The Simulated Source Apportionment of Light Absorbing Aerosols: Effects of Microphysical Properties of Partially-Coated Black Carbon

Biomass burning (BB) fractions estimated with the aethalometer model differ from 0 even for pure aerosols from fossil fuel (FF) combustion. We used black carbon (BC) aerosols partially-coated with non-absorbing materials to represent typical FF aerosols, and the BB fractions were determined with the aethalometer model. Thus, the estimated BB fractions are the fractions that FF aerosols are incorrectly attributed to BB aerosols. The BC morphology and mixing state have significant effects on the estimation of BB fractions. For freshly emitted BC, the BB fractions do not deviate significantly from 0, and the BB fraction is generally in the range of -1%-10% based on an FF absorption Angstrom exponent (AAE) of 1 and a BB AAE of 2. The BB fraction deviates substantially from 0 when BC becomes compact and is coated. The absolute values of the deviations sometimes can be close to 100% for heavily coated BC. The BB fraction is generally greater than 0 for fluffy BC while compact BC generally exhibits a negative BB fraction. The BB fractions of partially-coated BC are very sensitive to the size distribution and coating ratio, which is consistent with the results of the core-shell sphere model. We have also performed a series of studies with different configurations for BC morphologies, coating ratios, and mixing states, which show the variations of the estimated BB fractions with atmospheric aging. We also explain the reasons for the uncertainties in the BB fraction estimates and provide suggestions for using different AAE pairs.