This paper shows some VTDMA data from China, with the intent of investigating factors affecting the mixing state of refractory material in a polluted environment. The results are interesting and within the remit of ACP, and the manuscript is reasonably well written. Also, because measurements of this nature are particularly common, there is an element of novelty in its own right. However, this paper is slightly let down by the fact that the results are interpreted in a very self-contained manner, without really considering the wider body of knowledge. Addressing this should be fairly straightforward, however this could potentially change the character of the paper, therefore I recommend publication after 'major' corrections.

Major comments:

The authors give an interesting discussion investigating the potential reasons for the phenomena they observe, however they do not place this in the context of wider atmospheric implications. In particular, this paper would benefit from a comparison with equivalent measurements in other locations, or alternative methods of measuring BC mixing state (e.g. doi: 10.5194/acp-20-3645-2020). This will allow for a deeper insight into the processes and phenomena under investigation.

It would also better justify this as an ACP research article (as opposed to a measurement report) if either novel implications for wider atmopsheric science could be specifically identified, or if newly-identified phenomena could be singled out.

Minor comments:

Line 23: Taken in isolation, “weaken the volatility of soot particles” is a strange statement to make because many use the term “soot” synonymously with the refractory components like black carbon. I would rephrase.

Page 69: Co-emitted organic carbon from biomass burning can be refractory (sometimes referred to as ‘tar’ or ‘tarballs’).

Line 157: This repeats a statement already made earlier.

Line 166: I presume the factory calibration was used to calculated MBC, but this should still be stated.

Line 217: “better atmospheric diffusion conditions” needs to be better explained

Line 222: This paragraph doesn’t really say anything substantial and can probably be removed.

The authors present a five-month aerosol volatility measurement at a suburban site in North China Plain, focusing on analysis and interpretation of data from a volatility tandem differential mobility analyzer (VTDMA). The manuscript is like a measurement report. Throughout the manuscript, the authors tried to explain their measurements by some reasons that they could not demonstrate, using the sentences “likely caused by…”，“likely due to…”，“likely because of….” to interpret their results. Some discussions and conclusions are not reasonable and even wrong. For example, the authors conclude that anthropogenic emissions could weaken the volatility of soot particles and enhanced their degree of external mixing, which can not be supported by the measurement results that show increased fractions of non-volatile mode soot (i.e., externally mixed BC) and decreased coating depth with size increase. These measurement results follow the diffusion growth theory, namely condensation process of secondary aerosol components (i.e., coating materials) on soot surface is more sensitive to smaller size particles, in other word, the coating growth is effective for smaller soot particles.

General Comments:

The authors took 40-nm and 80-nm particles to explore volatility of nucleation-mode soot particles. The VTDMA measurement show that the particles in nucleation mode (represented by 40-nm particles) had strong volatility and a high degree of internal mixing with shrink factor of ~0.4, meaning that the residual particle size after heating was ~16 nm. These residual materials after heating at 300 degree for 40-nm particles are dominated by extremely low-volatile components rather than soot, taking into account that a carbon spherule of soot agglomerates has size of 15-30 nm. The authors claimed that the number concentration of 40-nm and 80-nm particles increased quickly due to the influence of new particle formation (NPF) events. Previous studies (e.g., Ehn et al., 2014) have demonstrated that the importance of extremely low-volatile organic components for the initial growth of new formed particles. These extremely low-volatile organic components remain in the particle phase after heating at 300 degree. How the authors to demonstrate that the residual materials after heating at 300 degree for 40-nm particles are soot rather than extremely low-volatile organic components?

Specific comments:

1. Abstract: I don’t think the May, September and October with temperatures in the range of 15-30 degrees are cold months.
2. Introduction (Page 3/Lines 61-62): The authors stated that “However, studies on the mixing state of BC or soot particles in the actual atmosphere are few due to limited observations.” To my knowledge, there have been many field measurements to investigate the mixing state of ambient BC particles using a single-particle soot photometer (SP2) and a soot particle aerosol mass spectrometer (SP-AMS) in recent years.
3. 2.2 Measuring BC: The raw data of AE33 measurement have a large uncertainty due to filter-loading and multiple-scattering effects. However, the authors did not make any corrections for measurement data.
4. Page 7/Line 188：Average temperatures in warm (June, July, and August) and cold (May, September, and October) months only have a difference of ~6 degree. The authors should reconsider classification criteria to discuss monthly variations.
5. Page 8/Lines 218-219: The similar changes in concentrations of BC and PM can not suggest the non-trivial role of BC in the formation processes of aerosol pollution. Both concentrations of BC and PM strongly depend on planetary boundary layer height.
6. Page 9/Lines 246: should be “non-volatile”
7. Page 12/Lines 326-328: There are only two sizes in nucleation mode. The authors made conclusion that the volatility of nucleation-mode soot particles became larger with increasing particle size, which is not solid.
8. Figure 10: The coating depth and temperature have a poor correlation (R2=0.03-0.04), which can not support that coating depth depends on temperature as discussed by the authors.
9. Page 17/Lines 441-442: Which measurement results can demonstrate enhanced nocturnal liquid chemical reactions?

