A Revised Framework for Classifying Organic Aerosols using Wavelength-dependent Absorption Properties

Mathai, Susan; Schuster, Gregory L.; Shook, Michael A.; Ziemba, Luke D.

doi:10.5194/egusphere-2026-1917

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https://doi.org/10.5194/egusphere-2026-1917

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20 May 2026

| 20 May 2026

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

A Revised Framework for Classifying Organic Aerosols using Wavelength-dependent Absorption Properties

Susan Mathai, Gregory L. Schuster, Michael A. Shook, and Luke D. Ziemba

Abstract. The representation of organic aerosols (OA) in global climate models fails to account for the wide range of species found in the atmosphere. Previous studies have observed that the optical parameters of OA species vary depending on the source from which they are emitted, as well as on their physical and chemical characteristics. However, accounting for all OA species in climate models is not practical. Hence, we have grouped OA species according to their optical parameters and physico-chemical characteristics. We classified OA as strongly absorbing brown carbon (S-BrC), moderately absorbing brown carbon (M-BrC), weakly absorbing brown carbon (W-BrC) and very weakly absorbing brown carbon (VW-BrC). We defined thresholds based on the imaginary refractive index (IRI) for a broad wavelength range from 300 to 550 nm. The classification demonstrates clear optical separation at 350–500 nm, with mass absorption coefficient (MAC) values spanning two orders of magnitude from VW-BrC (0.004 m²/g) to S-BrC (1 m²/g) at 400 nm. Representative species from each category were suggested as surrogates. This choice of species includes both absorbing and scattering OA and enables more accurate representation of OA in climate models and satellite retrievals, improving aerosol radiative forcing estimates.

Received: 03 Apr 2026 – Discussion started: 20 May 2026

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Susan Mathai, Gregory L. Schuster, Michael A. Shook, and Luke D. Ziemba

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RC1:
'Comment on egusphere-2026-1917', Anonymous Referee #1, 15 Jun 2026 reply
The manuscript presents a revised framework for classifying organic aerosols based on wavelength-dependent absorption properties. By compiling refractive index data from previous studies and translating the proposed IRI thresholds into MAC and SSA space, the authors attempt to provide a practical optical classification scheme for organic aerosols and brown carbon. The topic is relevant to aerosol-radiation interactions, climate-model parameterization, and satellite retrieval applications. The manuscript is generally well structured, and the effort to extend previous BrC classification concepts from a single reference wavelength to a broader spectral range is valuable. However, several issues need to be addressed before the manuscript can be considered for publication.
The current classification boundaries appear to be empirically selected from literature data, and additional quantitative evidence is needed to demonstrate their robustness and universality.

The current framework is derived from a relatively limited number of OA species, and its applicability to the broader diversity of atmospheric organic aerosols remains uncertain.

Since the refractive index data originate from multiple measurement techniques, the consistency and comparability of these datasets should be addressed.

Although the manuscript introduces a wavelength-dependent extension of existing BrC/OA classification concepts, related classifications of absorbing and weakly absorbing organic aerosols have been discussed in earlier studies (e.g., https://doi.org/10.1007/s40726-020-00139-3; https://doi.org/10.1002/2013JD019912; https://doi.org/10.1029/2007JD009756). The authors should therefore more clearly identify the unique contribution of the present work and emphasize the added value of incorporating wavelength-dependent optical information into the classification framework.

The rationale for adopting wavelength-dependent thresholds should be strengthened. Additional discussion is needed regarding the scientific advantages of multi-wavelength optical characterization relative to traditional single-wavelength approaches.

The manuscript would benefit from a clearer discussion of how the proposed classification framework could be implemented in aerosol optical databases, satellite retrieval algorithms, and climate model parameterizations.

Although the proposed classification is based on IRI, the corresponding MAC and SSA thresholds are ultimately derived from prescribed particle size distributions. Given that aerosol absorption, SSA, and radiative forcing are all highly sensitive to particle size characteristics, particle size distribution may represent a source of variability comparable to refractive index itself (https://doi.org/10.5194/acp-26-3107-2026, https://doi.org/10.1029/2025GL117418). Consequently, the robustness and transferability of the derived MAC- and SSA-based thresholds across different aerosol size regimes and atmospheric conditions remain unclear and warrant further discussion.

Table 5 appears to contain a typographical error: the SSA range for W-BrC at 300 nm is listed as “0.87-92”, which should likely be “0.87-0.92”.

The statement that the proposed framework will improve climate-model and satellite-retrieval estimates should be softened unless additional quantitative evidence is provided.

The manuscript should add a short discussion comparing the proposed multi-wavelength IRI classification with previous approaches based on k₅₅₀, AAE, MAC, or SSA.

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Citation: https://doi.org/10.5194/egusphere-2026-1917-RC1

Susan Mathai, Gregory L. Schuster, Michael A. Shook, and Luke D. Ziemba

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Short summary

There are several organic aerosols being emitted into the atmosphere through different sources. Often, it's challenging to account for these wide variety of species in global climate models due to a lack of clarity about their classification thresholds. In this study, we define thresholds based on the imaginary refractive index to classify organic aerosols into strongly, moderately, weakly, and very weakly absorbing. The classification shows clear optical separation from 350–500 nm wavelengths.


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