Apportioning Light Absorption of Ambient Aerosols to Black Carbon, Brown Carbon, and Lensing Effect Using a PAX-ISS Hybrid Method: Insights into Absorption Enhancement

Shen, Yi; Zhi, Guorui; Jin, Wenjing; Kong, Yao; Zhang, Yuzhe; Li, Zhengying; Sun, Jianzhong; Wang, Yuankai; Liang, Zhijian

doi:10.5194/egusphere-2026-734

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https://doi.org/10.5194/egusphere-2026-734

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16 Mar 2026

| 16 Mar 2026

Apportioning Light Absorption of Ambient Aerosols to Black Carbon, Brown Carbon, and Lensing Effect Using a PAX-ISS Hybrid Method: Insights into Absorption Enhancement

Yi Shen, Guorui Zhi, Wenjing Jin, Yao Kong, Yuzhe Zhang, Zhengying Li, Jianzhong Sun, Yuankai Wang, and Zhijian Liang

Abstract. Accurately apportioning aerosol light absorption to black carbon (BC), brown carbon (BrC), and the lensing effect is crucial for constraining aerosol radiative forcing, yet existing methods often fail to resolve all three components simultaneously. Here, we introduce and demonstrate an integrated measurement framework that couples photoacoustic spectroscopy (PAX) with an integrating sphere system using solvent mediation (ISS). This PAX-ISS hybrid method quantifies the total absorption of ambient aerosols (B_{abs_coated}) and removes the lensing effect via solvent-mediated coating removal to obtain the lensing-free absorption (B_{abs_uncoated}). The absorption solely due to the lensing effect (B_abs,lensing) is then directly quantified as their difference: B_abs,lensing=B_{abs_coated}−B_{abs_uncoated}. The lensing-free absorption (B_{abs_uncoated}) is further spectrally decomposed into BC and BrC contributions (B_abs,BC and B_abs,BrC) using a dual-wavelength iterative algorithm. Applied to seasonal samples in Beijing during 2023, the method revealed that BC dominated light absorption, with BrC contributing approximately 10 % annually. The apparent lensing-induced enhancement averaged 40 % of total absorption but exhibited strong seasonal (4.6–52.0 %) and spectral variations, contracting sharply at shorter wavelengths – a pattern indicative of a BrC "blocking effect" that offsets lensing enhancement. Our field measurements provide, for the first time, direct observational evidence supporting this blocking effect, which was initially proposed by other researchers based solely on numerical simulations. The annual wavelength-averaged absorption enhancement factor (E_abs) was 1.69 ± 0.10. This methodology provides a robust, observationally constrained approach to apportion aerosol absorption, offering refined insights for climate modeling.

Received: 06 Feb 2026 – Discussion started: 16 Mar 2026

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Yi Shen, Guorui Zhi, Wenjing Jin, Yao Kong, Yuzhe Zhang, Zhengying Li, Jianzhong Sun, Yuankai Wang, and Zhijian Liang

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-734', Anonymous Referee #1, 07 Apr 2026
This manuscript presents a novel hybrid framework that couples in situ PAX with ISS to apportion aerosol light absorption into BC, BrC, and lensing-related contributions. The topic is relevant and timely because accurate separation of these components is indeed important for constraining aerosol radiative forcing and for interpreting absorption enhancement by internally mixed carbonaceous particles. However, the current version still has room for improvement in several aspects, particularly in the clarity of the methodological framework, the organization of key definitions and equations, and the treatment of assumptions and uncertainties. Strengthening these points would improve both the rigor and readability of the manuscript and help support the main conclusions more convincingly.
The manuscript treats the ISS-derived absorption as a lensing-free state containing only BC and BrC absorption. This assumption is fundamental to the entire framework. However, the paper does not provide direct validation that the ISS treatment removes lensing without simultaneously altering the intrinsic optical behavior of BC and BrC, or changing the original particle/coating structure in a way that biases the result. The authors should more rigorously justify the physical meaning of this “uncoated” state and discuss possible artifacts introduced by solvent processing.

Could the authors clarify how much coating is actually removed and discuss whether the observed waveband-dependent differences may partly reflect size changes rather than lensing alone? This issue may be particularly relevant in light of recent studies emphasizing the importance of size-resolved constraints and particle microphysics for aerosol absorption estimates (doi:10.1029/2025GL117418; doi:10.1002/2017JD027833).

