Review of “Measurement report: Size-resolved mass concentration of equivalent black carbon-containing particle larger than 700 nm and its role in radiation” by Zhao et al.

The present manuscript reports the equivalent BC (eBC) mass concentrations contained in the particles with the aerodynamic diameter range of 150 nm to 1.5 um and argues the importance of an abundance and radiative effects of eBC contained in larger (> 700 nm) particles. Because the radiative effect of BC-containing particles is one of the significant uncertainties in numerical models that assess the aerosol direct radiative forcings (DRF), the data presented in this study is valuable. However, some critical points (e.g., the definition of eBC mass size distribution, assumptions of mass absorption cross section (MAC) values to derive eBC mass concentration, and assumptions for estimating the DRF of eBC) are unclear in the current manuscript. The reviewer believes major revisions are necessary before its publication in ACP.

Major comments:

Throughout the paper, it is unclear whether the authors are discussing the whole diameter of the BC-containing particles or the diameter of the BC core within the BC-containing particles. In the reviewer's opinion, “BC mass size distribution (BCMSD)” usually refers to the size distribution of BC cores. However, this study seems to refer to the size distributions of whole particles containing BC and non-BC material, which is difficult to understand unless it is clearly explained. When the authors state that “the mass size distribution of BC is bimodal," the readers may assume that the mass size distribution of BC core is bimodal. In the case of this paper, can it mean that the BC core size is not bimodal, but there are two distinct groups of BC cores containing thin and thick coatings?

More explanations on assumptions of mass absorption cross section (MAC) values to derive eBC mass concentration are needed, although a previous study is referred to in the manuscript. For example, even though the whole particle size is 700 nm, the MAC value should differ depending on whether the BC core is 700 nm (bare BC) or the BC core is 200 nm (thickly coated BC). If the uncertainty in the size-resolved MAC assumed in this study is large, the associated uncertainty in the size-resolved eBC mass concentrations is also considerable. Since the size-resolved absorption data is closer to the raw data, are these data more reliable and can be more central to the discussion in this paper?

In section 2.3, the authors explain the assumptions needed for estimates of DRF, but it is unclear whether these assumptions are consistent with the observations obtained in this study. How are the temporal changes in size distribution and mixing states of BC-containing particles incorporated into the estimate?

Clarifications of these points and English proofreading throughout the manuscript are required.

Specific comments:

L24: “Size distribution” cannot be expressed as high or low. Concentrations are high or low.

L37: “Absorption of BC increases light extinction”. The wording needs to be corrected.

L51: “reported that” is repeated.

L101: In this study, BC-containing particles smaller than 200 nm are not considered when calculating the “bulk” eBC mass concentrations. However, their contribution to total eBC mass concentrations is not negligible (as suggested by Figure 2). This can lead to an overestimate of the mass fractions of eBC>700 within the total eBC mass concentrations. Clarifications are needed.

L103: D_p0 is not defined.

L146-149: Are these assumptions consistent with the observations obtained in this study? It is difficult to understand how the measurement values are incorporated into the DRF estimate.

L169: The size-resolved absorption (Figure 5a3) is monomodal, but the eBCMSD (Figure 5a1) is bimodal, indicating a strong influence of the assumed size-dependent MAC. Some more explanation should be given for deriving the size-dependent MAC.

L174-175: These are dm/dlogD values rather than eBCMSD.

L193-194: It seems obvious since the eBC mass concentration ranges for clean and transition periods were defined as such.

