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

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.