Inferring the absorption properties of organic aerosol in Siberian biomass burning plumes from remote optical observations

Light-absorbing organic matter, known as brown carbon (BrC), has previously been found to significantly en- hance the absorption of solar radiation by biomass burning (BB) aerosol. Previous studies have also proposed methods aimed at constraining the BrC contribution to the overall aerosol absorption using the absorption Ångström exponents (AAEs) derived from the multi-wavelength remote observa- tions at Aerosol Robotic Network (AERONET). However, representations of the BrC absorption in atmospheric mod- els remain uncertain, particularly due to the high variabil- ity in the absorption properties of BB organic aerosol (OA). As a result, there is a need for stronger observational con- straints on these properties. We extend the concept of the established AAE-based methods in the framework of our Bayesian method, which combines remote optical observa- tions with Monte Carlo simulations of the aerosol absorp- tion properties. We propose that the observational constraints on the absorption properties of BB OA can be enhanced by using the single-scattering albedo (SSA) as part of the ob- servation vector. The capabilities of our method were first examined by using synthetic data, which were intended to represent the absorption properties of BB aerosol originat- ing from wildfires in Siberia. We found that observations of AAEs and SSA can provide efficient constraints not only on the BrC contribution to the total absorption but also on both the imaginary part of the refractive index and the mass absorption efficiency of OA. The subsequent application of our method to the original multi-annual data from Siberian AERONET sites, along with the supplementary analysis of possible biases in the a posteriori estimates of the inferred absorption properties, indicates that the contribution of BrC to the overall light absorption by BB aerosol in Siberia at the 440 nm wavelength is most likely to range, on average, from about 15 % to 21 %, although it is highly variable and, in some cases, can exceed 40 %. Based on the analysis of the AERONET data, we also derived simple nonlinear param- eterizations for the absorption characteristics of BB OA in Siberia as functions of the AA