Review of “A Study on the Key Factors Determining the Hygroscopic property of Black Carbon” by Su et al.

In this study, the authors investigate the hygroscopicity of five types of black carbon (BC) samples in the laboratory by measuring the change in mass of each sample as a function of relative humidity. They also try to interpret the hygroscopicity data by analyzing BC samples with the OC/EC analyzer, GC-MS, IC, and Raman spectroscopy. Because the hygroscopicity of BC-containing particles is an important parameter affecting the atmospheric behavior and climatic impacts of BC, the data presented here is valuable. However, many clarifications are required so that the readers can understand their experimental procedures and the validity of their results and interpretation. The reviewer believes major revisions are necessary before its publication in ACP.

Major comments:

Due to a lack of information and explanation, the reviewer could not understand the details of the experiments. For example, were the BC particles generated by the combustion of n-hexane, decane, and toluene collected on filters for further experiments? Is the U black carbon (UBC) sample originally suspension or a dry powder? No details of the aging experimental setup were also provided. Are there any previous studies using the same setup? What are the diesel engine operating conditions for collecting diesel BC (DBC) samples? It is difficult for the reader to reproduce their experiments with only the information provided in this manuscript.

The definition of the term “BC” should be clearly stated. In this study, the authors seem to refer to a BC particle as an entire soot particle that can contain other organic or inorganic compounds. However, this treatment can make various descriptions unclear. For example, an expression like "OC in BC" (Line 112) would be confusing for many researchers in the community. As discussed in previous studies (e.g., Petzold et al., 2013), it may be clear if mixed particles containing a BC fraction are termed “BC-containing particles” instead of “BC particles.” Please consider the terminology used in this study and state it clearly.

In the reviewer's understanding, this study analyses bulk samples, which means that the information on the mixing states of the particles collected on filters (e.g., mixing states of the particles collected from diesel exhaust) is not obtained. Because this study discusses the hygroscopicity based on the increase in the mass of each sample, does it implicitly assume that all non-BC components are internally mixed with BC? In other words, is the measured hygroscopicity considered an upper limit of the hygroscopicity of BC-containing particles? (If some non-BC components are externally mixed with BC, then the hygroscopic growth (mass increase) of the non-BC component should not contribute to the hygroscopic growth of BC-containing particles.) Explanations should be given.

A clearer description of this study's new findings should be provided. The authors point out “a lack of comprehensive understanding regarding the factors that determine the hygroscopic properties of fresh BC” in the abstract and introduction (Lines 15 and 75). From that perspective, however, aging experiments with SO2 are not intended to gain insight into fresh BC and are, therefore, not directly relevant. Also, it is well known that coatings of inorganic substances increase the hygroscopicity of BC-containing particles.

The title is too broad and should be more specific. English proofreading throughout the manuscript is also desired.

Specific comments:

L34: There have been many studies on the radiative effects of BC after 2001, and their understanding has been much updated. Please consider adding or updating the references.

L51: Is “commercial BC” not “fresh BC”? Is the UBC a commercial BC in this study?

L111: EC = EC1+EC2 + EC3?

L166: P has already been defined in L165.

L183: v and v_m are lowercase letters in Eq. (3) but uppercase letters in Eq. (1). 

L195: Table 2 lists the concentrations of SO42- and NO3-. Can the mass of EC and OC also be listed in the same Table?

L213: Five different samples were used in this study, but the results from the OC/EC, GC-MS, and Raman analyses are only shown for the three samples. Why is that?

L273: If I_D is the sum of I_D1 and I_D4, why are some values of I_D1/I_G larger than I_D/I_G in Table 3?

L295: Under the experimental RH conditions lower than the deliquescence RH of ammonium sulfate, ammonium sulfate would not contribute to hygroscopic growth even if present in abundance?

