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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. Line 225: The phrase “…with inorganic secondary species such as biomass burning” is unclear. Please revise.
8. 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.
9. 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

1. 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.

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.