This manuscript by Yu et al. introduced the aerodynamic size-resolved chemical composition and CCN activity of aerosols in the Beijing suburban region. The study combined an aerosol aerodynamic classifier (AAC) with a set of aerosol physical and chemical measurements and focused on the properties of refractory black carbon-containing particles (rBCc). The study found that rBCc are relatively spherical at sizes above 300 nm, and the number fraction of rBC increases as a function of particle size. Due to the coating properties and their larger sizes, a relatively large fraction of the rBCc could also be activated to contribute to cloud formation. The manuscript is well organized. I recommend the publication of the manuscript after the following minor revisions.

General comments:

1. I found the higher number fraction and larger MMD of rBC at larger sizes very interesting. The authors attributed this phenomenon to particle coagulation. Is this coagulation happening among the rBC particles or between rBC and other larger particles? Considering that fresh soot particles directly generated from engines are relatively small (~ 100 nm), how fast is this coagulation process and how does the involvement of other chemical species affect the evolution of rBC?
2. The coating thickness Dp/Dc showed a minimum value at the size of around 300 nm. The authors imply that traffic emissions may play a role in this change of coating properties. Could the authors elaborate more on the detailed mechanisms?

Detailed comments:

1. Page 4 Line 103: There are a few more studies on size-resolved CCN activity and aerosol physiochemical properties in Beijing, such as the following ones. Probably the authors want to stress that this study focused on rBC and used an AAC to size-classify the aerosols.

Gunthe, S.S., Rose, D., Su, H., Garland, R.M., Achtert, P., Nowak, A., Wiedensohler, A., Kuwata, M., Takegawa, N., Kondo, Y. and Hu, M., 2011. Cloud condensation nuclei (CCN) from fresh and aged air pollution in the megacity region of Beijing. Atmospheric Chemistry and Physics, 11(21), pp.11023-11039.

Fan, X., Liu, J., Zhang, F., Chen, L., Collins, D., Xu, W., Jin, X., Ren, J., Wang, Y., Wu, H. and Li, S., 2020. Contrasting size-resolved hygroscopicity of fine particles derived by HTDMA and HR-ToF-AMS measurements between summer and winter in Beijing: the impacts of aerosol aging and local emissions. Atmospheric Chemistry and Physics, 20(2), pp.915-929.

Wu, Z., Zheng, J., Wang, Y., Shang, D., Du, Z., Zhang, Y. and Hu, M., 2017. Chemical and physical properties of biomass burning aerosols and their CCN activity: A case study in Beijing, China. Science of the Total Environment, 579, pp.1260-1268.

2. Page 4 Line 113: One of the disadvantages for the AMS measuring the size-resolved composition is that the AMS cannot measure CCN related sizes (50 to 100 nm). But it appears that the AAC was used in size range of 90 to 1100 nm, which is not significantly better than the AMS measurement range.
3. Page 7 Eq. (2): How was rou_NR calculated? It may be better to include a table of nomenclature to introduce each of the parameters and how they are measured (by which instrument) or calculated.
4. Page 7 Eq. (5): How was epsilon_coating,i measured or calculated? Also, in Fig. 5h, the kappa values for rBCc are sometimes higher than those of all particles. Is this reasonable?
5. Page 9 Line 252: “idea” change to “ideal”
6. Page 9 Line 254: Was the scanning of the SS done in this work? In the methods section, the SS was fixed at 0.2%.
7. Page 10 Line 296: “… non-refractory aerosol mass concentrations during light pollution periods show limited size-dependent variation.” There are indeed significant variations as a function of particle size, although the absolute concentrations are relatively low.
8. Page 10 Line 306: Why was the Org highly oxidized in Beijing suburban? It may be better to elaborate this observation in more details.
9. Page 13 Line 400: “Fine rBC condensed …” Since rBC are in the particle phase, not in the vapor phase, they cannot “condense” onto pre-existing particles.
10. Page 15 Line 456: remove “and”.
11. Fig. 5b, 5c, 5d, 5e and 7a, 7b: Should the labels on the y-axes “dM” and “dN” be “dM/dlogDp” and “dN/dlogDp”? Also, why are the error bars shown in the positive direction only?
12. Fig. 8: Please show the legends in panels b and c (black curve is not introduced).