L306: Since this study examines not the ice nucleating property but the hygroscopicity of BC-containing particles, the comparison with previous studies focusing on the ice nucleating properties of BC may not be appropriate here. I suggest adding more explanation or eliminating the description of the ice nucleation. In addition, since it is now common to refer to ice nucleating particles (INPs) rather than IN, I suggest modifying or eliminating the use of the term IN in the abstract.

Reference: Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A., and Zhang, X.-Y.: Recommendations for reporting "black carbon" measurements, Atmos. Chem. Phys., 13, 8365–8379, https://doi.org/10.5194/acp-13-8365-2013, 2013.

Comment on “A Study on the Key Factors Determining the Hygroscopic property of Black Carbon”

Black carbon is a crucial component of aerosols in the atmosphere and the corresponding hygroscopicity is important for studying their CCN, IN and lifetime properties. In this study, the hygroscopic properties of BC parties from different fuel and aging process were measured. The results is interesting and convincing. I recommend this manuscript to be published after some major revisions.

Major comments:

1 One of the major concerns is the basic microphysical properties of the generated BC particles from different types. Are there any size distribution and morphology information about the DBC and UBC? These properties are important for understanding the hygroscopicity of the BC.

2 The VSA measure the hygroscopic properties of the BC. I didn’t get the information that the measured results represent the BC particles of bulk information or single particle? The author mentioned that the mass of BC under dry conditions was typically 1-5 mg. Please give us the size information of BC particles as the size is a very important parameter that relate the hygroscopicity with the mass increment.

Minor comments:

1 Maybe a table is enough for figure 3.

2 In section 3.1 there are five types of BC particles. Why are there only three type of BC presented in section 3.2?

3 As best as I know, the hygroscopic properties of BC particles were not directly related with the ice nucleation activation of BC in this study. The author mentioned the ice nucleation activation for many times but I don’t think they necessary.

Publisher’s note: this comment was edited on 15 November 2023. The following text is not identical to the original comment, but the adjustments were minor without effect on the scientific meaning.

This paper presents the hygroscopic behavior of BC particles generated from different types of fuel and aged with SO2 for varying durations and explore the key factors that influence the hygroscopic of BC. They found that the presence of water-soluble substances in BC facilitates increase the number of water adsorption layers at high relative humidity. And the hygroscopicity of BC can be enhanced by the formation of sulfate ions due to heterogeneous oxidation of SO2. I believe that the topic is interesting and it could be useful to the scientific community. However, some modifications are needed before they can be accepted.

Major comments:

1. The hygroscopicity of five different types of BC and the influencing factor was explored. However, only the aging of UBC was measured. The finding that low relative humidity has a limited effect on BC hygroscopicity is suitable for other black carbon (such as n-hexane flame BC, decane flame BC)? For example, will OC present in toluene flame BC lead to a decrease in the humidity turning point?
2. the key factor influencing the BC hygroscopicity was analyzed, including water-soluble ions, organic carbon content, and microstructure. However, BC size is very important for explore its hygroscopicity, but author seems to have neglected this. Please clarify.
3. why the microstructure of DBC and UBC was not measured?
4. The author has analyzed the influence of different factors on the hygroscopicity of BC, but in the ambient atmosphere, the main driving factor was difficult to identify. Could the author further discuss this through the results of this study?
5. For the prepared BC, the Raman spectra suggested that the ID/IG of n-hexane BC, toluene BC and decane BC increased in turn, and the hygroscopicity of BC showed the same tendency. But it is not very valid to conclude that the ID/IG was positively correlated with hygroscopicity. The discussion should be expanded to support your conclusion.

Some specific comments:

Line 17 “were” should be revised as “was”.

Line 284: Author found an increase in sulfate ions on UBC with longer aging times while the MRH of UBC remains relatively unchanged with SO2 aging, does this mean the sulfate coating have no effect on the hygroscopicity of black carbon under low humidity?

Line 302 How do you define this “high” RH levels? Here the “noticeable augmentation” and “high RH” should be specified.

Line 320: “ And provides a basis for improving our understanding………..”, means what provides….?