Unraveling the Impact of Heterogeneity and Morphology on Light Absorption Enhancement of Black Carbon-Containing Particles

Wei, Jing; Ding, Jin-Mei; Song, Yao; Wang, Xiao-Yuan; Pei, Xiang-Yu; Xu, Sheng-Chen; Zhang, Fei; Xu, Zheng-Ning; Tian, Xu-Dong; Xu, Bing-Ye; Wang, Zhi-Bin

doi:https://doi.org/10.5194/egusphere-2025-2844

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https://doi.org/10.5194/egusphere-2025-2844

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30 Jun 2025

| 30 Jun 2025

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Unraveling the Impact of Heterogeneity and Morphology on Light Absorption Enhancement of Black Carbon-Containing Particles

Jing Wei, Jin-Mei Ding, Yao Song, Xiao-Yuan Wang, Xiang-Yu Pei, Sheng-Chen Xu, Fei Zhang, Zheng-Ning Xu, Xu-Dong Tian, Bing-Ye Xu, and Zhi-Bin Wang

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 (E_abs). In this study, we employed a Centrifugal Particle Mass Analyzer and Single Particle Soot Photometer tandem system to characterize mass ratio (M_R, coating-to-BC) and morphology of BC-containing particles in Hangzhou, China. Fortunately, low, medium, and high E_abs values were observed during a single field campaign. Results show that the uniform core-shell Mie model overestimated E_abs especially in clean conditions (low E_abs). A morphology-dependent correction scheme was developed to improve optical property estimates of BC in the “transition state.” This improved model better reproduces measured E_abs in different pollution conditions and reveals that the concentrations of particle chemical composition affect the M_R threshold defining this state. Our findings highlight the need to account for real-world particle complexity in climate-relevant BC modeling.

Received: 15 Jun 2025 – Discussion started: 30 Jun 2025

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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Jing Wei, Jin-Mei Ding, Yao Song, Xiao-Yuan Wang, Xiang-Yu Pei, Sheng-Chen Xu, Fei Zhang, Zheng-Ning Xu, Xu-Dong Tian, Bing-Ye Xu, and Zhi-Bin Wang

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'Comment on egusphere-2025-2844', Anonymous Referee #1, 13 Oct 2025 reply
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
The definition of “transition state” is vague, and it should be quantified. The criteria used are not clear.

How the three cases (1,2, and 3) were defined is not clear.

The link with local conditions, pollution level, and contribution from different sources is not well developed. It is probably possible and relatively straightforward for the authors to use existing monitoring data to corroborate some of their discussions. I would also suggest generating some more discussion of the back-trajectory simulations to determine transport path and time, and potential sources. The authors do show some of this work in the SI, but in the main paper, there is little to no mention, and more discussion should be added.

The SI also shows an ACSM, which might provide further information that I did not see discussed in the paper. Maybe I missed it, but was Figure S7 based on the ACSM data? Maybe that could be made clearer in the SI as well as in the main text. Small note: In the same figure, CPAS should be CAPS (?)

It is not clear to me what the “improved model” is and how it is operationally defined. More details should be provided on this topic as it seems to be central to the discussion.

The literature cited is somewhat lacking and perhaps a bit biased; especially lacking is the comparison with other single-particle techniques such as microscopy.

Even though the paper is mostly clearly written, the grammar should still be improved; some limited examples are provided in the next section.

Specific comments
Line 35: space after models

Line 51. I think Fierce’s paper also discussed the effect of morphological deviations at low M_R

Line 86: “an BC” -> “a BC”

Line 111: “was” -> “were”

Line 120: How were the concentrations of PSL measured?

Line 127: “a” in front of “single”. Also, what do the authors mean by “without any assumptions”?

Line 129: “to compare the” -> “to be compared with”

Line 134: Mie is good only for spherically symmetric particles. How do the authors account for that, considering the premise of the paper is the deviation from spherical symmetry?

Line 137: The refractive index used here seems quite high with respect to other studies in the literature. Additionally, the coating is sometimes slightly absorbing, and that can make a difference, especially at shorter wavelengths. A sensitivity analysis using a range of indices of refraction would be helpful.

