Measuring Light Absorption by Organic Aerosols: Correction Factors for Solvent Extraction-Based Photometry Techniques

Abstract. Recent studies have shown that organic aerosol (OA) could have a non-trivial role in atmospheric light absorption at shorter visible wavelengths. Good estimates of OA absorption are therefore necessary to accurately calculate radiative forcing due to these aerosols in climate models. One of the common techniques used to measure OA light absorption is the solvent extraction technique from filter samples which involves the use of a spectrophotometer to measure bulk absorbance of the solvent-soluble organic fraction of particulate matter. Measured bulk absorbance is subsequently converted to particle-phase absorption coefficient using correction factors. The appropriate correction factors to use for performing this conversion under varying scenarios of organic carbon (OC) to total carbon (TC) mass ratios has been an unexplored area of research. The conventional view is to apply a correction factor of 2 for water-extracted OA based on Mie calculations. Here, we performed a comprehensive laboratory study involving three solvents (water, methanol, and acetone) to investigate the corrections factors for converting from bulk-to-particle phase absorption coefficients ( b abs,OA / b abs,bulk ) for primary OA emitted from biomass burning. We parametrized these correction factors as a function of OC / TC mass ratio and single scattering albedo (SSA). We observed these correction factors to be a function of the OC / TC ratio of the aerosol, and that the conventionally used correction factor of 2 for water-extracted OA could severely underpredict OA absorption at high EC mass fractions. We recommend using b abs,OA / b abs,bulk values between 2 and 11 for water extracts and values between 1 and 4 for methanol extracts based on OC / TC ratios, for EC mass fractions less than 0.25. Furthermore, a linear correlation between SSA and OC / TC ratio was also established. Finally, from the spectroscopic data, we analyzed the differences in Absorption Angstrom Exponents (AAE) obtained from bulk- and particulate-phase measurements. We noted that AAE from bulk measurements deviate significantly from their OA counterparts.