Fractal Dimensions and Mixing Structures of Soot Particles during Atmospheric Processing

Soot particles strongly absorb sunlight and hence act as a short-lived warming agent. Atmospheric aging of soot particles changes their morphology and mixing state and consequently alters their optical properties. Here we collected soot particles at tunnel, urban, mountaintop, and background sites in the North China Plain and analyzed their mixing structures and morphology using transmission electron microscopy. Soot particles were further classified into three types: bare-like, partly coated, and embedded. Bare-like soot particles were dominant at the tunnel site, while most soot particles were of the partly coated or embedded type at other sites. Fractal dimensions (D_f) of different types of soot particles ranged from 1.80 to 2.16 and increased in the following order: bare-like < partly coated < embedded. Moreover, their average D_f changed from 1.8 to 2.0 from the tunnel to the background site. We conclude that the D_f can characterize the shape of soot aggregates reasonably well, and its variation reflects soot aging processes. Compared with the reported D_f of soot particles, we found that the D_f value of 1.8 used in previous optical models primarily represents freshly emitted soot aggregates, rather than the ambient ones.