This manuscript by Heuser et al. “The synergy of morphology and mixing state on the absorption of coated black carbon soot” presents interesting and relevant new results on how coating and particle morphology affect black carbon absorption and further climatic implications. The data are highly valuable and original, the study is well written and the topic is important. I find the manuscript suitable for publication after the authors address a few points that would help clarify the interpretation of the results. My main comments concern the interpretation of the mobility-diameter changes and low Eabs values, the role of particle-to-particle heterogeneity, and the uncertainty characterization of the extinction-minus-scattering absorption method.

Major comment 1: Interpretation of restructuring, heterogeneity, and low Eabs

The main conclusion of the manuscript is that the BC absorption response cannot be explained by morphology alone but it follows from synergy between morphology and particle-to-particle mixing-state heterogeneity. I think this is a plausible and interesting conclusion, but in the current version the underlying evidence could be tied together more explicitly. At times, it is difficult to identify which results the authors base their conclusions on.

In particular, the SOA experiments appear to show an initial decrease in mobility diameter, which may indicate restructuring or compaction of soot aggregates. However, it is not fully clear to me whether this behavior should be interpreted primarily as aggregate collapse / void filling, or connected to changes in mass loading or evolving coating distribution. It would therefore help if the authors could discuss more explicitly the physical interpretation of the mobility-diameter decrease, including its timing relative to SOA formation and whether the data suggest a step-by-step process (e.g. initial restructuring followed by continued coating growth or aging).

Related to this, the manuscript emphasizes the importance of particle-to-particle heterogeneity in mixing state, but this aspect remains somewhat qualitative. If possible, I encourage the authors to add at least one simple quantitative characterization of heterogeneity from the available single-particle measurements (for example, fraction of heavily coated particles, or evolution of the rBC mass distribution) to help the reader to connect the discussed heterogeneity to the observed absorption response.

This point becomes especially important in the SOA experiments where Eabs appears to decrease strongly and in some cases even fall below 1. Since the coatings are non-absorbing, it is difficult to interpret the result physically. I would therefore ask the authors to discuss more explicitly whether such low values are connected to true population-level microphysical effects, or whether they may partly reflect normalization choices, dilution, wall losses, evolving size distributions, or other measurement uncertainties.

Major comment 2: Measurement description and uncertainty of EMS-derived absorption

A second point that would benefit from clarification is the absorption derived from the extinction-minus-scattering (EMS) approach. Because EMS uncertainty can increase substantially at high SSA I think the manuscript should describe more explicitly the instrumental setup, the applicable measurement range, and the uncertainty limits relevant for the experiments discussed here.

The manuscript mentions multiple CAPS and nephelometer instruments, but it is not fully clear which instrument combinations were used to derive absorption in different experiments. I would therefore ask the authors to specify this more clearly and to provide, if possible, a concise uncertainty characterization of the EMS method in the conditions relevant to this study (e.g. table2 in supplement). In particular, it would be useful to indicate over what ranges of absorption coefficient and SSA the EMS method was considered reliable.

I would also appreciate a short clarification of the statement that EMS absorption was “validated against reference filter-based measurements”. Why was the filter-based method treated as the reference and how well did the two methods agree? Over what concentration and SSA ranges was this comparison performed?

More generally, a few additional experimental details could help the interpretation: for example, whether the aerosol was dried (since both sulphuric acid and organic aerosol can take up water over a broad RH range) and what the relevant soot number concentrations / absorption coefficients / SSA values were during the key experimental stages. Some of these details are in the supplementary material (SSA), but they could be linked more explicitly to the discussion of uncertainty.

Major comment 3: More quantitative comparison to previous chamber studies

The manuscript would also benefit from a clearer comparison with previous chamber studies that have also reported larger absorption enhancement, including studies where restructuring of soot aggregates was also observed. Although these are qualitatively discussed in the introduction, the reasons for the differences are not further discussed.

I suggest that the authors add a brief but more quantitative comparison discussing which differences in experimental design, soot generation, coating protocol, definition of Eabs, or measurement approach could possibly explain the discrepancies. This would help readers place the current findings in context and better understand whether the results reflect very different soot/coating regime or methodological differences.

