The manuscript investigated the mixing state and light absorption enhancement of BC-containing particles at Beijing and Gucheng during winter. They found that coating of second inorganics has a larger enhancement than organic coatings. Overall, this study can improve the knowledge of BC light absorption enhancement and evaluation of BC light absorption during the atmospheric aging process. This can help reduce the uncertainties in BC climate effects. However, this manuscript is not well written. Many cases require additional information to clarify motivation, methodology, results, and interpretation. My impression of this paper is that it could be improved by considering the following suggestions. The revised paper should discuss these points, not just answer in the authors' response. I am willing to review the revised manuscript. Therefore, my recommendation for the editor is that this manuscript needs major revision.

General comments:

1. Captions of figures do not include all necessary information. Also, some figures are very confusing. Please see my specific comments below.
2. In the Methods section, the authors could provide more information about sampling, analysis methods, instrumentation, etc. I understand some methods have already been published and are widely used in the literature (e.g., L125-127, “The detailed … in factor i.”). However, it is better to provide short summaries of these methods in the main manuscript or SI. It is not clear how these methods work for me, and I have to go back to the original references.
3. Many discussions are not clear to me. Please see my specific comments.
4. I did not see any discussion about uncertainties. Please add these like uncertainties in instruments and data.

Specific comments:

1. In this manuscript, you used terms such as ‘coating’ and ‘internally mixed’. In this case, I assume you mean particles are internally mixed and core-shell morphology. However, it could also be partially coated or aggregated together, which you cannot tell from AMS or SP-AMS. Do you have any TEM images, EDX mapping, or shape factor measurement? If you do not have evidence to support core-shell morphology, I would like to see some discussions about the effects of different morphologies.
2. I understand that you used 870 nm PAX to measure light absorption properties since many studies believe only BC can absorb at 870 nm. However, many studies have pointed out that brown carbon (BrC) can also absorb at 870 nm, leading to overestimating your eBC. Moreover, BrC can also scatter light at 870 nm, leading to overestimating your BC scattering properties. These two can result in different results of ΔF_R. I did not see any discussion about these. Please add discussions.
3. In this study, the max temperature of the thermodenuder (TD) is 250 °C. However, this temperature might not remove all BrC and inorganics (see "Two-stage aerosol formation in low-temperature combustion" and "The Brown-Black Continuum of Light-Absorbing Combustion Aerosols"). How do you account for that? Also, did you count particle loss in the TD?
4. In your calculation of absorption enhancement (E_abs=b_abs,total/b_abs,BC), b_abs can be different for the same component if their concentration is different before and after TD. It is better to use MAC or imaginary part of the refractive index.
5. L114-118, “Seven particles … are shown in Table S1.” How many signals of each fragment do you need to classify a particle to a type? How did you decide the thresholds?
6. L130-131, “ BC-containing … (Xie et al., 2020).” I am curious why there is a discrepancy between your study and Xie et al.?
7. L135-137, “Comparatively, … at the urban site.” This is ture for large particles that SO₄ is generated from fog or cloud processing. SO4 could be generated from anthropogenic sources such as coal combustion for small particles. Do you have any measurements to show that these SO₄ are coming from the aging processes?
8. Section 3.2 is not clear to me. I might misunderstand some concepts. Please clarify my following questions:
   1. L163-165, “Moreover, the number … in urban region.” Why do you say this? The correlation between increased BC_N concentration and their role in pollution formation is not clear to me. First, do you have any particle concentration and air quality measurements to show these days are highly polluted? Second, what important roles are you mentioned here? I can see the potential correlation between BC_N formation and RH, but you need to explain a little more between BC_N and urban region pollution. Also, I suggest you do some statistic analysis to show correlations. Similar to any comparison you did in the manuscript.
