Comment on “ Comparison of the LEO and CPMA-SP2 techniques for black-carbon mixing-state measurements”

The manuscript gave the comparison results of the measured black carbon mixing states from LEO and CPMA-SP2 techniques. The articles fall well into the scope of AMT. In general, the results are interesting and important to the related researchers. The manuscript can be accepted after some major revisions.

Major Comments:

1 The LEO detection limits results and the corresponding impacts of measured black carbon mixing states were mentioned before by Zhao, Shen et al. (2020). The new findings of this manuscript relative to that article should be detailed.

2 The Gassion fit of LEO fits of the LSD signals may still be useful even though the signals detected by the SP2 are saturated. If these particles are considered, the difference between the LEO and CPMA-SP2 methods would not be so different as shown in Figure 4.

3 Around line 360, iff the BC core were not compacted, ie the BC core is filled with some air, then the effective density of the BC core may be reconsidered as shown in (Zhang, Zhang et al. 2016, Zhang, Su et al. 2018). This effects should also be dissucssed.

Minor Comments:

1 Line 209, it should be mentioned that some lag time of the detailed information can be found in Figure 8 or some following discussion.

Zhang, Y., H. Su, N. Ma, G. Li, S. Kecorius, Z. Wang, M. Hu, T. Zhu, K. He, A. Wiedensohler, Q. Zhang and Y. Cheng (2018). "Sizing of ambient particles from a Single Particle Soot Photometer measurement to retrieve mixing state of Black Carbon at a Regional site of the North China Plain." Journal of Geophysical Research: Atmospheres 123(22): 12778-12795.

Zhang, Y., Q. Zhang, Y. Cheng, H. Su, S. Kecorius, Z. Wang, Z. Wu, M. Hu, T. Zhu, A. Wiedensohler and K. He (2016). "Measuring the morphology and density of internally mixed black carbon with SP2 and VTDMA: new insight into the absorption enhancement of black carbon in the atmosphere." Atmospheric Measurement Techniques 9(4): 1833-1843.

Zhao, G., C. Shen and C. Zhao (2020). "Technical note: Mismeasurement of the core-shell structure of black carbon-containing ambient aerosols by SP2 measurements." Atmospheric Environment 243(117885).

Naseri et al. compare several techniques to derive the mixing state from black carbon measurements obtained by the single particle soot photometer (SP2). Their results show that coupling the SP2 with a centrifugal particle mass analyzer provides the best results.

Overall the paper is well written and clearly explains the origin and application of the different methods. This manuscript presents a valuable addition to groups that seek to experimentally determine the mixing state of black carbon using similar techniques. I therefore recommend publication.

Minor comments:

“normalized derivative of the scattering signal is used in the ND approach to obtain analogous information” -> Please explain what the normalized derivative is here.

“The upper coating thickness limit of LEO (line ii), was ∼ 285 nm in our study, due to saturation of the scattering detector” -> was it not possible to utilize the low gain scattering detector? This should extend this upper limit further.

“morphological assumptions” -> please rephrase to “assumption regarding the morphology” or something like that.