results with PMF-based identification.
Line 187-188: Please clarify whether the identified NOA is of primary or secondary origin.
Line 212-213: The abbreviations for OOAs have already been introduced earlier.
Line 267: The abbreviation "OA" has also been defined earlier.
Line 268: ... observations at where?
Line 292: Section 3.3?
Line 302-303: Please add a few references to situate your results in the context of previous literature.

Citation: https://doi.org/10.5194/egusphere-2025-3738-RC2
- AC3: 'Reply on RC2', Hwajin Kim, 11 Oct 2025
  
  Thank you to the anonymous reviewer for the thoughtful comments and suggestions, which were very helpful in improving our manuscript. We have carefully considered each point and revised the manuscript accordingly. Please find our point-by-point responses in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3738', Anonymous Referee #1, 05 Sep 2025
Kim et al. investigated the characteristics of organic aerosol (OA) in Seoul during wintertime haze events. In contrast to previous studies conducted in the region, this work successfully identified a nitrogen-containing OA (NOA) factor, likely associated with biomass burning, in addition to five other OA factors using positive matrix factorization (PMF). A notable finding was that the oxidation state of oxygenated OA (OOA) did not align with volatility: the less-oxidized OOA (LO-OOA) was found to be less volatile than the more-oxidized OOA (MO-OOA). This observation challenges the conventional classification of OOA as semi-volatile or low-volatile and supports the use of LO-OOA and MO-OOA terminology.
The study provides insights relevant to understanding air quality in Seoul, one of the world’s largest metropolitan areas, where haze events are increasingly linked to nitrogen-related pollutants. Although NOA contributed only a minor fraction of total OA, its enhancement offers clues about particle-phase chemistry that may worsen air quality and could inform the design of mitigation strategies. Furthermore, this study represents the first attempt in Seoul to connect the oxidation state of OA with volatility, adding useful information to the literature.
However, many of the findings presented are not conceptually new within the broader atmospheric chemistry field. In addition, several interpretations remain speculative, with limited supporting evidence. In particular, Section 3.3 does not convincingly explain why MO-OOA in Seoul appears more volatile than LO-OOA. For these reasons, the work may be more appropriate for publication as a Measurement Report in its current form, unless the discussion and interpretation are substantially strengthened.
Additional comments are provided below.
Major comments:
Section 3.2: This study is presented as a characterization of wintertime aerosol, yet comparisons are made with fall data. Please clarify the source of the fall dataset—was it obtained from your previous work or from another published study? If it is adopted, this needs to be explicitly stated earlier in the section and consistently noted throughout the manuscript, including in figure captions.

Section 3.3: The discussion in this section could be substantially strengthened. For instance, the relatively high volatility of MO-OOA may represent a distinct feature of Seoul compared to other megacities. A more thorough comparison with results from other TD-AMS studies would help contextualize your findings. Given that one of the stated objectives of this work is to “improve understanding of wintertime OA in Seoul,” highlighting how Seoul’s OOA differs from or aligns with other urban environments would significantly enhance the discussion.

Line 163: The claim that enhanced MO-OOA and NOA formation during haze is due to stagnation is not convincing without supporting meteorological data. Please provide wind speed/direction or back-trajectory analyses.

Line 172: Could factor resolution differ between V- and W-mode? Please show whether NOA is detected in both modes and perform PMF sensitivity tests to mode/resolution.

Line 194: Since amines can also originate from solvents, might some portion of NOA be linked to solvent emissions? Given Seoul’s solvent usage, this possibility should be addressed.

Line 199: Where do you expect biomass burning that could influence Seoul’s air to occur? Since NOA (linked to BBOA) is a central finding, more discussion on plausible sources and transport pathways would be valuable.

Line 225: The phrase “…with inorganic secondary species such as biomass burning” is unclear. Please revise.

Line 226: Earlier you noted that LO-OOA lacks m/z 60, but here you attribute it partly to combustion-related activities. This appears contradictory. Please clarify or rewrite.

Line 257–258: The text seems to imply that NOA could be detected as nitrate. However, most reduced nitrogen species (other than nitro-aromatics, based on Xu et al., 2021, AMT) are unlikely to be detected as nitrate by AMS. Please clarify.