L243: Any reference for interpretation or comparison? (e.g., Liu, D.et al., Contrasting physical properties of black carbon in urban Beijing between winter and summer. Atmospheric Chemistry and Physics 19, 6749-6769, https://doi.org/10.5194/acp-19-6749-2019, 2019)

Zhao et al. presented equivalent black carbon (BC) mass size distribution (eBCMSD) and bulk absorption coefficient based on aerosol size measurements and tandem AAC-AE31 measurements. They found that eBC larger than 700 nm is critical for accurately estimating aerosols' direct radiative forcing (DRF). This study is very interesting and has the potential to help improve the relevant uncertainty in the climate models. However, this manuscript misses several critical points (e.g., how to calculate eBCMSD, how to calculate MAC, and how to calculate BC mass fraction), which makes me unable to validate the results. Moreover, the manuscript needs to be better organized and needs lots of proofreading. Therefore, the manuscript should be rejected and resubmitted after revising the manuscript. I am happy to review the resubmitted manuscript. Please see my comments below to support my decision:

Major comments:

1. The method section needs to be clearer to me, making me unable to validate the results. First, the AE33 measurement needs to be clarified. How did you retrieve σ_abs? Did you correct for multi-scattering? How do you calculate the mass fraction of BC from bulk since you do not know the MAC of bulk? How do you get MAC of eBC? How did you separate iron dust and BC? AE33 measures transmission, which is used to calculate the σ AE33 uses the default MAC to calculate wavelength-depended BC mass concentration. Thus, I do not understand why you need to estimate the MAC of bulk to get σ_abs. I also read Zhao et al. 2021, but I do not know how you could use that method since you do not know the BC core size. Moreover, it is also unclear how you calculated m_eBC, bulk, and estimated the eBC fraction since you do not have an OC-EC analyzer or other measurements to measure organic and BC mass fractions. Moreover, you have different size measurements (APS, SMPS, and AAC), measuring different aerodynamic sizes (APS: optical aerodynamic size, SMPS: electrical mobility diameter, AAC: aerodynamic). How do you combine them together? Furthermore, how do you estimate the particle densities?
2. The trend in mass and σ_abs seem statistically the same, which makes me feel your size-resolved MAC is not different across the different sizes. Please comment on that. I want to see a plot of size-resolved MAC.
3. Results and Discussion section needs to add uncertainties when you show data. Your value is very low, which might be instrument noise or data uncertainty. I suggest having a summary table.
4. When you discuss MAC and σ_abs, you should always put the wavelength. It is meaningless to discuss light absorption properties without mentioning the wavelength.
5. The paper needs to be better organized. The figure number is chaotic. They should be in the same order in the text. Captions of Fig. 3 and Fig. 4 are not acceptable. You should provide all the necessary information in the caption. Also, your subplot label should always be distinct from your plots.
6. This paper needed to provide a detailed explanation of the discrepancy between Changzhou and Pekin. Is that due to different eBC sources, seasons, geographical locations, or something else? What can we learn from your results? How can we utilize your results?
7. The manuscript needs to be proofread by professional proofreaders before resubmission. There are lots of grammar issues.

Specific comments:

1. L56-59, “It should be noted … (chow et al., 2001). I suggest discussing "Soot superaggregates from flaming wildfires and their direct radiative forcing." Offline microscopy technology can be used to analyze large soot particles.
2. Figure 5 a2 and a4: Why PDF mode overlaps with lower quartiles, not the median?
3. L179-181, “In order to … 1.0 µg m^-3.” How did you define the thresholds of these three periods? Based on statistical analysis or literature?
4. L218, “It could be … ubiquitous.” When you say something is ubiquitous, abundant, etc., you should show the fraction with uncertainties. Please check the manuscript and revise it accordingly.
5. L242-243, “It could be … boundary layer.” What do you mean by Level of eBCMSD? Total mass or particle size? Why do you say that the planetary boundary layer regulates it? There should be more emission of eBC during the daytime, especially at morning and evening traffic peaks.
6. L260-261, “The large spread … on absorption.” How about different BC mass in different size bins?
7. L320-323, “The increase in … that in Beijing.” Please explain the difference in detail (see my previous major comment).
8. Section 3.4 Case study. Why do you have this section here? It should be discussed in your previous sections. Moreover, what’s the advantage of using mean diameter compared with geometric mean diameter?
9. The conclusions section needs to add more discussion about environmental implications.