the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
Experimental investigation of soot morphological transformation and its impact on size-resolved light absorption
Abstract. This study experimentally investigated the influence of aggregate morphology on soot light absorption. Fresh soot was generated using an inverted-flame burner and compacted through controlled humidification–drying cycles to isolate the effects of structural transformation from those of chemical composition or coating. Size-resolved absorption measurements were performed at three wavelengths (440, 516, and 635 nm) using a cantilever-enhanced photoacoustic spectrometer (CEPAS) coupled with a differential mobility analyzer (DMA) and a centrifugal particle mass analyzer (CPMA). For small particles, the absorption cross-section increased after compaction but decreased for larger particles, with the wet-to-dry absorption ratio transitioning from values above unity to below unity as particle mass increased. The tipping point occurred in the 1–2 fg mass range, corresponding to a volume-equivalent diameter of approximately 102–129 nm for spherical particles at a soot material density of 1.8 g cm-3. This behavior suggests a competition between near-field dipole–dipole coupling, which enhances optical absorption in moderately compact aggregates, and optical shielding, which suppresses absorption in highly compact structures. The findings are consistent with theoretical predictions and numerical studies of aggregate optics, providing experimental evidence for morphology-dependent absorption transitions. These results emphasize the importance of accurately representing soot morphology in optical and climate models and motivate further controlled experiments to disentangle the effects of structure, coating, and composition on soot radiative properties.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research. Dr. Tuomas Hieta is employed by Gasera Ltd., the company that manufactures the photoacoustic cell used in the CEPAS. The authors have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Review of Kuula et al., ar-2025-38', Anonymous Referee #1, 12 Jan 2026
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RC2: 'Comment on ar-2025-38', Anonymous Referee #2, 20 Jan 2026
The authors present results of measurements of black carbon absorption cross-sections, differentiating between absorption by freshly produced ‘fractal’ BC and collapsed BC. They induce particle collapse by exposing particles to high relative humidity. The observe that the absorption cross-section, for a given particle mass, decreases as mass increases. They conclude that their results are consistent with theoretical predictions.
Overall, I have one major concern that leads me to question the results presented and to conclude that the only appropriate outcome is that this paper must be rejected. In short, nowhere have the authors addressed the issue of multiply charged particles. It is exceptionally well established that size selection using a DMA does not produce a truly monodisperse particle distribution with respect to physical particle size. This is because a DMA selects according to particle mobility, which depends on the number of charges on a particle. For a given particle mobility, the fraction of particles that have 1, 2, or 3 (or more) charges tends to vary with particle size, and also depends on the nature of the underlying polydisperse size distribution. Particles having more than one charge are larger than those having only one charge. These particles with multiple charges will have larger absorption cross-sections than those having only one charge. It is not feasible to consider particle absorption for size-selected particles absent consideration of the influence of multiply charged particles. That the authors have not given this any consideration is a major weakness that likely influences the results.
The authors’ results already show that, for a given mobility diameter, the per-particle mass differs between their dry (fractal) and wet (collapsed) cases. Consequently, the properties of the particles having 2 or 3 charges and the same mobility diameter will also differ between the dry and wet cases. Moreover, it may very well be that the fraction of particles, for a given mobility diameter, having one charge versus two or three differs between the dry and wet cases; I would expect this to be the case since the distribution will have shifted between the dry and wet cases since collapse affects the mobility diameter for a given mass. To quantitatively characterize how particle collapse influences the absorption cross-section as a function of particle mass it is critical that the issue of multiple charging is accounted for. As the authors have not made such a consideration I have no ability to judge whether their results are true or not. They are undoubtedly wrong (for the above reasons), but whether the general trend observed here would be preserved with a robust and full analysis is an open question.
Consequently, I must recommend that this paper is rejected. It may be that the authors have sufficient data to allow for consideration of multiply charged particles (for example, the CPMA scans), and if so I encourage them to leverage such data and appropriately revise and resubmit the paper. But, the current version must, unfortunately, be rejected. (I will, on a positive note, mention that the data quality in terms of precision seem to be very good.)