This paper presents online, size-resolved measurements of black-carbon-containing particles and associated properties collected in a suburb of Beijing. The strength of this work is that it simultaneously probes size-resolved composition and other properties, which makes this work potentially useful for modeling studies in terms of providing ground truth of “what is out there”. The work fits well within the scope of ACP. I have two major comments and a number of minor comments that should be addressed before the paper can be accepted for publication.

Major comment:

1. Section 2.2: An important assumption for the calculation of density and D_v,all is that mineral dust is not present in the samples, since dust cannot be detected by the instrumentation used for this study. This should be mentioned here, and a discussion is needed to what extent this assumption can be justified and what error is introduced by this assumption.
2. The large, heavily coated particles reported here are interesting. I would assume that it takes a long time to grow such thick coatings on such large cores by condensation of low-volatile vapors. The authors mention coagulation as another possibility of how such these particles could be formed. However, coagulation is usually also a slow process in the atmosphere (depending on number concentrations). Could such thick coatings be formed by in-cloud aqueous phase chemistry and resuspension of the coated aerosol after the cloud evaporates? Are the coatings of these particles really secondary in nature or are they possible at least partially primary? It would strengthen the paper if the authors provide could some back-of-the-envelope estimates of how these particles could be formed.

Minor comments:

1. L. 36: “By applying the mixing state of refractory black carbon containing particles …”. The word “applying” sounds strange here. Do you mean “considering”?
2. L. 73-81: A relevant reference to motivate the work presented in this paper may be Ching J, Kajino M. Aerosol mixing state matters for particles deposition in human respiratory system. Scientific reports. 2018 Jun 11;8(1):1-1.
3. L. 150: Since different kinds of diameters are used in this paper depending on measurement technique, please specify what kind of diameters are Dp and Dc.
4. It would be good if the results found for Beijing could be contrasted to other environments, for example see Motos G, Schmale J, Corbin JC, Zanatta M, Baltensperger U, Gysel-Beer M. Droplet activation behaviour of atmospheric black carbon particles in fog as a function of their size and mixing state. Atmospheric Chemistry and Physics. 2019 Feb 20;19(4):2183-207 and Motos G, Schmale J, Corbin JC, Modini R, Karlen N, Bertò M, Baltensperger U, Gysel-Beer M. Cloud droplet activation properties and scavenged fraction of black carbon in liquid-phase clouds at the high-alpine research station Jungfraujoch (3580 m asl). Atmospheric Chemistry and Physics. 2019 Mar 25;19(6):3833-55.
5. Section 2.2: The notation in this section is unclear. Equation (1) refers to a per-particle quantity, but this is not how the authors use it here, since Line 179 mentions “for all particles”, which I assume means “averaged over all particles”. However, Figure 6 presents size-resolved graphs of shape parameter, which implies that equation (1) is calculated as an average for each size bin. Please clarify and improve the notation so that this becomes clear.
6. Line 276: “To test whether the mixing state varies according to ambient pollution concentrations…” Do you mean mixing state as in “internally/externally mixed” (i.e., within a given size range)? This sounds like an interesting idea, but I don’t think it is answered in the results section since this is looking at the dependence of various quantities on particle size – I would not call this mixing state.
7. Line 292: “no significant difference”: suggest rephrasing this since it is not meant in the sense of “statistically significant”
8. Section 3.2: It would help the reader if you could add to this section references to the individual figures that you are referring to.
9. Definition of rBC-containing particles: What is the minimum core size that can be detected?
10. Line 374: Is the more spherical-like morphology for larger particles because the coating material forms a spherical coating around the non-spherical core, or does the core itself become more spherical because it collapses? This may seem a minor point, but it is important when justifying the use of Mie calculations for BC-containing particles, which assumes a spherical core.
11. Line 470: Why were optical properties evaluated for a wavelength of 880 nm? Unless this was done to compare to measurements (which is not the case here), it is more common to do this for ~500 nm (closer to the peak of the spectrum from sunlight).
12. The use of English language is appropriate for the most part although some paragraphs/sentences would benefit from being proof-read by a native English speaker.