A study of the morphology and effective density of externally 1 mixed black carbon aerosols in ambient air using a size-2 resolved single-particle soot photometer ( SP 2 ) 3

16 The morphology and effective density of externally mixed black carbon (extBC) 17 aerosols, important factors affecting the radiative forcing of black carbon, were studied 18 using a tandem technique coupling a differential mobility analyzer (DMA) with a 19 single-particle soot photometer (SP2). The study extended the mass-mobility 20 relationship to large extBC particles with a mobility diameter (dmob) larger than 350 nm, 21 a size range seldom included in previous tandem measurements of BC aggregates in 22 the atmosphere. The experiment was conducted at an urban site in Beijing during a 1923 day winter period from 23 January to 10 February 2018. Ambient dry particles were 24 selected by the DMA, and the size-resolved extBC particles were distinguished from 25 particles with a thick coating (internally mixed) according to the time delay between 26 the incandescence signal peak and the scattering peak detected by the SP2. The masses 27 of the extBC particles were then quantified. The time differences between the DMA 28