Size–resolved mixing state of ambient refractory black carbon aerosols in Beijing during the XXIV Olympic Winter Games

Atmospheric loading of black carbon aerosols influences air quality and visibility, and their mixing state and aerosol chemistry were sensitive to both emission scenarios and regional transport. In this study, the aerodynamic size-resolved mixing state of refractory black carbon aerosols (rBC) and elemental composition were investigated using a novel tandem system consisting of an aerodynamic aerosol classifier (AAC), a single particle soot photometer (SP2), and single-particle-aerosol mass spectrometry (SPA-MS) during the XXIV Olympic Winter Games (OWG) at an urban site in Beijing. We found that rBC-containing particles with an aerodynamic diameter (D_ae) of 300 nm were characterized by a rBC core with a count median diameter (CMD) of 108±4 nm that typically have regular morphology ( , higher relative coating thickness (RCT: mean ~1.8), and strong light absorption enhancement (E_abs: mean ~1.4). The rBC-containing particles with D_ae = 200 nm normally had irregular shapes and weak E_abs. Classification based on air mass clustering and air pollution level revealed that the coating thickness of rBC particles only exhibited unimodal patterns under clean air conditions but bimodal distributions under heavy pollution conditions, implying multiple origins of rBC particles, even during OWG periods. Furthermore, the chemical compositions of the rBC coating quantitatively determined by SPA-MS were used to estimate the possible origins of rBC with D_ae = 300 nm. Our results showed that rBC particles were primarily mixed with organics, nitrate, and sulfate during the XXIV OWG period. The organic components have a limited role in increasing rBC coatings under polluted conditions, while the alkali metal ions (K and Na) associated with traffic emissions and the secondary inorganic species favor the formation of thick coatings. Higher RH makes limited contributions to rBC mixed with sulfate and organics only (BCOC*_S) and pure BC. The relatively fresh BC could directly mix with organics and sulfate at low RH levels while evolving to mix with nitrate at high RH conditions. In addition to high concentrations of locally emitted precursors and high RH that tend to form thick coatings of nitrate-related rBC aerosols, secondary transport from the west and southwest also contributes to rBC aerosols mixed with nitrate with relatively thick coatings.