Yoo et al. present a concise and well-written investigation of individual particles collected in Seoul as part of the KORUS-AQ campaign, with the goal of investigating the different particle types that might be found as a result of different atmospheric events. The authors furthermore specifically highlight the composition of particles in the supermicron size range. Some of the core findings aren’t particularly novel (e.g., from the abstract: “atmospheric aerosol 34 particles in urban area vary depending on their size, sources, and reaction or ageing status..”), and have been well known to the atmospheric community at large for some time. However, the authors don’t attempt to sell this as the novel aspect, and instead emphasize the need to establish these kinds of relationships for particle types that aren’t generally well characterized in a variety of environments (e.g., supermicron). The relatively high abundance of heavy metal particles that were observed is particularly interesting as well. I believe that this study will be publishable in ACP pending the addressment of several minor concerns listed below:

Page 4 Lines 120 – 123: Please use the HYSPLIT references that NOAA specifically requests (Stein et al; Rolph et al; https://www.ready.noaa.gov/HYSPLIT_traj.php).

Page 4 Lines 126 – 128: The vacuum conditions needed to analyze particles via SEM should be acknowledged, and caveats should be listed for how this affects the final particle characterization (e.g., bias against more volatile components).

Page 5 Line 149: The referred to particle classification scheme in the SI is somewhat unclear – the particle classification descriptions appear rather qualitative at times (e.g., ‘particles containing many heavy metals’) rather than a more precise, quantitative description (e.g., what quantitative abundance of which specific heavy metals do the authors use to assess a particle as one containing heavy metals?). As a result, it is somewhat difficult to get a true appreciation for how particle classification was done. Even if the authors are reproducing a previously published classification scheme, the precise information should be reproduced in the SI and if necessary, referenced accordingly.

Figure 3-4: Consider using a different color scheme that is more amenable to color-blindness.

Page 11 Lines 313 – 314: While the authors suppositions here may be true, they should caveat them by noting the extremely number of total particles that lead them to these observations.

Figure 4: Consider labeling what the various stages are in the figure, or alternatively, at least list it in the figure caption.

Page 14 Lines 369 – 370: Have the authors investigated the potential of biomass burning/wildfires specifically instead of just combustion in general? Consider the NASA fire map resource.

This study investigated physicochemical properties of individual particles during the KORUS-AQ campaign using a quantitative electron probe X-ray microanalysis. This study classified these individual particles into seven types and atmospheric situations into five types. This study showed relative abundances of different types of particles during five types of atmospheric situations. This study found that Zn-containing particles were mostly sourced from local vehicle exhausts and waste incinerations, while Mn, Ba, and Cu-containing particles were mainly attributed to metal-alloy plants or mining emissions. This study suggested that the morphology and chemical compositions of particles were affected by their sources and ageing processes. Although this study clearly expressed the results, some problems listed below need to be addressed before the manuscript could be published.

1. L16: Please indicate the full name of ‘KORUS-AQ’.
2. L117: The second sampling time (15:00 ~ 16:00) should be afternoon. I don't think it's appropriate for the authors to consider this period as an evening.
3. L117-118: What is the range of sampling duration? The authors should show it.
4. Section 3.1: The authors classified individual particles into seven types, but most of these seven types of particles are externally mixed. As we know, particles are usually internally mixed in aged air, especially during polluted periods. Many researchers have found this phenomenon in urban air using high-resolution TEM, such as internal mixing of carbonaceous and secondary inorganic aerosols (Adachi & Buseck, 2013; Li et al., 2021; Zhang et al., 2022). I suggest the authors to further consider the internal mixing of secondary aerosols and other particles (e.g., carbonaceous or metal-containing particles) to better evaluate the source of particles.
5. Some data sources did not be indicated, such as ‘73.2% and 44.5%’ on L181, ‘71.0%’ on L189, and etc. A table may be appropriate.
6. L272: ‘As’ is not a heavy metal element. Correspondingly, some results in section 3.1.5 and figures (e.g., Figure 3) need to be changed.
7. L340: During period I, the air mass is more likely to be long-range transported based on backward trajectories in Figure S4a. How did the authors determine the period I as a local polluted event?
8. L375-384: The sentence is repeated.
9. L398-399: This result confuses me. Based on backward trajectories in Figure S4b, air masses at high altitudes (1000 m A.G.L) should be transported from northeastern China rather than that at low altitudes showed by the authors. Air masses at low altitudes are mainly from the local area.

Reference list:

Adachi, K., & Buseck, P. R. (2013). Changes of ns-soot mixing states and shapes in an urban area during CalNex. J. Geophys. Res.-Atmos., 118(9), 3723-3730. https://doi.org/10.1002/jgrd.50321

Li, W., Teng, X., Chen, X., Liu, L., Xu, L., Zhang, J., Wang, Y., Zhang, Y., Shi, Z., 2021. Organic Coating Reduces Hygroscopic Growth of Phase-Separated Aerosol Particles. Environmental Science & Technology 55 (24), 16339-16346.

Zhang, J., Wang, Y., Teng, X., Liu, L., Xu, Y., Ren, L., Shi, Z., Zhang, Y., Jiang, J., Liu, D., Hu, M., Shao, L., Chen, J., Martin, S.T., Zhang, X., Li, W., 2022. Liquid-liquid phase separation reduces radiative absorption by aged black carbon aerosols. Communications Earth & Environment 3 (1), 128.