Measurements of the Diversity of Shape and Mixing State for Ambient Black Carbon Particles

The complex particle morphology of black carbon (BC) induces considerable uncertainties in constraining its optical properties. In particular, the diversity of how the BC core and coating are combined is yet to be resolved due to the lack of direct ambient quantification. In this study, by concurrent measurements of the electrical mobility diameter (D_m) and optical volume-equivalent diameter (D_ve), the dynamic shape factor (χ = D_m/D_ve) of ambient BC-containing particles at different mixing states was quantified for the first time. BC in urban Beijing showed a widely distributed χ of 1.4 ± 0.43. Particles with larger refractory BC (rBC) cores showed higher non-sphericity and higher χ. A ten-fold increase in the volume ratio of the coating over rBC (VR) could decrease the fraction of fractal BC by 36%. This study proposed a χ-VR space to describe the particle shape, which is able to include all ambient quantified BC morphologies in optical model.