Morphology and Fractal Dimension of Size‐Resolved Soot Particles Emitted From Combustion Sources

Databases of the morphological parameters of fresh size‐resolved soot particles and their systematic comparisons among various combustion sources are important to trace the soot aging process and evaluate their optical properties. Here, the mixing state, effective density (ρeff), monomer number and diameter (N and dp), and fractal dimension (Df) of size‐resolved soot particles from vehicle emissions (VE), biomass burning (BB), coal combustion (CC), tunnel air (TA) and urban air (UA) were characterized based on electron microscopy observations and analysis. We determined that freshly‐emitted soot particles from combustion sources contain not only bare‐like soot but also some coated‐soot particles. ρeff (157–689 kg/m3) decreased while N (46–1,500) and dp (24–42 nm) increased with the increased diameter of soot particles. The Df of BB and CC were independent of the diameter changes and fluctuated between 1.65 and 1.80, while the Df of VE and TA (1.62–1.71) increased and UA Df (1.87–1.80) decreased with increasing diameter. Based on Df obtained in this study, we found that the ensemble Df of VE, BB, CC, TA, and UA could not represent the finer (600 nm) in VE, TA, and UA. This study highlights that the morphological parameters of freshly‐emitted soot particles vary among different combustion sources and have a size effect, particularly for finer and coarser soot particles. The database of size‐resolved soot particles will be helpful to improve soot models based on numerical simulation and better evaluate atmospheric optical properties. Plain Language Summary The complex morphology of soot particles causes uncertainties in simulating their optical effects in the atmosphere. By applying a new image recognition technique based on electron microscope observations and field studies, we set up a database that displays the morphological parameters of size‐resolved soot particles freshly emitted from vehicle emissions, biomass burning, and coal combustion and those collected in tunnel and urban air. The results of fractal dimension, monomer diameter, and number of individual soot particles show that the morphological parameters reported in previous studies cannot represent well soot particles in all size bins. Our findings not only play an important role in determining the morphology of real atmosphere soot particles but also provide a soot parametric database to improve the accuracy of atmospheric soot models from d