Line 261: If metallic sulfates contribute to PM₁, then defining PM₁ as NR-PM₁ + BC may underestimate total mass. Please comment.

Line 289: Since MO-OOA is described as more volatile, the phrase “consistent with its aged, highly condensed nature” seems contradictory. Consider deleting or rephrasing.

Line 308: What alternative oxidation mechanisms or environmental conditions could explain the observed inverse relationship between O:C ratio and volatility? Please specify and cite relevant studies.

Line 311: The authors attribute reduced OH to suppressed O₃ photolysis under haze. However, in polluted boundary-layer conditions, OH production often depends strongly on HONO photolysis in addition to O3 photolysis. If the authors want to keep this statement, I suggest add discussion and relevant reference on this.

Line 312: Under conditions of reduced OH, how are RO₂ radicals formed and how might they contribute to particle-phase autoxidation? Please elaborate and provide references.

Line 315–316: To support the interpretation that fragmentation contributes to the high volatility of MO-OOA, please cite previous studies that compared OA composition using both online and offline techniques.

Line 317: Please revise “semi-volatile species” to “semi-volatile/intermediate-volatility organics.” Additionally, please clearly comment that functionalized but low-molecular-weight compounds can fall in the SVOC–IVOC range and may contribute to the high volatility of MO-OOA. Please provide supporting references.

Line 327–328: A comparison of average OA mass spectra between haze and non-haze periods would be informative. Do non-haze periods show relatively more high-molecular-weight fragments?

Specific comments:
2 Experimental methods
Line 97: Since you are presenting quantitative results, please specify what collection efficiency (CE) value was applied in the AMS data analysis. In addition, could the use of the thermal denuder (TD) influence CE, for example by altering particle phase state or mixing characteristics? Please clarify whether you assumed the same CE with and without TD and provide justification or relevant references. Finally, if you define PM₁ mass as NR-PM + BC, it would be important to cite studies demonstrating that PM₁ in Seoul contains only limited amounts of metals or other refractory components (e.g., dust, sea salt), otherwise this definition may underestimate the total PM₁ mass.

3 Results and discussion
Line 149: Please clarify why a PM level of ~28 µg m⁻³ is considered “moderate.” A reference or comparison to regional air quality standards would help.

Line 186: A reference regarding the atmospheric lifetime and reactivity of amines would strengthen this discussion.

Line 242: Please italicize all “m/z ##” and “f_##” notations throughout the manuscript for consistency.

Line 246: Instead of describing the correlation between NOA and BBOA as “moderate,” please quantify the correlation coefficient and state whether it is higher relative to other factor pairs.

Line 248: The phrase “such as biomass burning” may be redundant here, since this point is already discussed in the BBOA section. Consider removing it.

Line 300–303: Please provide supporting references for this discussion.
Citation: https://doi.org/10.5194/egusphere-2025-3738-RC1
- AC1: 'Reply on RC1', Hwajin Kim, 11 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3738/egusphere-2025-3738-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC1
- AC2: 'Reply on RC1', Hwajin Kim, 11 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3738/egusphere-2025-3738-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC2
RC2:
'Comment on egusphere-2025-3738', Anonymous Referee #2, 14 Sep 2025

Kim et al. present a study of aerosol chemistry during wintertime in Seoul in 2019. Building upon their earlier publication in 2017, the authors conducted a dedicated field campaign focusing on aerosol volatility and obtained several intriguing results that are highly relevant to understanding and modeling aerosol processes in this megacity. One particularly interesting finding is that highly oxidized organic aerosols were shown to be highly volatile, providing observational evidence for autoxidation and fragmentation processes occurring in the particle phase. The dataset is well analyzed, and the manuscript is clearly written. The study fits well within the scope of ACP, and I consider it suitable for publication as a research article, rather than a measurement.
Section Introduction: The Introduction could be further strengthened by expanding the background on aerosol volatility. Including more context on previous volatility-related studies would help frame the contribution of this work.
Section 2.1: This section currently begins with a citation to the authors’ earlier study, which may lead readers to assume that the dataset is the same. However, the actual description of the 2019 field study only appears several sentences later. To improve clarity and logical flow, I suggest first presenting the details of the current field campaign and then referring back to the earlier study for context.
Line 146-150: This paragraph relies on information from the Supplementary Material, which makes it awkward as an entry point into the main results. I suggest either removing it or integrating the content later in the manuscript, once the main results are introduced.
Section 3.1.1: The identification of nitrogen-containing organic aerosols (NOA) could be better supported. I encourage the authors to provide additional evidence, for instance through mass spectral comparison with previous studies, or by applying the NO/NO₂ ratio approach to assess NOA, and then comparing the results with PMF-based identification.
Line 187-188: Please clarify whether the identified NOA is of primary or secondary origin.
Line 212-213: The abbreviations for OOAs have already been introduced earlier.
Line 267: The abbreviation "OA" has also been defined earlier.
Line 268: ... observations at where?
Line 292: Section 3.3?
Line 302-303: Please add a few references to situate your results in the context of previous literature.