The authors use PAX to measure total coated absorption at 870 nm and then extrapolate that value to 380-880 nm using a single AAEcoated. This reconstructed coated spectrum is then directly used in the subtraction that defines the lensing contribution. The concern is that coated aerosol absorption in this study is itself a mixture of BC, BrC, and enhancement effects, all of which may have different spectral behavior. It is therefore unclear whether a single AAE is sufficient to represent the full spectral dependence.

The DWIC method uses 420 and 650 nm as the two anchor wavelengths, described as BrC-sensitive and BC-dominated, respectively. However, the manuscript does not explain why this pair was selected over other possible combinations, nor whether the results depend strongly on this choice. This should be justified more carefully.

The DWIC separation relies on carbon black and humic acid sodium salt as surrogate standards for BC and BrC. This is practical, but the paper does not discuss how representative these materials are for ambient Beijing aerosol in different seasons. The limitations of these proxies should be addressed explicitly.

The apparent lensing term is derived as the difference between the extrapolated coated absorption and the ISS-derived uncoated absorption. Because this residual may be sensitive to accumulated uncertainties, especially at short wavelengths, could such uncertainties substantially affect the retrieved lensing-related term and thereby the robustness of the conclusions?

The hybrid framework combines real-time in situ PAX observations with offline filter-based ISS measurements collected over 23.5-hour sampling periods. The authors implicitly assume that both datasets represent the same aerosol population and can be merged into a common spectral decomposition framework. The concern is that the manuscript does not discuss possible mismatch between online and offline observations, since the method depends on cross-platform consistency, this issue deserves much more attention.

The CF is important because it converts ISS absorption into a quantity comparable with PAX. Is CF wavelength dependent? A brief summary of the uncertainty introduced by CF in the final apportionment results would also be helpful.

Section 3.3.2 compares the derived enhancement factors with previous literature values from different sites and methods. Could the authors clarify more explicitly whether the definitions of enhancement are fully consistent across these studies?

Some key terms and formulas are explained repeatedly after already being defined, which affects the clarity of the presentation. It would be helpful to streamline these repeated explanations and better organize the related equations and terminology so that the manuscript is easier to follow. Please check carefully
Citation: https://doi.org/10.5194/egusphere-2026-734-RC1
RC2: 'Comment on egusphere-2026-734', Anonymous Referee #2, 25 Apr 2026

Major Comments:
This manuscript combines in situ and filter-based techniques to address important questions about aerosol optical properties. They present long term sampling of Beijing with important insights about how seasonal changes can affect light absorption by aerosol beyond the first-order changes in black carbon concentrations.
However, while it is clear that great care was taken with the calibration and execution of each individual analyses, the propagation of uncertainty for each method is not presented. Without a thorough discussion of the uncertainties in each step of the process, the conclusions remain unsupported. This is a common challenge in trying to establish the relative contributions of different aerosol absorption mechanisms, and one that I hope that this dataset and the ISS-PAX Hybrid method can help resolve with additional analysis.
Referee Comment #1 provides an excellent list of potential sources of uncertainty that need to be addressed; I have nothing to add.

Minor comments:
Graphical abstract: “(offsets lensing at short __?_)”
Line 50: Instead of “stability”, maybe “vapor pressure”?
Line 51: Swap “former” and “latter”
Figure 1. Bottom left-most box should be – B_abs,lensing = B_{abs_coated} - B_{abs_uncoated}

Citation: https://doi.org/10.5194/egusphere-2026-734-RC2

Yi Shen, Guorui Zhi, Wenjing Jin, Yao Kong, Yuzhe Zhang, Zhengying Li, Jianzhong Sun, Yuankai Wang, and Zhijian Liang

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https://doi.org/10.5194/egusphere-2026-734-supplement

Yi Shen, Guorui Zhi, Wenjing Jin, Yao Kong, Yuzhe Zhang, Zhengying Li, Jianzhong Sun, Yuankai Wang, and Zhijian Liang

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Short summary

A novel PAX-ISS method apportions aerosol light absorption to black carbon (BC), brown carbon (BrC), and the lensing effect. Field measurements reveal BC dominance, ~10 % annual BrC contribution, and a 40.0% average lensing role in light absorption with strong spectral-seasonal variations. We provide the first observational evidence for BrC's shortwave "blocking effect", which offsets lensing. This approach offers robust constraints for aerosol optics and climate modeling.


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