   2. L166-167, “This result … in rural area.” See comment 8(a).
   3. L166-171, “In addition, we … zhang et al./. 2021).” I do not understand this. Higher RH has lower BCOC_N. Does that mean more BCOC_N formation depends on photochemical? Moreover, how could sulfate formation not affect BC mixing state and light-absorption? This does not make sense to me.
   4. In figure 3, why did you name the earlier case as case 2? How do you define P0-P5?
   5. L175, “As a consequence, … in half-day.” You showed that E_abs increased by decreasing BCOCS and increasing BOCO_N. Could that be because BCOC_S is less volatile than BOCO_N, so that after TD, more BOCO_N (coating) can be removed?
   6. L180-187, “As shown in Fig. 3d, … at high PM level.” First, how do you conclude the mountain valley winds influenced the concentration of the fine particles? Do you have any evidence to show the wind direction, or have any references explained that before? Moreover, since you mentioned the fine particles were dominated by fossil fuel OA, which indicates local traffic emission based on my knowledge. Then, should the diurnal variations cause by the changes in traffic conditions? Second, You did not show the diurnal cycle of E_abs­. Moreover, I also do not understand the effect of mountain valley winds since you did not clearly explain that. Furthermore, could the variation in Eabs be due to changes in chemical composition? Third, You mentioned that after P5, BBOA was stable, and FFPA increased. However, I found BBOA decreased, and FFOA increased until midnight of 11/10, then decreased. I do not see you providing BBOA and FFOA concentration at other times. It will be interesting to see that. Last, you said the BCOC_N and BC were high in the last sentence. I am not sure what the period is. Could you clarify that? Moreover, I also do not understand how you conclude that fresh fossil fuel BC might mix with OC and nitrate at a high PM level.
   7. L190-191, “Such differences were … during daytime.” Have you considered the heterogenous nitro reactions during nighttime?
9. L198, should it be Fig. S1 instead of Fig. S2? Also, Fig S2 is very confusing. What are the solid line and dashed line?
10. Figure 4: what are these triangles in the figures? For figures 4 a and b, I suggest switching the x and y axis since the x axis depends on the Eabs, which depends on the (NO₃+SO₄)/C_n and (OC+Metal)/C_n.
11. L208-209, “These results indicated … exceeded ~ 6.” This is not clear to me. Can you explain a little bit more? You are not showing a time series of these two ratios. Does eBC increase with aging time? Also, I do not know how you calculate (NO₃+SO₄)_AMS/eBC. Based on your context, it seems like this ratio will keep increasing due to aging (although you did not provide the time evolution of that ratio). Then if that keeps increasing, why will the (NO3+SO4)/Cn ratio stop increasing? I guess that eBC is initially increased due to increasing NO₃ and SO₄, which have higher E_abs (eBC=b_abs/MAC=E_abs*b_{abs_BC}/MAC). NO₃+SO₄ increased at the beginning and then reached a steady state. Then something happened, which decreased the overall Eabs (maybe coating of organics?). Anyway, I might be wrong. However, please show me more evidence.
12. L209-211, “Different from … at both sites.” What does this tell you? What is your purpose in comparing these two variables, and why do you not use the same format as Fig. 4a?
13. L211-212, “Moreover, … (R²=0.95)”. Where are these results presented?
14. L223-225, “These results … at the urban site.” Did you compare mass spectra of TD and bypass? Could that be because OC and metals are less volatile, they were not removed by TD efficiently?
15. L227-229, “Combined with … at the rural site.” How do you find that? Do you have any TEM images or EDX mapping? Mass spectra only can tell you these species exist in the same particle. Based on mass spectra, you cannot get morphology (partically coating or aggregate or core-shell).
16. Equation 1-6: How did you get these equations? Why are there so many variables? What is the difference between 1 and 2, 3 and 4, 5 and 6? You did not provide any explanations for these equations.
17. L232-233, “ The predicted … (Fig. S4).” Have you done any statistic analysis to show the correlation between predicted and measured E_abs?
18. L239-240, “Note that … Factor5 in BJ.” It is not clear which factor is which. Please clarify that either here or in the Methods section.