Separate from the above fundamental concern, I will note that I find the comparison theory weak. It would be much stronger if the authors were to use their data to perform specific calculations that illustrate quantitative agreement. As it stands, I find the comparison to the literature a bit vague.
Citation: https://doi.org/10.5194/ar-2025-38-RC2
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The manuscript "Experimental investigation of soot morphological transformation and its impact on size-resolved light absorption" addresses an interesting question, using a novel light absorption sensor based on photoacoustic spectroscopy with a cantilever transducer, at 3 wavelengths.
This is an interesting topic and the manuscript is quite well written. Unfortunately, because the samples were not adequately constrained in the "size-resolved" dimension, which is the focus of the paper, I cannot recommend this manuscript for publication.
First, the authors' results for untreated soot are inconsistent with literature. This discrepancy is neither discussed nor justified by the authors, despite its significance for the interpretation of the results.
Second, the inconsistency is likely due to multiply charged soot particles during the experiments. The authors did not size (or mass) classify the measured soot particles prior to measurement. Instead, they simply operated their photoacoustic instrument after a DMA (DMA-CEPAS and DMA-CPC). They also performed DMA-CPMA-CPC experiments to estimate single-particle mass. The second measurement does not adequately account for artifacts in the first.
DMA-CPMA-CPC will give a count-based distribution, while the DMA-CEPAS will give mass-weighted signals (since absorption is proportional to mass). Thus, the DMA-CPMA-CPC will be an inadequate correction for multiple charging, as was demonstrated by Radney and Zangmeister (Aerosol Sci Technol 2016, http://dx.doi.org/10.1080/02786826.2015.1136733). That paper demonstrates uncertainties of over 100% in the resulting mass-specific extinction coefficient. These occur purely because of the number- versus mass-weighting of CPC and CEPAS signals.
Recent work has demonstrated ways to avoid these artifacts. For example, the combination of an AAC-DMA instead of DMA alone can produce singly charged particles (Sapkota et al., 2025 https://doi.org/10.1080/02786826.2025.2519093). Alternatively, two other approaches were cited by the authors, one using DMA-CPMA in tandem (Radney et al 2014 (https://doi.org/10.1021/es4041804)) and the other using a corona charger before a CPMA (Corbin et al 2022 (https://doi.org/10.1016/j.carbon.2022.02.037)). One of these approaches should be used and the experiments repeated.
Other comments:
1. Why is the reported absorption cross-section given in "A.U." if "the CEPAS was calibrated using nigrosin reference particles and Mie-modelling" (line 80)? The authors have just published and cited a validation study (https://doi.org/10.5194/ar-3-1-2025). It is therefore unclear why calibrated, physically meaningful units are not reported here. This raises the concern that the calibration may not yield sensible results under the present experimental conditions, potentially due to multiple charging artifacts.
2. The results should have been discussed quantitatively in the context of previous mass absorption cross-section (MAC) measurements. The manuscript cites previous papers at lines 265-275, but the discussion remains qualitative and overlooks important nuances. For example, it is stated that Radney et al 2014 (https://doi.org/10.1021/es4041804) observed no size-dependent absorption but Corbin et al 2022 (https://doi.org/10.1016/j.carbon.2022.02.037) did. Looking at those papers, one sees that the size dependence in the latter paper begins at < 1 fg, a region that the former paper did not measure. Therefore, the two papers are consistent: both show no size dependence above 1 fg. So, the authors' observation of a MAC that increases continuously from 1 to 10 fg is inconsistent with literature and needs to be explained.
3. The authors present the manuscript as a study of "soot morphological transformation" but, on closer inspection, the authors have exposed hydrophobic soot to >80% humidity. This caused "relatively weak morphological collapse of the soot aggregates, which was likely influenced by their inherent hydrophobicity." If the soot morphology has not been strongly transformed, why title the paper "investigation of soot morphological transformation"?