Line 154: MAC units?

Line 164: Why are these refractive index values different from those in Figure 137? Is it because of the wavelength? The wavelengths should be specified clearly in every instance.

Figure 1: The core shell Mie model is for 200 nm, what is that a radius or a diameter, and is it mass equivalent, or something else? Also, why 200 nm exactly? More discussion on the model would be useful.

Line 215: I would not use the word “closely” here; the agreement is reasonable, but not that close.

Line 227: What does “at different period” refer to exactly? Perhaps “periods”.

Line 232: Why log₁₀?

Line 241-242: I am not sure what “due to stems” means.

Lines 264-265: I am not clear how these ranges are defined.

Line 265: I do not recall the authors discussing the pollution regimes of the three cases. I would give that more emphasis early on because that is probably at the root of the observed behaviors.

Line 269: What do the authors mean by heterogeneous aging?

Line 272: While plausible, this explanation (on why “BC required less coating material to evolve into a core-shell structure”) is not completely clear to me, and the authors might want to elaborate more.

Line 292: Again, I think “closely” is too strong.

Lines 300-302. I believe in the paper by Fierce et al., the discrepancy was suggested to be due to M_R heterogeneity but mostly for Large coatings, while for low coatings the deviations were associated with non-core-shell configurations, so I do not see a clear discrepancy between the two studies, on the contrary, it seems to me that the results are quite similar when considered for similar coating regimes.

Figure 4(b), 4(c), and 4(d), what is on the x axis? Please put an axis title and units if applicable.

Line 323: Why inorganic specifically? Why not also organics?

Line 337: The authors should clearly describe the “improved method”. Here, it is not clear at all what that is. How was the fit performed, and what did they do with the fit?

Lines 360 to 368: How do the authors suggest developing such a unified model?

Figure S2: It is not clear how the CPC would measure a size distribution, being a simple counter. The caption mentions a DMA; perhaps that should be clearer from the legend. What are SCHG and BBHG (scattering and broadband high gain?). Spell out acronyms in the caption.

Figures S4 and S5: the signal ranges are very different. Why is that? Can the nephelometer and the CAPS measure scattering coefficients below 0.01 Mm^-1 accurately and precisely? That is hard to believe. Also how were the Mie simulations being informed, in terms of concentrations, size, and index of refraction?

Figure S7: What does the horizontal pink dashed line indicate, the mean E_abs?

Figure S8: Some more information is needed. How were the trajectories being calculated? What is the time span, what is the elevation used, can the authors also provide vertical trajectories to see whether the air masses are coming from the boundary layer or from aloft to understand if it is near or long-range transportation?

Figure S9: Are these based on the SP2?

Figure S10: The units on the x-axis are not critical, but still, it would be nice to have them.

Figure S11: Units on x axes (fg?).

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.

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Citation: https://doi.org/10.5194/egusphere-2025-2844-RC1

Jing Wei, Jin-Mei Ding, Yao Song, Xiao-Yuan Wang, Xiang-Yu Pei, Sheng-Chen Xu, Fei Zhang, Zheng-Ning Xu, Xu-Dong Tian, Bing-Ye Xu, and Zhi-Bin Wang

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https://doi.org/10.5194/egusphere-2025-2844-supplement

Jing Wei, Jin-Mei Ding, Yao Song, Xiao-Yuan Wang, Xiang-Yu Pei, Sheng-Chen Xu, Fei Zhang, Zheng-Ning Xu, Xu-Dong Tian, Bing-Ye Xu, and Zhi-Bin Wang

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Short summary

Black carbon (BC) is a light-absorbing particle that contributes to atmospheric warming, but its radiative impact remains highly uncertain. We conducted field measurements in Hangzhou, China, to examine how mass ratio (coating-to-BC) and morphology influence light absorption. Our results show that widely used optical models overestimate absorption especially under clean conditions. A new morphology-based method improves model accuracy and reduces this uncertainty.


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