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| 30 Jun 2025
Unraveling the Impact of Heterogeneity and Morphology on Light Absorption Enhancement of Black Carbon-Containing Particles
Abstract. Black carbon (BC) is a strong climate forcer, but considerable uncertainty remains in estimating its radiative impact, largely due to persistent gaps between observed and modeled light absorption enhancement (Eabs). In this study, we employed a Centrifugal Particle Mass Analyzer and Single Particle Soot Photometer tandem system to characterize mass ratio (MR, coating-to-BC) and morphology of BC-containing particles in Hangzhou, China. Fortunately, low, medium, and high Eabs values were observed during a single field campaign. Results show that the uniform core-shell Mie model overestimated Eabs especially in clean conditions (low Eabs). A morphology-dependent correction scheme was developed to improve optical property estimates of BC in the “transition state.” This improved model better reproduces measured Eabs in different pollution conditions and reveals that the concentrations of particle chemical composition affect the MR threshold defining this state. Our findings highlight the need to account for real-world particle complexity in climate-relevant BC modeling.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2844', Anonymous Referee #1, 13 Oct 2025
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RC2: 'Review of on egusphere-2025-2844', Anonymous Referee #2, 28 Oct 2025
Review of Jing Wei et al., "Unraveling the impact of heterogeneity and morphology on light absorption enhancement of black-carbon-containing particles"The manuscript by Jing Wei et al. presents [charger-]CPMA-SP2 measurements, from which measurements of the distribution of rBC fraction can be inferred. The manuscript is carefully written and shows that the authors have thought carefully about the interpretation of their results. The literature context is good, and the data set represents a significant contribution to the literature. However, there is one major gap which must be addressed before this manuscript is published (multiple charge correction) and another big opportunity for improvement (compare the SP2 with the analytical grade nephelometers, not only with the SP2-LEO analysis). There are also a few smaller opportunities for improvement, described below.(1) The major gap is that this work does not cite, nor implement, the advances in [charger-]CPMA-SP2 data inversion which have been published since the original work by Liu et al., 2017 (cited by the authors). Those advances have been published in a series of papers, most recently Naseri et al. 2024, which are accompanied by open-source code for performing the calculations. The key feature of these calculations is to acknowledge the aerosol bipolar charger, which is necessarily placed before the CPMA to make sense of its output. Without this charger, the SP2 would see a large fraction of neutral particles downstream of the CPMA.With this charger, the SP2 would see not only the desired particles (q=1, the CPMA setpoint) but also a large fraction of doubly (q=2) and triply (q=3) charged particles. The purpose of the CPMA-SP2 data inversion code cited above is the account for these q>1 particles. For example, if the CPMA was set to 2 fg/e (2 femtograms per charge) and the SP2 observed a particle with 1 fg of rBC, that particle might be a 50%/50% rBC/coating particle (q=1, 2fg/1e), a 25%/75% rBC/coating particle (q=2, 4fg/2e), or even a 16% rBC (6fg/3e). A full data inversion will account for this.Implementing this data inversion would affect:- Eq 1, the authors write "without any assumptions", but there is an assumption of singly charged particles.- All figures, which currently do not distinguish "doubly charged particle with BC fraction 0.5" from "singly charged particle with half the coating".Other major comments(2) The MAC vs M_R plot of Figure S6 shows that MAC did not increase even at M_R of 5.5. Mie theory and lab experiments show that coated soot has E_abs 2.0 at M_R of 5.5 (Cappa et al., 2012; doi:10.1126/science.1223447). So the fundamental premise of this paper seems to be violated by Figure S6.(3) The abstract claims a morphology dependent correction scheme but none of the data are morphology dependent.(4) The introduction could be enhanced by adding a discussion of how morphology can be measured. Or, the word morphology could be removed from the final paragraph. I suggest removing it, as the true measurement here is M_R, not morphology.(5) Line 154, the MAC of 9 m2/g is 3 standard deviations than the expected value of 8 +- 0.7 m2/g at 550 nm (Liu et al 2020). Please comment.(6) Figure 3: Is the positive correlation truly statistically significant? Please use prediction bands and add uncertainty in the fit coefficient. Is it truly different from 1? It seems to me that there is not a significant relationship.(7) Line 181 why not model the nephelometer instead of the SP2? You have already assumed a wavelength independent rBC refractive index.