Citation: https://doi.org/10.5194/egusphere-2025-3738-RC2
- AC3: 'Reply on RC2', Hwajin Kim, 11 Oct 2025
  
  Thank you to the anonymous reviewer for the thoughtful comments and suggestions, which were very helpful in improving our manuscript. We have carefully considered each point and revised the manuscript accordingly. Please find our point-by-point responses in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3738-AC3

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Hwajin Kim on behalf of the Authors (11 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (21 Oct 2025) by Theodora Nah

RR by Anonymous Referee #2 (23 Oct 2025)

RR by Anonymous Referee #1 (03 Nov 2025)

Suggestions for revision or reasons for rejection

Kim et al. have substantially improved the manuscript, and the dataset is valuable. However, several structural, interpretive, and technical issues remain that should be addressed before acceptance in Atmospheric Chemistry and Physics. Below I list Major (structural/scientific) items followed by Specific & Technical (line- or figure-level) points.

Major comments
1. Restructure Section 3.1 (clarity and reader flow).
o Because resolving NOA is central to the manuscript and many readers may be unfamiliar with this factor, I recommend reorganizing Section 3.1 as: Overview → SOA → POA → NOA. This order (from broadly known to newly resolved) will help readers understand the factors that are consistent with previous studies before introducing the novel NOA factor. As NOA is closely related to POA, discuss NOA immediately after POA.

2. Clarify how NOA was identified and why it appears now.
o In Section 3.1.1, first explain the diagnostics that establish the NOA factor (spectral markers, HR signatures, diurnal behavior, correlations). After that, discuss why NOA was resolved during this campaign but not in prior campaigns in Seoul (e.g., instrumental mode/resolution differences, episodic source enhancement, seasonality, PMF settings). And lastly, compare NOA characteristics with observations in other cities to contextualize uniqueness vs. commonality.

3. Reorder Section 3.3.1 and avoid overreaching language.
o Present results first, then discussion. Limit speculative material and ensure that mechanistic claims are supported by measurements or cited modeling. The current Section 3.3.1 contains speculative text that is not directly supported by the manuscript’s measurements. Retain the essential discussion (the paragraph that addresses reasons for higher volatility of MO-OOA) but shorten and focus the first paragraph; place tentative hypotheses in the Discussion with clear labeling as speculation.
o I recommend avoiding the word “Mechanism” in the subsection title unless direct mechanistic evidence is presented. A more suitable and descriptive title would be something like “High-volatility nature of MO-OOA in Seoul wintertime.”

4. Revise the Conclusion to emphasize NOA and the volatile MO-OOA observation.
o The first paragraph of the Conclusion currently omits a balanced discussion of NOA’s mixed primary/secondary nature and overemphasizes biomass burning. Combine the first two paragraphs and use that space to emphasize the manuscript’s principal takeaways: (a) reliable identification of a NOA factor and its mixed origin, and (b) the observation that MO-OOA in Seoul exhibits unexpectedly high volatility. Frame outstanding uncertainties and recommended follow-up analyses/measurements as clear next steps.

Specific & technical comments (line- and figure-level)
Manuscript-wide editorial/formatting
• There are numerous minor formatting and grammar issues (missing spaces after punctuation, double punctuation, inconsistent font sizes, inconsistent spacing around “m/z”, missing italics for m/z and f##). Please perform a careful copy-edit before the next submission.

