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. The morphology and effective density of externally mixedblack carbon (extBC) aerosols, important factors affecting the radiativeforcing of black carbon, were studied using a tandem technique coupling adifferential mobility analyzer (DMA) with a single-particle soot photometer(SP2). The study extended the mass–mobility relationship to large extBCparticles with a mobility diameter (dmob) larger than 350 nm, a sizerange seldom included in previous tandem measurements of BC aggregates inthe atmosphere. The experiment was conducted at an urban site in Beijingduring a 19 d winter period from 23 January to 10 February 2018. Ambientdry particles were selected by the DMA, and the size-resolved extBC particleswere distinguished from particles with a thick coating (internally mixed)according to the time delay between the incandescence signal peak and thescattering peak detected by the SP2. The masses of the extBC particles werethen quantified. The time differences between the DMA size selection and theSP2 measurement were processed previously. The normalized number sizedistributions were investigated at the prescribed dmob sizes in therange of 140–750 nm to provide the typical mass of extBC at each dmob. Onthis basis, the mass–mobility relationship of the ambient extBC wasestablished, inferring a mass–mobility scaling exponent (Dfm) (animportant quantity for characterizing the morphology of fractal-like BCaggregates) with a value of 2.34±0.03 in the mobility rangeinvestigated in this study. This value is comparable with those of dieselexhaust particles, implying a predominant contribution of vehicle emissionsto the ambient extBC in urban Beijing. Compared to the clean period, a higherDfm value was observed in the polluted episode, indicating a morecompact BC aggregate structure than that in the clean period. The effectivedensities (ρeff) of the extBC in the same dmob range were alsoderived, with values gradually decreasing from 0.46 g cm−3 at 140 nmmobility to 0.14 g cm−3 at 750 nm mobility. The ρeff valueswere slightly lower than those measured using the DMA–aerosol particle massanalyzer (APM) system. The difference in ρeff values was likelydue to the lower BC masses determined by the SP2 compared to those measuredby the APM at the same mobility, since the SP2 measured the refractory BC(rBC) mass instead of the total mass of the BC aggregate, which consists ofboth rBC and a possible fraction of nonrefractory components measured by theAPM. The ρeff values in the 280–350 nm dmob range were muchcloser to the values for soot aggregates reported in the literature. Itmight be related to the more compact structure of BC aggregates in thisrange, resulting from the reconstruction effect by volatile and/orsemivolatile components in the atmosphere. The reconstruction effect mightalso result in a hiatus in the increased dynamic shape factor in the rangeof 200–350 nm, which presented an overall increase from 2.16 to 2.93 in the140–750 nm dmob range.

atmospheric measurement techniques

A 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)