Understanding the surface and pore features of soot nanoparticles is important for predicting their behavior in combustion environments and atmospheric processes. Here, we present a novel computational framework combining reactive molecular dynamics simulations with detailed atomistic analysis to characterize the morphology of over 2000 incipient soot particles formed from acetylene pyrolysis at 1350–1800 K. The surface and pore features of these nanoparticles are explored directly using three-dimensional atomic surface mesh for the first time. The nanoparticles are found to have a highly irregular shape, with an average sphericity of 0.57 and a surface fractal dimension (D_S) of approximately 2.22, in excellent agreement with experimental data. The particles exhibit significant internal porosity (Φ≈0.22) dominated by micropores (≤2 nm). Micropores contribute to a very high specific surface area of approximately 2652. Three distinct pore types – tunnels, pockets, and isolated cavities – are identified in the incipient soot primary particles. The internal pore network is found to have a fractal dimension (D_VC) of approximately 2.15. Strong positive correlations between pore volume and surface area (R²≈0.70) are observed. The findings point to a complex and irregular external and internal structures of incipient soot nanoparticles and a complex pore network within them.

Soot inception by acetylene pyrolysis at 1350-1800 K is investigated using reactive molecular dynamics. The composition and chemical structure of soot precursor molecules formed during inception are elucidated. During soot inception, increasing the process temperature leads to faster depletion of C₂H₂ molecules and faster formation of C₂H₃, C₂H₄, C₂H₆, CH₄, and C₂ with the concurrent appearance of H₂ molecules. Small molecules consisting of 1-5 C atoms (C₁-C₅) are formed due to reactive collisions and grow further to larger hydrocarbon compounds consisting of 6-10 C atoms. At initial stages of inception, prior to the formation of incipient soot, three-member rings are formed, which are associated with the formation of compounds with fewer than 10 C atoms. Once incipient soot is formed, the number of C₁-C₁₀ compounds and the number of three-member rings drop, while the number of five- and six-member rings increases, indicating that the formation of larger rings is associated with the growth of soot clusters. The chemical structure of soot precursor molecules obtained by bond order analysis reveals that molecules with up to 10 C atoms are either linear or branched aliphatic compounds or may contain three-member rings fused with aliphatic components. Molecules with more than 10 C atoms often exhibit structures composed of five- or six-member C rings, decorated by aliphatic components. The identification of molecular precursors contributing to soot inception provides crucial insights into soot formation mechanisms, pinpointing potential pathways of soot formation during combustion.