The manuscript by Sun et al. investigated the chemical composition and mixing state of BC-containing particles and explored the driving factors of their evolution at an urban and a rural site. The results showed that E_abs was highly dependent on the secondary inorganic aerosol coated on BC at both sites, while high primary coated OC also resulted in light absorption enhancement for relatively fresh BC particles at the rural site. E_abs showed a similar evolutionary process at both sites: E_abs was negligible in BC particles from fresh primary emissions and then increased significantly when BC is aged rapidly with increased coatings of OC-nitrate or nitrate. Finally, E_abs could reach the highest value as sulfate involved in BC aging. This study novelly combined the mass concentration of chemical components with the peak area of individual particles to investigate the effect of the actual atmospheric chemical components on the black carbon mixing state as well as the light absorption enhancement. It is informative for exploring the relationship between the aerosol bulk composition and individual particle mixing state. I recommend it for publication on Atmospheric Chemistry and Physics after the authors consider several minor revisions to the manuscript.

Comments:

1. To better understand this study, it can be highlighted in section 2.1 that the SPAMS was deployed independently of TD. In section 3.3, section 3.4, the calculated optical properties of BC for Gucheng rural site could be comparable with a previous work conducted on a rural mountain site in Beijing (doi: 10.5194/acp-21-681-2021) while another study simultaneously performed both in IAP and on a mountain site (doi:10.1029/2020JD033096) may also be a good reference to your results on BC Eabs considering one identical experimental location in both. Additionally, SOA playing an important role in BC absorption enhancement (Line 225) has been previously reported by an aircraft observation whereby more light cloud be reflected by clouds underneath BC aerosol layers resulting in more SOA formation, thus thicker coatings on BC (doi: 1088/1748-9326/ab4872). I suggest making more discussions on these according to the above works.
2. line 79, ‘Ambient aerosols with a flow rate…, then aerosol particles (∼2 L min⁻¹) were…’ should be better divided into two sentences.
3. line 103-104, ‘2 269 659 and 3 399 565 BC-containing particles are identified in Beijing and Gucheng, respectively.’ Should be ‘2 269 659 and 3 399 565 BC-containing particles were identified in Beijing and Gucheng, respectively.’
4. line 122, there is no need to emphasize Table S1.
5. Line 136, ‘suggesting that BC particles were more aged at the urban site.’ should be ‘suggesting that BC particles are more aged at the urban site.’
6. line 137-138, ‘which is twice than’ should be ‘which was twice than’, notice the consistent tense in a same sentence. Please check the full text.
7. line 155, ‘… diesel vehicle emissions at the rural site especially nighttime.’ should be ‘… diesel vehicle emissions at the rural site especially at nighttime.’
8. line 173-174, ‘During the initial stage of haze episode (P0)’ is better written as ‘During the initial stage of haze episode case1 (P0)’.
9. line 179, “Similar to haze episode 1” should be ‘Similar to case 1’ which was mentioned in this study.
10. line 198, ‘“Fig. S2” should be “Fig. S1”.’
11. line 212-213, ‘… quantify the impacts of POA and SOA on BC-coated OC.’ is better written as ‘… quantify the impacts of POA and SOA on OC coated on BC.’
12. line 242-243, ‘FactorB is the major type of aged BC in the rural area which was dominated by BCOC_N.’ should be ‘FactorB is the major type of aged BC in the rural area which is dominated by BCOC_N.’
13. line 253-254, ‘BC could be …, and lead to …’ should be ‘BC could be …, and led to …’, please check the tense of the full text.
14. line 280-281, ‘to above 1.30 after aged and internally mixed with nitrate’ should be ‘to above 1.30 after aging and internally mixing with nitrate’.
15. line 96&253, “~” format should be consistent, please check the full text.
16. Please keep the abbreviation of the single particle aerosol mass spectrometer consistent in this study (e.g. HR-SPAMS in line 89, SPAMS in line 205).