(8) Figure 4 shows "E_abs,improved" but this paper does not actually demonstrate an improvement. The results here do not seem generalizable. Rename to "E_abs,this work"Minor commentsLine 56, you might cite Radney et al. 2014 and Corbin et al. 2023.Line 58, please cite a paper for "or located on the particle surface" or for the entire sentence.Line 99, "noisy scattering signals" or "noisy incandescence signals"? The section is discussing M_R? Please also explain "noisy" with a few more words.Line 103, does this aquadag correction factor trace back to Gysel et al. (2011)? Either way, that early paper should be cited.Line 124, why compare 2 difference averaging times?Line 125, Did you calibrate the CAPS extinction measurement with this scattering calibration?Line 137, this refractive index is very precise (1.48) would you not expect variability (1.5 to 1.6?). Also, a different RI is cited later on line 164.Line 157, please add Generalized Mie Model to the list.Several examples: Please change "(Method)" to a section reference.Line 181, mathematical -> empiricalLine 255-256, is there any experimental evidence to quantify the M_R the rBC would become fully embedded?Figure S1, CPAS not defined. ACSM not defined. Define all acronyms in caption.Line 323, "during the haze period... chemical reactions produced a lareg number of inorganic substances..." How do you know this?Line 327, please define "embedding pattern"Line 331, either the formation of BC coatings was "unfavourable" or the plume was less aged.ReferencesA. Naseri, Corbin, J. C., and J. S. Olfert. Comparison of the LEO and CPMA-SP2 techniques for black-carbonmixing-state measurements. Atmos. Meas. Tech., 17(12):3719–3738, 2024.Corbin, J. C., et al. (2023). "Mechanisms of soot-aggregate restructuring and compaction." Aerosol Sci. Technol. 57(2): 89–111.Gysel, M., et al. (2011). "Effective density of Aquadag and fullerene soot black carbon reference materials used for SP2 calibration." Atmospheric Measurement Techniques 4(12): 2851-2858.Radney, J. G., et al. (2014). "Dependence of Soot Optical Properties on Particle Morphology: Measurements and Model Comparisons." Environ. Sci. Technol. 48(6): 3169-3176.Liu, F., et al. (2020). "Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function." Aerosol Science and Technology 54(1).My apologies to the authors for my late review.Citation: https://doi.org/
10.5194/egusphere-2025-2844-RC2 -
RC3: 'Comment on egusphere-2025-2844', Anonymous Referee #3, 29 Oct 2025
The manuscript treats an important aspect of black carbon optical properties with innovative techniques. Overall, the topic and novelty fulfil the requirements for ACP publication. Some work still to be done improve the readability of the manuscript and especially ensure that robustness of the measured properties. The article requires consistent modification.
NOMENCLATURE AND READABILITY: The use of abbreviations and symbols requires careful attention. Several parameters (particularly those distinguishing measured from modeled quantities) are difficult to follow, which hinders comprehension, especially in the results section. I strongly recommend introducing a summary table listing each property, its abbreviation, and whether it refers to a measurement or a modeled value. Improving consistency here would greatly enhance clarity for readers unfamiliar with the measurement framework.
INSTRUMENTAL DESCRIPTION AND UNCERTAINTIES: The methodology section currently lacks sufficient citations and discussion of measurement uncertainties. Given that the study combines multiple instruments in a novel configuration, these details are crucial. The uncertainties of the CPMA–SP2 tandem system, as well as of derived quantities such as Mₚ and M_BC, should be clearly stated and discussed in the context of previous literature. Since many key quantities in the analysis are expressed as ratios, unquantified uncertainties may propagate and influence the reported variability. In line with other reviewers’ remarks, I encourage the authors to describe the CPMA–SP2 system and data processing steps in greater detail, including how calibration and error propagation were handled.
RESULTS AND INTERPRETATION:The results section shows promising potential for impact, but several points require clarification. The authors could strengthen the manuscript by expanding the discussion on how the proposed “transition-state” correction scheme might be generalized to other atmospheric environments. For example, conditions in rural or biomass-burning regions, or during other seasons, may produce distinct coating compositions and morphology evolution pathways. Explaining how the correction parameters (e.g., MR thresholds or morphology indicators) could adapt to such conditions would enhance the broader applicability of the method. The method for defining the three “cases” should also be revisited. It appears that only Case 2 represents a specific or anomalous event compared to the rest of the campaign. I suggest first describing the overall meteorological and bulk aerosol conditions and then examining how the optical properties vary under those regimes. This would provide a more physically grounded interpretation of the case classification. At present, the combined issues of unclear nomenclature and insufficient methodological detail reduce the understanding of the results, being the ultimate limiting factor of the manuscript.
