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 (ρ_eff), monomer number and diameter (N and d_p), and fractal dimension (D_f) 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. ρ_eff (157–689 kg/m³) decreased while N (46–1,500) and d_p (24–42 nm) increased with the increased diameter of soot particles. The D_f of BB and CC were independent of the diameter changes and fluctuated between 1.65 and 1.80, while the D_f of VE and TA (1.62–1.71) increased and UA D_f (1.87–1.80) decreased with increasing diameter. Based on D_f obtained in this study, we found that the ensemble D_f of VE, BB, CC, TA, and UA could not represent the finer (<150 nm) soot particles and coarser soot particles (>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.