The paper investigated size-resolved aerosol composition during a transboundary pollution event and concluded that a significant amount of secondary aerosols (mostly inorganic) in the accumulation mode (particle diameters ranging from 0.94 to 2.5 μm) can be formed during the severe haze episode. Their main conclusion is that the composition apportionment of submicron particles (PM0.94) is essentially the same as that of supermicron particles (PM2.5–0.94) if inorganic aerosols are the main constitution, which is most likely the case of a severe haze. This result may add value to the relevant research field as a new finding. However, this work focuses on mostly delivering their measurement results without in-depth analyses. So, in my opinion, this work may be meaningful in terms of reporting another measurement datasets in the early spring season. However, I do not think that this paper advances our fundamental understanding of secondary aerosol formation under favorable meteorological conditions (i.e., humid and cold environments). A large amount of secondary inorganic aerosols (sulfate, nitrate and ammonium) under stagnant conditions characterized by weak wind, shallow boundary layer, high humidity and/or low temperature, has been reported in numerous studies as the authors introduced. Large increases in sulfate and nitrate aerosols during the pollution event are therefore nothing special, even though a much large increase in nitrate aerosol is observed due to the lower air temperature as compared to the result found from the previous campaign (KORUS-AQ campaign). The latter is also an expected and already known result. I personally wish they could focus more on oxalate (and/or organic acids) and aqueous secondary organic aerosol (SOA), which is much less known compared to secondary inorganic aerosol.

1. Throughout the paper, they stated multiple times the potentially important role of local emissions from Seoul in the higher levels of PM2.5 in Seoul than in Incheon or Sungi. Considering the larger population in Seoul, this is an obvious statement. It would be much better if they could provide some quantification of contributions of local emissions vs. transboundary transport to the higher PM levels in Seoul. With regard to local emissions, on the other hand, I wonder why PM2.5 concentration in Inha University (= 43.6 μg m^–3) or Sungi station (=36.2 μg m^–3) during the clean period is higher than that in Seoul (= 30.5 μg m^–3) (Table 3 and Table 4). Under clean conditions, it is expected that local emissions would contribute most to PM2.5 levels. I agree that emissions from Seoul would be higher than those in Incheon, but how can these results be interpreted?

2. line 37, the high correlation between oxalate and sulfate is not directly provided with figures or tables in the manuscript. I presume that this is an important finding as they explicitly mentioned this in the abstract. So, I would suggest presenting a figure that directly shows timeseries of SO4, NO3, NH4, and oxalate (at least in supplement). In addition, further discussion seems to be still required. The authors noted that the high correlation between the two species indicates a secondary aqueous aerosol formation. Many studies reported elevated levels of SOA under humid or foggy conditions and speculated that these are associated with aqueous phase SOA formation. The higher oxalate concentration during the polluted period (0.7 μg m-3, Table 3) than that during the clean period (0.2 μg m-3) seems meaningful with a factor 3.5 enhancement, which is not negligible. However, OA in Seoul is only enhanced by a factor 1.7 (line 313-314). In fact, this enhancement actually reflects OC enhancement because they applied a constant factor (1.8) to OC to calculate OA. Although the authors said the mass closure is good, the remaining mass (PM2.5 minus the sum of all aerosol species except for OA) relative to OC is largest in the polluted period, followed by transition and clean periods. That is, for example, the sum of all aerosol species except for OA is 100.3 μg m^–3 in the polluted period, and the ratio of the differential mass (= 127.2 – 100.3 = 26.9 μg m^–3) to OC (= 9.4 μg m^–3) is 2.86 while it is 1.75 in the clean period (Table 4). I understand that all of the remaining mass would not be OA, but part of it should be OA. I wonder if the authors can provide some insight into (or evidence) the increased level of OA or SOA during the pollution event. Besides, I doubt if a constant factor of 1.8 can be applied to both polluted and clean episodes.

3. In Fig. 5, the sum of PM0.94 and PM2.5-0.94 (e.g., 66.9 μg m^–3 in polluted case) and the sum of all species listed in Table 3 (= 73.3 μg m^–3 in polluted case) do not match. Any explanations?

Minor Revisions:

• Line 106: Fig. S1 should be in the main text and a note on the distance between Seoul and Incheon. Figure S1 should include the location of the Incheon Met Site
• Line 151: I am not that familiar with the OPC-Grimm 1.109, but I believe it does not actually measure aerosol mass. It measures size distribution and then a density and calibration is used to calculate mass (from an online manual this may be done with dolomite dust). This should be addressed in the text. In addition, it gives a reason to use the Sungi PM2.5 in subsequent analysis as it is the same method as used in Seoul (which you typically did).
• Line 415-418: You state that the reductions in CO from the polluted to clean periods could be due to Chinese influence. However, if this were solely the case, wouldn’t CO be higher (or comparable) at Incheon than Seoul. Seoul always has higher CO than Incheon. Maybe some of the enhancement is due to Chinese transport but in addition, the shallow boundary layer may allow for an increase in CO due to local emissions. Looking at CO/CO2 would help in the future.
• Line 434-436. It is unclear what you are calculating by relative fraction in supermicrometer to all sizes. Does the 43% mean 43% of the sulfate is in the supermicron and 57% is in the submicron? Please clarify.
• Line 496-499: a statement is made that based on this campaign that PM1 composition measurements can be assumed to be the same as PM2.5 composition for modelling purposes. A cautionary note is included starting on line 501 stating that this is not the case for cleaner periods. While this is true for this case, it may not be the same for other haze events such as those mentioned when temperatures are higher and therefor the composition is different (less nitrate). It isn’t clear how frequent these low temperature haze events are and in fact on line 524-527 you state this was a fairly uncommon event. The cautionary note here should be expanded to show the limitations of analysis from this one event for understanding the PM1-to-PM2.5 connection (here and also in the conclusion – line 557-558).

Typos/Suggestions:

• Table 1: include elevation; I think all are near sea level but it should be included
• Line 142: “Furthermore, the” should be “The”
• Line 199/208: BAM is an abbreviation for Beta Attenuation Monitoring
• Line 251: how can secondarily-produced species include primary organic aerosols? Should it be “includes primary organic aerosols and secondarily-produced species (i.e., SO₄^2- and SOA)”
• Table 3: caption repeats the dates related to the polluted, transition and clean time periods. This has already been stated so do not repeat it (it just makes the caption more cumbersome).
• Table 3 and following: ozone, NO2 and SO2 should be reported in ppb. CO can be in either ppb or ppm
• Line 284-285: For comparison. What is the Seoul - Sungi value for the entire polluted period, entire transition period and entire clean period.
• Line 360: The sentence starting with “Oxalate is produced” should be moved before the sentence starting “The strong correlation”
• Line 455-457: it is confusing to discuss both NOR and SOR value at the same time. Discuss nitrate in one sentence. And then sulfate.
• Line 558: “for haze pollution.” should be “for this haze pollution event.”