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

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.