This study  calculated the optical properties and radiative effects of black carbon (BC) particles with different absorption Ångström exponent (AAE) methods, and further estimated their effect on brown carbon (BrC). The manuscript explained how the microphysical properties of BC particles determine wrong BC’s and BrC’s absorption and radiative properties under two AAE methods. The study is interesting and the results are helpful for understanding why the optical and radiative properties of BrC is deviated to BC. Overall, the manuscript can be revised and then may be published in ACP. The problems are addressed as following:

Main:

1. The author calculated the absorption deviation for BrC by using non-absorbing coating. If these calculations make sense, we must assume that the true AAE of the BC particles with BrC coating is the same with those with non-absorbing coating. However, is that true? And hence I concern whether the “b_{abs_BC_440_Estimated} - b_{abs_BC_440}” can represent the Δ I’m very confused here.

Besides, refractory index is one of the factors determining the absorption. The deviation of BC absorption will be also affected by the refractory index. All the discussion in this manuscript is carried out under the 1.95+0.79i for BC and 1.55+0i for coating. The results for “the estimated BrC absorption should be the absorption from BC that is incorrectly attributed to BrC” is not comprehensive because the refractive index varies.

1. The study may have an important implication on the estimation of BrC’s optical properties and radiative effect. This should be addressed in the manuscript. The author only summarized the main conclusions and did not give impressive implication.
2. The author gave the results for F<=0.3. However, the aged BC particles have F ranges in 0-1 in the atmosphere. It seems the present results in this study are not complete. I think it is impossible to construct a shape model with large F and small coating thickness due to the limitation of the MSTM method (The coating must be sphere). If this is the situation, then why the author did not use DDA to calculate the results? If the data for F > 0.3 can not be supplemented, please clarify this in the main context.

Specific:

1. Line 6: The term ABS_BrC was not well explained. It is not “the estimated BrC absorption”. The ABS_BrC seems to be a critical parameter to understand the whole manuscript. In the Abstract, the meaning of ABS_BrC should be clearly explained to help readers to understand the results mentioned in the Abstract.
2. Line 9: The full name of “WDA” was not mentioned before using this abbreviation.
3. Lines 181-182: “the corresponding r_max and r_min are 0.0342 μm and 0.2 μm” seems a wrong sequence for r_max and r_min.
4. Lines 235-237 and 238-239: It seems that the AAE_{440_870} = 1 method often has larger ABS_BrC than AAE_{440_675} = 1 method. Why this happens? The results should be tried to explain here.
5. Lines 243-244: The sentence is too complicated. Maybe the author miss some punctuation marks.
6. Line 246: “can be observed” I don’t think “observed” is a suitable word here.
7. “On the other hand, the AAE increases with Dp/Dc when BC has a fluffy structure. Thus, the AAE can be greater than 1 when the fluffy BC is partially coated with a thick coating (Zhang et al., 2020b; Luo et al., 2023), resulting in ABS_BrC of less than 0.” The author tried to explain why the ABS_BrC < 0, but there is no direct causal relationship between the former and the latter sentence. The author needs to give more reasonable explanation.
8. Line 256: “The more compact structure can also represent another process of atmospheric aging”. The compact structure was caused by many aging processes. I wonder the author means which process?

The study aims to quantify how the microphysical properties of BC affect BrC absorption estimates, providing insight into the uncertainties surrounding the BrC assessment through absorption measurements at various wavelengths. This study is very useful and significant which may bring an important advance in the estimates of BrC absorption.

Main: The results obtained in this work are impressive and significant, however, the presentation of these results should be better. Be more succinct and direct, highlight the main results, improve the figures (tables?), and worry about making better captions in the figures and not just explaining them in the middle of the text, this will make reading pleasure.

Below are some comments and specific suggested adjustments to the text.

1 Introduction

line 22- Carbonaceous aerosols are a major contributor to climate “change”.

line 55 - A quick and effective way to differentiate BC, BrC, and dust using Scattering Angstrom exponent x Absorption Angstrom Exponent ratios is described in studies such as (CAPPA et al., 2016; CAZORLA et al., 2013; RUSSELL et al., 2010).

line 70 – “while more recent studies based on measurements and simulations have shown a wide range of AAE values”. Which studies? Please mention it here.

2 Estimating the BrC absorption

Lines 118-120 – “we assumed the following cases for aged BC aerosols: (1) fluffy BC cores partially coated with other materials; (2) compact BC without coating materials; (3) compact BC partially coated with other materials; (4) compact BC fully coated with other materials”. In my opinion, they are very promising choices.

Figure 1 – Please explain in more detail each of the 4 cases in the figure legend.

Line 157 - of nonabsorbing

3 Results

The results text is quite challenging to understand, with a lot of technical data and few graphics, and tables. It would be great to explain the results more pleasantly. Maybe doing better figures.

Did you use AERONET data? which sites? make it clear and explicit in the text.

Figure 2. ABSBrC at different Dp/Dc estimated using the AAE = 1 methods. What different Dp, Dc and F indicate? Please clarify at legend too. Example: The increases in F may represent a process of atmospheric aging

Figure 3 – 4, Add the explanation on the legend: “we see that ABSBrC increases with rg when AAE is fixed. This is caused by a decrease in AAE with increasing rg for fluffy BC”. The article will be easier to read

Figure 5. “Similar to Figure 3, but using the WDA method”. Please it

Lines 303 – 304 “The estimated ABSBrC based on the WDA using the bare sphere model and the core-shell model has comparable values and can vary in a range from about -40% to 36%”. What does it mean? Explain.

Figure 6 - Please explain the figure better, what does this purple bar mean for example? What significant result did you get? highlight this in the caption.

Lines 388-389 “The estimated DRF in this work may be about +0.216 – +0.612 W/m2 , which is generally in the range of values reported by previous studies”. How did you estimate it?

Figure 9. “The global mean BC AAOD calculated using different models”. What is the most important result of this figure? explain. Perhaps this is one of the most important figures in his work.

4 Conclusions and Summary

In conclusion, try to find explanations for the results obtained and not just repeat succinctly what was said in the results.

Will you prepare any material supplementary?

In summary, the work is very good and could mean a great scientific advance in BrC estimates from BC measurements. In addition, it can bring significant advances in the estimation of the radiative forcing of absorbing aerosols. Congratulations on a great job.