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. Refractory black carbon (rBC) in the atmosphere is knownfor its significant impacts on climate. The relationship between themicrophysical and optical properties of rBC remains poorly understood and isinfluenced by its size and mixing state. Mixing state also influences itscloud scavenging potential and thus atmospheric lifetime. This studypresents a coupling of a centrifugal particle mass analyser (CPMA) and asingle-particle soot photometer (SP2) for the morphology-independentquantification of the mixing state of rBC-containing particles, used in theurban 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 highlydynamic polluted environment with a wide variety of conditions that could beconsidered representative of megacity area sources in Asia. An inversionmethod (used for the first time on atmospheric aerosols) is applied to themeasurements to present two-variable distributions of both rBC mass andtotal mass of rBC-containing particles and calculate the mass-resolvedmixing state of rBC-containing particles, using previously publishedmetrics. The mass ratio between non-rBC material and rBC material (MR) iscalculated to determine the thickness of a hypothetical coating if the rBCand other material followed a concentric sphere model (the equivalentcoating thickness). The bulk MR (MR bulk ) was found to varybetween 2 and 12 in winter and between 2 and 3 in summer. This mass-resolved mixingstate is used to derive the mass-weighted mixing state index for therBC-containing particles ( χrBC ). χrBC quantifies how uniformly the non-rBC material is distributed across therBC-containing-particle population, with 100 % representing uniformmixing. The χrBC in Beijing varied between 55 % and70 % in winter depending on the dominant air masses, and χrBC was highly correlated with increased MR bulk andPM 1 mass concentration in winter, whereas χrBC insummer varied significantly (ranging 60 %–75 %) within thenarrowly distributed MR bulk and was found to be independentof air mass sources. In some model treatments, it is assumed that moreatmospheric ageing causes the BC to tend towards a more homogeneous mixture,but this leads to the conclusion that the MR bulk may onlyact as a predictor of χrBC in winter. The particlemorphology-independent and mass-based information on BC mixing used in thisand future studies can be applied to mixing-state-aware models investigatingatmospheric rBC ageing.

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