Evaluation of the absorption Ångstrom exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol

Equivalent black carbon (EBC) measured by a multi-wavelength Aethalometer can be apportioned to traffic and wood burning. The method is based on the differences in the dependence of aerosol absorption on the wavelength of light used to investigate the sample, parameterized by the source-specific absorption angstrom ngstrom exponent (alpha). While the spectral dependence (defined as alpha values) of the traffic-related EBC light absorption is low, wood smoke particles feature enhanced light absorption in the blue and near ultraviolet. Source apportionment results using this methodology are hence strongly dependent on the alpha values assumed for both types of emissions: traffic alpha TR, and wood burning alpha WB. Most studies use a single alpha TR and alpha WB pair in the Aethalometer model, derived from previous work. However, an accurate determination of the source specific alpha values is currently lacking and in some recent publications the applicability of the Aethalometer model was questioned.