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

Abstract

Abstract. Light-absorbing organic matter, known as brown carbon\n(BrC), has previously been found to significantly enhance the absorption of\nsolar radiation by biomass burning (BB) aerosol. Previous studies have also\nproposed methods aimed at constraining the BrC contribution to the overall\naerosol absorption using the absorption Ångström exponents (AAEs)\nderived from the multi-wavelength remote observations at Aerosol Robotic\nNetwork (AERONET). However, representations of the BrC absorption in\natmospheric models remain uncertain, particularly due to the high\nvariability in the absorption properties of BB organic aerosol (OA). As a\nresult, there is a need for stronger observational constraints on these\nproperties. We extend the concept of the established AAE-based methods in\nthe framework of our Bayesian method, which combines remote optical\nobservations with Monte Carlo simulations of the aerosol absorption\nproperties. We propose that the observational constraints on the absorption\nproperties of BB OA can be enhanced by using the single-scattering albedo\n(SSA) as part of the observation vector. The capabilities of our method were\nfirst examined by using synthetic data, which were intended to represent the\nabsorption properties of BB aerosol originating from wildfires in Siberia.\nWe found that observations of AAEs and SSA can provide efficient constraints\nnot only on the BrC contribution to the total absorption but also on both\nthe imaginary part of the refractive index and the mass absorption\nefficiency of OA. The subsequent application of our method to the original\nmulti-annual data from Siberian AERONET sites, along with the supplementary\nanalysis of possible biases in the a posteriori estimates of the inferred\nabsorption properties, indicates that the contribution of BrC to the overall\nlight absorption by BB aerosol in Siberia at the 440\u2009nm wavelength is most\nlikely to range, on average, from about 15\u2009% to 21\u2009%, although it is highly\nvariable and, in some cases, can exceed 40\u2009%. Based on the analysis of\nthe AERONET data, we also derived simple nonlinear parameterizations for the\nabsorption characteristics of BB OA in Siberia as functions of the AAE.\n