A novel study of the morphology and effective density of externally mixed black carbon aerosols in ambient air using a size-resolved single-particle soot photometer (SP2)

Abstract. The morphology of externally mixed black carbon (extBC) aerosols, an important factor affecting radiative forcing, was studied by a tandem method coupling a differential mobility diameter (DMA) with a single-particle soot photometer (SP2). Ambient particles were selected by the DMA, and the size-resolved extBC particles were distinguished from those of a thick coating (internally mixed) and quantified by the SP2. Time differences between the DMA size selection and the SP2 measurement were processed previously, as well as the effects of multicharged particles. Based on the mass-mobility relationship, the fractal dimension of the extBC particles was obtained, with a value of 2.36 ± 0.04. This value is comparable with those of diesel exhaust particles, implying a predominant contribution of vehicle emissions to the ambient extBC in urban Beijing. The effective densities (ρeff) of the extBC in the mobility diameter range of 140–750 nm were also derived, with values gradually decreasing from 0.34 g cm−3 at 140–160 nm to 0.12 g cm−3 at 700 nm. The ρeff values were generally lower than those measured using the DMA-aerosol particle mass analyzer (APM) system. This was most likely due to the lower BC masses determined by the SP2 compared to those from the APM, since the SP2 measured the mass of pure refractory BC instead of the entire BC aggregate consisting of both refractory BC and nonrefractory components measured by the APM.