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. The Aethalometer model has been used widely for estimating the contributionsof fossil fuel emissions and biomass burning to equivalent black carbon(eBC). The calculation is based on measured absorption Ångströmexponents (αabs). The interpretation of αabs isambiguous since it is well known that it not only depends on the dominant absorber but also on the size and internal structure of the particles, coresize, and shell thickness. In this work the uncertainties of theAethalometer-model-derived apparent fractions of absorption by eBC fromfossil fuel and biomass burning are evaluated with a core–shell Mie model.Biomass-burning fractions (BB(%)) were calculated for pure and coatedsingle BC particles for lognormal unimodal and bimodal size distributionsof BC cores coated with ammonium sulfate, a scattering-only material.BB(%) was very seldom 0 % even though BC was the only absorbingmaterial in the simulations. The shape of size distribution plays animportant role. Narrow size distributions result in higher αabsand BB(%) values than wide size distributions. The sensitivity of αabs and BB(%) to variations in shell volume fractions is thehighest for accumulation-mode particles. This is important because that iswhere the largest aerosol mass is. For the interpretation of absorptionÅngström exponents it would be very good to measure BC sizedistributions and shell thicknesses together with the wavelength dependencyof absorption.

Modeled source apportionment of black carbon particles coated with a light-scattering shell