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 (BrC), has previously been found to significantly enhance 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 observations at Aerosol Robotic Network (AERONET). However, representations of the BrC absorption in atmospheric models remain uncertain, particularly due to the high variability in the absorption properties of BB organic aerosol (OA). As a result, there is a need for stronger observational constraints on these properties. We extend the concept of the established AAE-based methods in the framework of our Bayesian method, which combines remote optical observations with Monte Carlo simulations of the aerosol absorption 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 observation vector. The capabilities of our method were first examined by using synthetic data, which were intended to represent the absorption properties of BB aerosol originating 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 parameterizations for the absorption characteristics of BB OA in Siberia as functions of the AAE.

Impact of the Atmospheric Photochemical Evolution of the Organic Component of Biomass Burning Aerosol on Its Radiative Forcing Efficiency: A Box Model Analysis

“…The role of these effects in the evolution of BB aerosol and its optical and radiative properties is presently poorly known, especially in Siberia, and should be clarified in future studies. Further studies are needed to examine possible changes in the evolution of the ARFE due to BrC absorption [49,102,103] which has also been disregarded in the simulation presented in this paper.…”

“…That is, the contributions of inorganic salts and water to the particle composition were disregarded. Note that optically dense smoke plumes are typically observed in Siberia in a relatively dry atmosphere with RH less than 70% [49], whereas the water uptake by BB aerosol particles under such conditions is indeed likely to be small [50], and it is also known that inorganic salts typically constitute a minor fraction of BB aerosol [51]. Finally, we also disregarded the dilution.…”

“…The organic shell was assumed to be nonabsorbing, that is, the contribution of brown carbon (BrC) to absorption was neglected. Available estimates of BrC absorption in Siberian BB plumes indicate that it is highly variable [49,58] but, on average, is rather weak within the visible wavelength range. Hence, the BrC absorption is unlikely to be a key factor determining the evolution of the radiative forcing efficiency of Siberian BB aerosol, although the impact of BrC on the radiative properties of Siberian BB aerosol needs yet to be clarified in future dedicated studies.…”

“…Hence, our analysis primarily addresses the radiative properties of specifically Siberian BB aerosol. Note that the use of a simple VBS scheme is justifiable by the fact that there is presently no validated and generally accepted method to simulate the atmospheric evolution of BB aerosol, whereas different available methods yield diverse results [36,49]. Taking into account that our modeling framework is highly simplified, we do not attempt to obtain accurate quantitative estimates of the radiative properties of BB aerosol, but rather aim at identifying only qualitative features of the evolution of the radiative forcing efficiency of BB aerosol in an idealized smoke plume.…”

Optical Characterization of Fresh and Photochemically Aged Aerosols Emitted from Laboratory Siberian Peat Burning

“…Many of these effects are driven by the optical properties of the emitted aerosols; therefore, this has created significant interest in the optical properties of biomass burning emissions and their change during atmospheric processing. Extensive work has been conducted on optical properties, including light absorption, for freshly emitted [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] biomass burning aerosols and on the change of their optical properties during atmospheric processing [22][23][24][25][26][27][28][29][30][31][32][33]. However, work on the optical properties of peat emissions and their changes has been very limited [34].…”