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

Abstract

Abstract. 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 (χrBC). χrBC quantifies how uniformly the non-rBC material is distributed across the rBC-containing-particle population, with 100 % representing uniform mixing. The χrBC in Beijing varied between 55 % and 70 % in winter depending on the dominant air masses, and χrBC was highly correlated with increased MRbulk and PM1 mass concentration in winter, whereas χrBC 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 the MRbulk may only act as a predictor of χrBC in winter. The particle morphology-independent and mass-based information on BC mixing used in this and future studies can be applied to mixing-state-aware models investigating atmospheric rBC ageing.

Evolution of Organic Aerosol From Wood Smoke Influenced by Burning Phase and Solar Radiation

“…However, the coating volume in bulk calculated above can largely overcome such uncertainty, as the overall coatings are largely modulated by larger particles. Previous studies showed high consistency between the SP2‐only determined coatings and that by morphology‐independent measurements, with a max uncertainty of 25% from the SP2 measurement (Yu et al., 2020).…”

Vertical profile of particle hygroscopicity and CCN effectiveness during winter in Beijing: insight into the hygroscopicity transition threshold of black carbon

“…The setup has been described in detail in recent studies. 46,47 Briefly, the CPMA was placed upstream of the SP2. The monodisperse ambient particles with a certain nominal mass were selected by the CPMA and then introduced into the SP2 for analysis of the mass of refractory black carbon (rBC) in single particle.…”

Direct measurements of black carbon fluxes in central Beijing using the eddy-covariance method

“…The physical properties of BC in Beijing during this campaign for both seasons have been discussed by Yu et al (2020) and Liu et al (2019) based on the concurrent ground-level SP2 observations. The mass ratio of internally mixed non-BC material (the 'coating') to BC was quantified by Yu et al (2020), which varied from 2 to 12 in the winter season and 2 to 3 in the summer season. The higher variability during the winter was related to combination of frequent occurrences of heavy haze episodes and presence of additional sources compared to the summer seasons.…”

“…This study is part of the wider UK-China Air Pollution and Human Health (APHH) project, which had field campaigns in Beijing during winter (16 th Nov-10 th Dec 2016) and summer (15 th May-30 th June 2017) (Shi et al, 2019). Also as part of this project, ambient BC concentrations were measured using a separate SP2 connected to a conventional inlet line, as described by Liu et al (2019) and Yu et al (2020).…”