Dynamic shape factor and mixing state of refractory 1 black carbon particles in winter in Beijing using an 2 AAC-DMA-SP 2 tandem system 3

Abstract

15 Refractory black carbon (rBC) is one of the most important short-lived climate forcers in the 16 atmosphere. Light absorption enhancement capacity largely depends on the morphology of 17 rBC-containing particles and their mixing state. In this study, a tandem measuring system, 18 consisting of an aerodynamic aerosol classifier (AAC), a differential mobility analyzer (DMA) 19 and a single particle soot photometer (SP2), was adopted to investigate dynamic shape factor 20 (χ) and its relationship with the mixing state of rBC-containing particles at an urban site of 21 Beijing megacity in winter. The results demonstrated that the aerosol particles with an 22 aerodynamic diameter of 400 ± 1.2 nm normally had a mobility diameter (Dmob) ranging from 23 250 nm to 320 nm, reflecting a large variation in shape under different pollution conditions. 24 Multiple Gaussian fitting on the number mass-equivalent diameter (Dmev) distribution of the 25 rBC core determined by SP2 had two peaks at Dmev =106.5 nm and Dmev =146.3 nm. During 26 pollution episodes, rBC-containing particles tended to have a smaller rBC core than those 27 during clean episodes due to rapid coagulation and condensation processes. The χ values of 28 the particles were found to be ~1.2 during moderate pollution conditions, although the shell29 core ratio (S/C) of rBC-containing particles was as high as 2.7 ± 0.3, suggesting that the 30 particles had an irregular structure as a result of the high fraction of nascent rBC aggregates. 31 1 https://doi.org/10.5194/acp-2019-433 Preprint. Discussion started: 1 July 2019 c © Author(s) 2019. CC BY 4.0 License.