Liu et al. evaluated the use of HTDMA-measured size-resolved κ to predict the mixing state index (χ, YZ method) with χ predicted by a particle-resolved model. Overall, this paper suggested that the YZ method is valid, especially in urban and continental regions. Liu et al. also provided estimates of the uncertainties associated with the YZ method, which can be applied in the future by others who use it. Overall, Liu et al. provided very detailed evaluations and discussions. However, I feel this paper still has many areas for improvement. I would suggest a major revision.

Major comments:

I feel this paper needs to improve by adding more scientific discussions. Currently, it is a very good technical note with some scientific discussions. One particular section where I want to see more scientific discussion is Section 5. The authors presented long-term measurements but did not provide sufficient discussion. For example, I am interested in the impacts of different sources on κ and χ. What will be the impact of chemical compositions, since most sites have AMS or ACSM? Any seasonal variations? Are there any site dependencies? I am also curious if you have predicted χ using your particle-resolved models at these sites. How will the confidence intervals discussed in previous sessions apply to this session?

1. I am not fully convinced why marine aerosols are not included in both methods, and why including them will cause overestimation of χ. Three of your sites are in the coastal regions (EPC, CRG, and HOU). Excluding marine aerosols will case significantly uncertainties in predicting aerosol composition and κ. I understand that marine aerosols are typically in the coarse mode, but there is growing evidence that fine marine aerosols are also prevalent. It would be beneficial to add more discussion of the exclusion of fine marine aerosols in the paper.

2. I have some comments related to ARM data. It is not very clear to me where each measurement is located. Moreover, as I mentioned in the previous comment, there are many other measurements in these sites, including aerosol size distribution, aerosol chemical composition, and meteorological measurements. Please consider utilizing the open-source data to provide a comprehensive discussion of χ and κ.

3. Could you add more types of hygroscopic species to the YZ model? How will that affect your results?

4. Please make sure to define the acronyms in the first place in the main manuscript.

5. There are some places not well defined at first. Please refer to my minor comments.

6. The figure size is too small and hard to read. I suggest make them larger

Minor comments:

1. L53-56, “Accordingly, … is computed.” It will be helpful to define MH and LH here since it is the first place these two terms appear.

2. L 78-80, “For the aerosol … can be calculated as:” It is not clear to me what bin means here. Do you mean size bin or κ bin?

3. L151-155, “We then … Na, and Ca.” χPMC was not defined in the main text before. Please define ARO1, ARO2, ALK1, OLE1, API1, API2, LIM1, LIM2, OIN, and OC.

4. L179-180, “Under the … LH species.” Could you explain why this happens?

5. Would section 4.3 fit better after section 4.1?

6. Could you label which one is internally mixed and which one is externally mixed in Fig. 2 a and b?

7. Figure 6 should come first before Figure 5 to help understand the site locations.

8. L259-260, “The results indicate … up to 86%.” Where are your results, and why do you think χ is an overestimate, and compared to what?

9. L273-283. “The coexistence … internally mixed populations.” I have a hard time following these two paragraphs.