SPECIFIC COMMENTS
L1: title. I am not convinced by “heterogeneity”. What does it mean in this context? It is a very general term that, alone, does not convey a unanimous message.
L35-38: Eabs is not contextualized, not properly described. Enhancement with respect to? Please provide a short description.
L51: what do you mean with heterogeneity?
L67: please avoid the use of “fortunately” , it undermines the preparation and thoughts behind your research. Leaving the reason of the positive outcome of your work to luck.
L80-84. If available, I suggest adding 1 or 2 references describing the site and its representativity.
L86-109. I suggest describing a bit more each instrument alone. With the use of references, which is limited. Here some old works about SP2 describing its principle: (Stephens et al., 2003; Moteki and Kondo, 2010) and calibrations: (Gysel et al., 2011). I suggest a recent paper exploring the operational limits of the SP2 (Schwarz et al., 2022). This is the major reference of the CPMA: (Olfert and Collings, 2005). For the tandem combination of SP2 with mass analysers I suggest a relatively old review (Cross et al., 2010) and a more recent set of papers (Liu et al., 2022; Naseri et al., 2022; Zanatta et al., 2025). DMT 2011. This is an odd reference. The ACQUADAG scaling factor to fullerene should be slightly better accounted for. DMT 2011 is and odd reference. The original reference should be (Baumgardner et al., 2012; Laborde et al., 2012). Please do the same for the ACSM. The CAPS-SSA is fully detailed by (Modini et al., 2021). Overall, these works report all the error associated with the single measurements, which will propagate substantially for the application intended in the current manuscript. None of these are described.
L93: I would expect the SP2 showing the multi charged particles. How exactly was the mass MBC calculated?
L115. What model and company? Please be consistent with previous notation.
L122: CAPS and nephelometer may well respond to PSL, especially small PSL. Truncation error may become more and more important with larger particles and especially irregular particles…such as ramified fresh BC. Please provide a small statement about it. Was truncation corrected?
L127: Well, we “assume” that everything is working properly. This is why is important to estimate, even roughly, the uncertainty.
L132-133: I have a couple of questions here. The CPMA is capable of selecting particles based on their mass to charge ratio. Hence, it is recommended, even by the manufacturer, to run the CPMA after a neutralizer/charger. From the schematics of Figure S1 it looks like there was no neutralizer. What it is the additional uncertainty of running the CMPA-SP2 setup without a charger? Single and multi-charged peaks, should be visible in the mass distribution provided by the SP2. I wonder if the authors quantified the single particle BC mass (MBC, by fitting the SP2 mass distribution, including only the first peak (single charge) or fitting the full distribution). This technical detail may influence all the result sections. Hence need to be fully and properly described. Regarding the sampling collection. Unfortunately, Figure S2 shows that the counting efficiency of the SP2 is far from 100%, especially below 100 nm (Figure S2a). It is also surprising that the SP2 counts systematically more than the CPC. I presume that some setting in the SP2 were not properly configured or that the CPC had some counting issues. Moreover, why the two incandescence detectors should have a different (linear and non-linear) mass/incandescence relationship?
L140, why distorted. It is attenuated due to the evaporation of absorbing-refractory material.
L142-146: the LEO-fit relies on many assumptions, as correctly stated by the authors. It would be nice if they could elaborate, shortly, about the reason behind these choices. Moreover, with a similar number of assumptions (density of coating and BC cores), the optical coating thickness could be derived directly from the Mp and Mbc was this performed? Are the results coherent?
L149: define the MAC.
L154: I like the approach of deriving the MAC of uncoated BC using this extrapolation. However, this MAC (no units) for uncoated BC results to be slightly higher than previous estimations in European urban (Savadkoohi et al., 2024) rural (Zanatta et al., 2016) and the canonical 7.5 m/ g of (Bond and Bergstrom, 2006). This could also be due to the high variability of MAC itself across sites and seasons, but also to strong uncertainties related with MAC (absorption and BC mass) and bulk MR (width of the distribution of particles exiting the CPMA and method to quantify the single particle mass with the SP2). Overall, I notice a lack in providing context to these findings and assumptions. The MACBC_core is fundamental to all the results presented in the paper, hence, even small errors may substantially modify the quantification of the enhancement. The authors must provide more details on their methods and uncertainties, and put all of these consideration with the context of recent literature.
L169-175: I am genuinely confused on how the MACbc introduced in equation 3 was calculated. Use a logic order when presenting variables. Is the MAC of equation 4 the same presented in equation 3?