The manuscript investigates the daily and seasonal variability of soot particle mixing state coupling black carbon measurements and volatility tandem differential mobility analyzer data collected in a suburban site of the North China Plain (NCP).

The introduction reports that several other studies investigated the mixing state of soot particles using volatility measurements in the NCP. The introductions underlines that the present study differs from the previous ones because it encompasses two different seasons during a 5-month period. It is important to highlight what are the novelty of the results of this study compared to the previous ones, thanks to the multiple season measurements.

Some of the conclusions are not supported adequately by observations. Figure 4 shows that VV nucleation particles are characterized by a higher volatility during warmer months. On the other hand, the conclusion concerning the seasonal variability of nucleation mode soot particles relies on the assumption that nucleation mode soot is totally internally mixed. This assumption is not adequately supported by the presented results. For example, Figure 3 shows that nucleation mode particles are characterized by a multimodal distribution of SF, and soot could be responsible for the SV and NV peaks, which do not present the temperature trend discussed by the authors (higher values in warmer months). A deeper discussion of the results is encouraged. At line 441 the authors state that “Moreover, enhanced nocturnal liquid chemical reactions were responsible for the enhanced volatility of accumulation-mode soot particles in the nighttime.” No clear evidence of liquid or heterogeneous phase reactions during night-time is provided in this study to support such a statement. Furthermore, soot particle coating is controlled by condensation of vapor phase compounds and coagulation with other particles (Bond et al., 2013; Ko et al., 2020). The link between soot coating and NPF events is quite speculative and is not clear (line 274). If the author are interested in investigating such a link, the particle number in the range 10 -100 nm should be investigated, rather than solely the change in particle number concentration at 40 nm and 80 nm, as done at line 274.

Specific comments:

Line 166. Please specify if BC concentration was retrieved using the MAC suggested by the manufacturer or a site-specific MAC. In addition, the fact that BC is retrieved form optical measurements and the dependency of MAC on BC coating introduce some limitations in discussing BC concentration variability. The authors should mention this limitation in the discussion of results.

Line 228: It is not clear what the authors mean when they report the wavelength-dependent size resolved SF-PDF. It looks like figure 3 reports the SF-PDF for different particle sizes, as selected by DMA1.

Line 239 – 240: I suggest the authors to be more accurate, indicating that previous studies observed fresh BC in the lower bound of the accumulation mode. In fact,  Levy et al. 2014 reports that the highest frequency of externally mixed fresh BC is observed at 150 nm, while Wu et al., 2017 reported that rBC size distribution measurements in Beijing peaked at about 200 nm, with a secondary less significant mode at 600 nm.

Paragraph 3.3.2 The statistical significance of the differences between day-time and night time observed in Figure 7 looks small. Could the authors comment on the observed variability?

Figure 10: Did the author explore a different type of fitting for the relationship between coating thickness (Dc) and PM_10-400 (for example a logarithmic fitting rather than a linear fitting)?

Technical corrections:

Line 71-72. This statement is true in polluted environments

Line 115. A list of the measured meteorological parameters should be added to complete this sentence.

Line 137: The scope of DMA1 is to select particles with a specific mobility dimeter, thus it would be more accurate to write: “the water-based condensation particle counter (WCPC, model 3787, TSI Inc.), measuring the number of particles ranging from 10 to 400 nm.

Line 192: From Figure 2 it looks like in July wind from southeast was present instead of prevalent. Please revise the sentence accordingly.

Line 194: please specify when wind speed is considered high. From figure 2 it is difficult to understand if winds in August were stronger than in the other months.

Line 204: please add a reference for the assumed particle density.

Line 439: coating of soot takes place for condensation of newly formed material and not newly formed particles.

References:

Ko et al., Atmos. Chem. Phys., 20, 15635–15664, 2020

Bond et al., J. Geophys. Res.-Atmos., 118, 5380–5552, 2013