Figures/visualization
• Line 186 / Fig. S8: Cluster 1 is not well visualized in Fig. S8. Provide a zoomed-in version (or higher-resolution panel) so the cluster interpretation is clear.
• Line 409 / f44 and f43: The high and stable f44 and f43 results are important and should be shown in a main-text figure rather than only in the supplement. If retained in the supplement, clearly reference them in the main text and ensure they are visible in the linked supplement figure.

PMF/factor robustness
• Line 201: Because the NOA mass fraction is low, please report PMF reproducibility metrics (bootstrap). If bootstrap results exist, include them and summarize whether NOA is reproducibly resolved. If not performed, run bootstrap tests to quantify factor stability.

Repetition/consolidation
• Lines 208–212 vs. 229–250: These passages appear repetitive (NOA relevance to combustion). Combine and streamline these discussions to avoid redundancy. Place the definitive explanation and supporting evidence in one location.

Clarifications/wording
• Line 257: Clarify “highly oxidized chemical composition.” Do you mean “oxidized relative to POA” or “oxidized relative to other OA factors”? Be explicit.
• Line 264: The statement that “LO-OOA is mostly related to primary sources, but not secondary” is ambiguous. Also, wouldn't LO-OOA vs. MO-OOA be more related to “less aged” versus “more aged”?
• Line 280: When defining COA, report the m/z 55/57 ratio used as the key tracer for COA identification and show those values or rationale in the text or table.
• Line 308: When you state “Nearly complete evaporation occurred by 200 °C (~2% remaining)”, indicate explicitly which species you refer to (e.g., nitrate, organics) and include the figure/table reference.
• Lines 311–314: Statements in this range make inferences not directly shown by measurements. Add appropriate references or align the text with the data; avoid speculative assertions without citation.
• Line 318: Specify which chloride species you mean (e.g., ammonium chloride vs. metal chlorides) and provide literature references for volatility comparisons.
• Lines 319–321: The current wording is unclear. If you intend to say that organics display a continuous decrease in mass fraction with TD temperature, indicating a range of volatilities, whereas some inorganic species show abrupt loss at specific temperatures, please rewrite explicitly and include references if appropriate.
• Line 334: Oxidation is typically more active during daytime photochemistry. The current statement that tries to explain the higher O:C of HOA is not convincing enough.
• Line 337: COA appears to be less oxidized than BBOA but also less volatile — please comment on this apparent discrepancy and add discussion.
• Lines 353–357: These sentences are repetitive. Condense to improve readability.
• Line 368: Clarify the phrase “strong overall OA volatility” — do you mean “higher volatility” or “greater mass loss upon heating”? Use precise language.

Other
• Line 418–421: Material here reads like a summary and is more appropriate for the Conclusion. Consider moving it there.

Hide

ED: Reconsider after major revisions (08 Nov 2025) by Theodora Nah

AR by Hwajin Kim on behalf of the Authors (12 Dec 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (16 Dec 2025) by Theodora Nah

RR by Anonymous Referee #1 (23 Dec 2025)

ED: Publish subject to minor revisions (review by editor) (26 Dec 2025) by Theodora Nah

AR by Hwajin Kim on behalf of the Authors (26 Dec 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (31 Dec 2025) by Theodora Nah

AR by Hwajin Kim on behalf of the Authors (04 Jan 2026) Manuscript

Journal article(s) based on this preprint

23 Jan 2026

Source-resolved volatility and oxidation state decoupling in wintertime organic aerosols in Seoul

Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

Atmos. Chem. Phys., 26, 1145–1161, https://doi.org/10.5194/acp-26-1145-2026,https://doi.org/10.5194/acp-26-1145-2026, 2026

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Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

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Hwajin Kim, Jiwoo Jeong, Jihye Moon, and Hyun Gu Kang

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We investigated how organic particles behave during severe winter haze in Seoul using advanced real-time measurements. We found that even highly oxidized particles can remain volatile, and a rare nitrogen-containing type emerges under cold, stagnant conditions. We interpret both as products of limited atmospheric processing and combustion-related sources. These findings help improve understanding of pollution formation and evolution.


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