Minor comments

• Please clarify the aims of the control experiments in lines 95–100 more explicitly: what specific effects were expected, and in which measured quantities?
• Please clarify how the SP2 was calibrated (e.g. calibration material and consistency across calibration approaches). Was size-selected mini-CAST soot or fullerine used? Were the results similar?
• Lines 193–194: please elaborate briefly on the comparison between EMS and the filter-based reference method.
• Was the aerosol dried before optical measurements?
• Please comment briefly on the atmospheric relevance of the selected miniCAST operating point.
• Line 30: the sentence “The range of measured Eabs … cannot be predicted using a fixed Eabs “ is not logical and should be rephrased.
• Line 73: “the” → “a”.
• Line 285: typo (“butv”?).
• In Fig. S11 and related figures, comparison between Ntot and rBC soot distributions would be easier if the color scales were harmonized, or if an additional panel were added for soot-mode total number.
• Could you please comment on the apparent drop in enhancement factor when the lamp is switched on in the supplementary (control experiment) figures?

This manuscript presents a systematic chamber study of absorption enhancement by coated soot, with particular emphasis on the combined effects of coating amount, morphological restructuring, and particle-to-particle mixing-state heterogeneity. The experimental design is comprehensive, including H2SO4 and α-pinene SOA coating experiments, multiple control experiments, and extended ageing periods. The combined use of SMPS, CPMA, SP2, TEM, nephelometer, and CAPS measurements provides a valuable multi-constraint dataset linking soot size, effective density, refractory BC mass, morphology, and optical properties. Overall, the manuscript is scientifically valuable and suitable for publication after minor revision. The following comments mainly request clearer process attribution, more quantitative use of the existing measurements, and a fuller description of uncertainty. They do not require additional experiments, and most can be addressed through clarification, reanalysis of existing data, or more cautious wording.

1. The timing of soot collapse should be clarified. Figure 2b suggests that SOA-Sref and SOAhigh-Shigh already undergo collapse during the initial coating stage, while Eabs increases, which appears inconsistent with the later interpretation that collapse reduces absorption enhancement. The authors could explain whether different mechanisms dominate the optical response during the initial coating stage and subsequent ageing. In addition, the continuous decrease in MR_bulk during processing should be clarified; is SOA evaporation responsible?
2. Figs in the manuscript show SOA and H2SO4 may have different impacts on BC collapse. This is interesting because previous studies typically using one type of coating materials, e.g., Peng et al. (2016, PNAS) and Zhang et al. (2020, PNAS). Could the author confirm this and make a short discussion?
3. 3. The morphology and mixing-state evidence should be presented more quantitatively. Figures S4 and S5 are selected images, yet the main text refers to the majority of particles and to mixing-state control of Eabs. Please report the number of particles analysed and, if available, morphology-class fractions; otherwise, soften the causal wording. Relevant studies, such as Huang et al. (2024, One Earth), could also be cited to strengthen the interpretation and improve the credibility of the morphology–optics linkage.
4. 4. The radiative-effect calculation should include a clearer uncertainty assessment. Table 3 reports relative RF differences without showing how uncertainty in measured SSA, extinction-minus-scattering absorption, assumed refractive indices, and retained parameters affects the range. It is suggested to propagate the dominant remaining uncertainties or provide a sensitivity interval.

Technical comments

• Around line 248, the Mie-derived Eabs is described as the ratio of uncoated to coated babs, whereas Sect. 2.3.1 defines Eabs as coated relative to prior/uncoated absorption.
• Please give the explicit conversion equation between SP2-derived mrBC and msoot and clarify whether the reported factor of 1.2–1.8 represents mtotal/mrBC or the inverse.
• Around line 247, “the the size distribution” should be corrected to “the size distribution.”
• Around line 285, “due to condensation; butv also coagulation” should be corrected to “due to condensation, but also coagulation.”
• Around line 292, “TEM images depicts” should be corrected to “TEM images depict.”
• Please use either “ageing” or “aging” consistently throughout the manuscript.
• The main text refers to Figs. S17–S25 for the control experiments, whereas the Supplement contains figures only through Fig. S23. Please correct the figure range.