L174: The nomenclature is a bit confusing here. MACBC-core of line 174 is the same used int Equation 3 ? Or the MA_core, presented in Line 154 is used in equation 3? So, the “Mie MACBC_core” is it similar to 9.08 m2 /g. This aspect is extremely important and influences with a different weight MACMie and MAC observed with a different weight, especially in figure 2d where the delta enhancement is presented.
L188: At what wavelength are these values provided. Could the authors state something about the absorption enhancement at different wavelengths? The high presence of organic material may change the Eabs at lower wvalenght?
L220-221: I recommend caution when mentioning morphology, especially in a section title. This scattering cross-section ratio is a far approximation for morphology assessment. It may be a proxy, but nothing more and must be confirm by real morphology observations such as microscopy fractal dimension or, at least DMA/CPMA density/fractal exponent measurements.
L222-223: what is the meaning of the sentence?
L239: The authors states that the difference between observed and modelled enhancement depends on the variability of the standard deviation of MR. First why the log10(Mr) was used ? Second, I all honesty, it is difficult to observe any sort of correlation in the scatterplot presented in figure 2d. Especially considering that correlation coefficient and slope changes substantially among the periods. In my opinion, Figure 2s does not support the claims of the authors.
L254: please provide the wavelength
L252-276: although the results shown in figure 3 are interesting, this section is very confusing. It is hard to understand what causes the decrease in the transition regime. Try to restructure your though in a more logic process. Could this “transition state” represent the compaction due to coating formation. This sort of natural process will reduce the optical and geometrical cross section of the particles. It is usually observed in chamber studies (e.g. Schnaiter et al., 2005; Zanatta et al., 2025) and rarely, up to my knowledge, observed in ambient conditions (Bhandari et al., 2019). This process description could be developed further.
Section 3.3 soffers a similar issue with readability. I am not fully convinced by the reasoning behind the period separation. Only period 2 looks different from the others.
F1: are these enhancement measured all at the same wavelength?
F2: please improve the labelling of the axis. Number counts and SD of…?
REFERENCES
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- 1
Review of the paper by Wei et al. ACP 2025
The paper presents an interesting analysis of measured absorption enhancement by black carbon particles mixed with other species. The data are interesting and quite valuable. However, several aspects of the manuscript are not well explained, and some are confusing or unclear. I think the authors can address most of these shortfalls, and if so, then the manuscript would deserve publication.
General comments
Specific comments
Some limited references that might be of relevance:
1. Beeler, P., et al., Light absorption enhancement of black carbon in a pyrocumulonimbus cloud. Nature Communications, 2024. 15(1): p. 6243.
2. Ueda, S., et al., Light absorption and morphological properties of soot-containing aerosols observed at an East Asian outflow site, Noto Peninsula, Japan. Atmos. Chem. Phys., 2016. 16(4): p. 2525-2541.
3. China, S., et al., Morphology and mixing state of individual freshly emitted wildfire carbonaceous particles. Nature Communications, 2013. 4.
4. Cross, E.S., et al., Soot Particle Studies—Instrument Inter-Comparison—Project Overview. Aerosol Science and Technology, 2010. 44(8): p. 592-611.
5. Corbin, J.C., R.L. Modini, and M. Gysel-Beer, Mechanisms of soot-aggregate restructuring and compaction. Aerosol Science and Technology, 2022: p. 1-48.
6. Adachi, K. and P.R. Buseck, Changes of ns-soot mixing states and shapes in an urban area during CalNex. Journal of Geophysical Research: Atmospheres, 2013. 118(9): p. 3723–3730.
7. Adachi, K., S.H. Chung, and P.R. Buseck, Shapes of soot aerosol particles and implications for their effects on climate. Journal of Geophysical Research-Atmospheres, 2010. 115.
8. Adachi, K., et al., Mixing states of light-absorbing particles measured using a transmission electron microscope and a single-particle soot photometer in Tokyo, Japan. Journal of Geophysical Research: Atmospheres, 2016. 121(15): p. 9153-9164.
9. leviChang, H. and T.T. Charalampopoulos, Determination of the Wavelength Dependence of Refractive-Indexes of Flame Soot. Proceedings of the Royal Society-Mathematical and Physical Sciences, 1990. 430(1880): p. 577-591.
10. Moteki, N., Measuring the complex forward-scattering amplitude of single particles by self-reference interferometry: CAS-v1 protocol. Optics Express, 2021. 29(13): p. 20688-20714.
11. Liu, S., et al., Enhanced light absorption by mixed source black and brown carbon particles in UK winter. Nat Commun, 2015. 6.