This study is a theoretical analysis about the influence of water content of coatings on complex refractive index of aged black carbon. Three models are used, which are closed-cell model, partially coated model and coated-aggregate model. Varied simulations were made since different values of the most noticeable parameters involved were selected. For instance, the influence of relative humidity at different wavelengths, or particle radius was investigated. Consequently, the paper may be accepted for publication in Atmospheric Chemistry and Physics after the introduction of some minor changes.

The main inconvenience that authors should correct is the lack of citations in the results section. Some references should be introduced to indicate that these results are not isolated and can be compared with those from previous analyses. This comparison is useful to discuss the results of this research.

Moreover, a link with real situations, for instance, real cases with high concentration of black carbon and high relative humidity where this theory could be applied, should be encouraged. Similar examples should reveal that presented calculations respond to real conditions.

Minor remarks.

Figure-1 caption should be revised and corresponding labels should be introduced.

Dots after “Figure” in figure captions should be suppressed (from Figure 2 to Figure 9).

Some dots after “Figure” and figure number in text should be suppressed. For instance, L. 188, L. 208, L. 220, L. 239.

L. 214. Introduce one space between “(b)” and “and”.

References should follow the journal style.

This study focuses on complex refractive index of black carbon with coatings under different RHs. They retrieved the refractive index and water content for the non-absorbing coating of BC particles, and then calculated the optical properties using MSTM. This study is very basic study to retrieved the optical properties of the aged BC with different RHs. Based on my opinion, the study had one certain innovation working on the water contents of coatings, because there is limitation on water contents on coatings of BC particles and their refractive index. Basically, I like this work which provide more information for future studies.

Of course, we should notice there is some limitation of this study. All the study did not consider the realistic particles in ambient air. Even the authors did not cite some from other studies which has been well described the coating thickness and mixing structures. I would strongly recommend the authors should considered this issue. Moreover, the authors used lots of long sentences and subordinate clause. It is difficult for people to understand this paper well. I would ask the revisions. Certainly, I would ask the authors revised the manuscript.

Abstract

Line 18-20: the number is difficult to be understood their meanings.

Introduction:

I would like to recommend the authors revised this part carefully.

L27-28 Fresh bare BC will be coated by inorganic salts or organics during aging process such as condensation and collision in the atmosphere, and hydrophobic BC aerosol becomes hydrophilic

This sentence should cite some references here. The BC aging process has been provided by the Li . J. Geophys. Res. 2016, 121(22): 13,784-713,798.

L30-31, the sentence should cite more references here. Certainly Luo et al. is one of this study. The general conclusions should cite more references. Such as Fierce et al., Nat. Commun. 2016, 7: 12361; npj Climate and Atmospheric Science 2024, 7(1): 65. Wang et al., J. Geophys. Res. 2021, 126(10): e2021JD034620. And others.

L60-75, the paragraph did not cover the recent studies using different methods. For example, Wang et al., Geophy. Res. Lett. 2021, 48(24): e2021GL096437. developed one EMBS method to calculate BC and their absorption; Fierce et al., Nat. Commun. 2016 developed the inhomogeneous thickness of coatings on BC particles and improved the absorption. Please make more to cover the knowledge there.

L205 please display the simply model on top of the figure. Then people can know the what are these models.

L260 deleted the well

L265-268, this sentence is too long. The similar sentence should be simplified.

Finally, I would ask what kinds of condition should be considered the water contents. In realistic, could you provide how much difference between water and non-water in coatings influence optical properties of aged particles? If the author could make such comparison, this could be important for the potential readers.