the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Apr 2026
The synergy of morphology and mixing state on the absorption of coated black carbon soot
Abstract. Black carbon (BC) strongly absorbs light and contributes to climate warming. Coating formation on fractal BC-containing soot is a ubiquitous atmospheric process that enhances its absorption, but the extent of this absorption enhancement (Eabs) remains uncertain and difficult to represent in models. In this study, we use large simulation chamber experiments to investigate BC absorption and its modification by coating formation and ageing. Polydisperse fractal BC-containing soot was mixed with non-absorbing atmospherically-relevant secondary aerosols (sulfuric acid and α-pinene secondary organic aerosols). Varying reaction dynamics produced different coating rates and masses, mimicking atmospheric processing. The evolution of soot and coating properties was tracked for up to 20 hours. The results of this study suggest a general positive correlation between Eabs and coating amount on soot. However, they also experimentally demonstrate the strong effect of morphological modifications and mixing state heterogeneity on BC absorption, both depending on the reaction dynamics of coating formation and ageing of the coated soot. Combined variations in morphology and mixing state affect the magnitude of Eabs and its further evolution, leading, depending on processing, to either an increase or decrease in absorption. The range of measured Eabs resulting from soot processing cannot be reliably predicted using a fixed Eabs value or by a core-shell optical model, two approximations commonly used in climate models. These results support the need for new formalisms that would take into account both morphological and mixing state heterogeneity to improve model predictions of BC absorption and its role on the radiative budget.
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Status: open (until 01 Jul 2026)
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CC1: 'Comment on egusphere-2026-1867', Martin Schnaiter, 25 Apr 2026
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AC1: 'Reply on CC1', Claudia Di Biagio, 27 Apr 2026
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We thank Dr. Schnaiter for this important comment. The key study by Schnaiter et al. (2005) is already cited in the introduction, where we aimed to synthesize the extensive body of literature on black carbon absorption enhancement, including previous simulation chamber experiments. However, as noted, it is not yet discussed in direct comparison with our findings. In the revised version, we will further contextualize our results within this framework, including the findings from the AIDA chamber study (Schnaiter et al., 2005), as suggested.
Citation: https://doi.org/10.5194/egusphere-2026-1867-AC1
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AC1: 'Reply on CC1', Claudia Di Biagio, 27 Apr 2026
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RC1: 'Comment on egusphere-2026-1867', Anonymous Referee #1, 24 May 2026
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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?
Citation: https://doi.org/10.5194/egusphere-2026-1867-RC1
Data sets
Soot + sulfuric acid + None - Aerosol study - optical properties - 2021-03-03 J. Heuser et al. https://doi.org/10.25326/76FN-TA55
Soot + sulfuric acid + O3 - Aerosol study - optical properties - 2021-03-04 V1.1 Public J. Heuser et al. https://doi.org/10.25326/PWHP-ED62
Soot + sulfuric acid + O3 - Aerosol study - optical properties - 2021-03-09 J. Heuser et al. https://doi.org/10.25326/A18P-JF85
Soot + alpha-pinene + O3 - Aerosol study - optical properties - 2021-05-18 J. Heuser et al. https://doi.org/10.25326/AK6B-ZA08
Soot + alpha-pinene + O3 - Aerosol study - optical properties - 2021-05-20 J. Heuser et al. https://doi.org/10.25326/N7S0-S115
Soot + alpha-pinene + O3 - Aerosol study - optical properties - 2021-05-26 J. Heuser et al. https://doi.org/10.25326/BGRM-3554
Soot + None - Aerosol study - optical properties - 2021-02-23 J. Heuser et al. https://doi.org/10.25326/8KVR-AA70
Soot + None - Aerosol study - optical properties - 2021-02-24 J. Heuser et al. https://doi.org/10.25326/M24W-V933
Soot + None - Aerosol study - optical properties - 2021-05-17 J. Heuser et al. https://doi.org/10.25326/2QRB-4E45
Soot + None - Aerosol study - optical properties - 2021-05-19 J. Heuser et al. https://doi.org/10.25326/8HT1-WA13
Soot + None - Aerosol study - optical properties - 2021-02-26 J. Heuser et al. https://doi.org/10.25326/WN6E-0272
Soot + None - Aerosol study - optical properties - 2021-03-01 J. Heuser et al. https://doi.org/10.25326/EAQE-VX80
Soot + O3 - Aerosol study - optical properties - 2021-03-02 J. Heuser et al. https://doi.org/10.25326/JRAC-4W36
Soot + O3 - Aerosol study - optical properties - 2022-12-12 J. Heuser et al. https://doi.org/10.25326/2BD3-Q151
Soot + O3 - Aerosol study - optical properties - 2022-12-14 J. Heuser et al. https://doi.org/10.25326/9G66-MG61
Soot + O3 - Aerosol study - optical properties - 2022-12-15 J. Heuser et al. https://doi.org/10.25326/XJKP-JD08
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This is a very interesting manuscript on the absorption enhancement of black carbon (BC) through coating with sulphuric acid and SOA mass. However, the manuscript lacks a discussion of these results in the context of earlier work, in particular the study by Schnaiter et al. (2005), who performed similar experiments in the AIDA chamber. The experimental setup and conditions in the AIDA chamber were very similar to those used in the present study in the CESAM chamber. Notably, Schnaiter et al. (2005) reported absorption enhancements of 1.8 to 2 even for BC aggregates that fully collapsed during the experiments, which is somewhat in contradiction to the low absorption enhancements presented in the present study.
Good scientific practice requires that new results be discussed in the context of existing literature. In this case, a comparison with Schnaiter et al. (2005) is essential, particularly given that the experiments were conducted under very similar conditions, namely in aerosol chambers with comparable instrumentation and experimental procedures. Without such a comparison, it remains unclear how this work advances our understanding of the BC absorption enhancement relative to studies conducted more than two decades ago.