Dynamic shape factor and mixing state of refractory black carbon particles in winter in Beijing using an AAC-DMA-SP2 tandem system

Abstract. Refractory black carbon (rBC) is one of the most important short-lived climate forcers in the atmosphere. Light absorption enhancement capacity largely depends on the morphology of rBC-containing particles and their mixing state. In this study, a tandem measuring system, consisting of an aerodynamic aerosol classifier (AAC), a differential mobility analyzer (DMA) and a single particle soot photometer (SP2), was adopted to investigate dynamic shape factor (𝜒) and its relationship with the mixing state of rBC-containing particles at an urban site of Beijing megacity in winter. The results demonstrated that the aerosol particles with an aerodynamic diameter of 400 ± 1.2 nm normally had a mobility diameter (D_mob) ranging from 250 nm to 320 nm, reflecting a large variation in shape under different pollution conditions. Multiple Gaussian fitting on the number mass-equivalent diameter (D_mev) distribution of the rBC core determined by SP2 had two peaks at D_mev = 106.5 nm and D_mev = 146.3 nm. During pollution episodes, rBC-containing particles tended to have a smaller rBC core than those during clean episodes due to rapid coagulation and condensation processes. The 𝜒 values of the particles were found to be ~ 1.2 during moderate pollution conditions, although the shell-core ratio (S/C) of rBC-containing particles was as high as 2.7 ± 0.3, suggesting that the particles had an irregular structure as a result of the high fraction of nascent rBC aggregates. During heavy pollution episodes, the 𝜒 value of the particles was approximately 1.0, indicating that the majority of particles tended to be spherical, and a shell-core model could be reasonable to estimate the light enhancement effect. Considering the variation in shape of the particles, the light absorption enhancement of the particles differed significantly according to the T-matrix model simulation. This study suggested that accurate description of the morphology of rBC-containing particles was crucially important for optical simulation and better evaluation of their climate effect.