Characterising mass-resolved mixing state of black carbon in Beijing using a morphology-independent measurement method

Refractory black carbon (rBC) in the atmosphere is known for its significant impacts on climate. The relationship between the microphysical and optical properties of rBC remains poorly understood and is influenced by its size and mixing state. Mixing state also influences its cloud scavenging potential and thus atmospheric lifetime. This study presents a coupling of a centrifugal particle mass analyser (CPMA) and a single-particle soot photometer (SP2) for the morphology-independent quantification of the mixing state of rBC-containing particles, used in the urban site of Beijing as part of the Air Pollution and Human Health-Beijing (APHH-Beijing) project during winter (10 November-10 December 2016) and summer (18 May-25 June 2017). This represents a highly dynamic polluted environment with a wide variety of conditions that could be considered representative of megacity area sources in Asia. An inversion method (used for the first time on atmospheric aerosols) is applied to the measurements to present two-variable distributions of both rBC mass and total mass of rBC-containing particles and calculate the mass-resolved mixing state of rBC-containing particles, using previously published metrics. The mass ratio between non-rBC material and rBC material (MR) is calculated to determine the thickness of a hypothetical coating if the rBC and other material followed a concentric sphere model (the equivalent coating thickness). The bulk MR (MRbulk) was found to vary between 2 and 12 in winter and between 2 and 3 in summer. This mass-resolved mixing state is used to derive the mass-weighted mixing state index for the rBC-containing particles (chi(rBC)). chi(rBC) quantifies how uniformly the non-rBC material is distributed across the rBC-containing-particle population, with 100% representing uniform mixing. The chi(rBC) in Beijing varied between 55% and 70% in winter depending on the dominant air masses, and chi(rBC) was highly correlated with increased MRbulk and PM1 mass concentration in winter, whereas chi(rBC) in summer varied significantly (ranging 60 %-75 %) within the narrowly distributed MRbulk and was found to be independent of air mass sources. In some model treatments, it is assumed that more atmospheric ageing causes the BC to tend towards a more homogeneous mixture, but this leads to the conclusion that theMRbulk may only act as a predictor of chi(rBC) in winter. The particle morphology-independent and mass-based information on BC mixing used in this and future studies can be applied to mixing-stateaware models investigating atmospheric rBC ageing.