Figure includes the final products collected from different precursors in the nanoreactors, which are mapped according to the precursor properties. These products are postprocessed using the TEMSIM package to obtain simulated TEM images, which can be directly compared to the structures seen in HRTEM experiments . For PAH-a and PAH-b, the PAHs are arranged in a gaseous form. Most radicals form cross-linked dimers or trimmers, and no large cluster is observed. For PAH-c and PAH-d, a prominent cluster is produced in the box center. The structure of PAH-c is disordered and loosely packed, similar to the covalently bound incipient particle predicted by Johansson et al. . For PAH-d, a coreshell-like structure is observed, where the core region shows disordered and cross-linked structures, and the shell region (upper right part) is ordered and closepacked stacks. For large radicals, such as PAH-e, -f, and -g, the final products are spherical stacked structures with cross-links between different layers. These results suggest that the morphology of the final products is highly related to the size of the precursors. At flame temperatures, no PAH cluster can form when the molecular mass of precursors is lower than 200 Da. This critical mass is lower than the value of 666 Da for physical nucleation of PAH molecules simulated by Mao et al. . This can be attributed to the higher reactivity of PAH radicals, which can form cross-links and lead to chemical nucleation. For PAH molecules without reactive sites, a higher temperature (2500 K) is required for chemical nucleation, which is too high for typical sooting flames. As the size of the precursors increases, the product particle changes from a disordered cluster into an ordered and close-packed structure. It is interesting to note that the product of PAH-d has a core-shell structure, which includes a disordered core and outer shell of stacked PAHs . Such structures are also reported in HRTEM experiments . Another piece of evidence is from a recent mass spectrum experiment , which shows that PAHs of 239-838 Da are the main component of soot particles, suggesting that physically stabilized soot inception of PAH radicals larger than 200 Da is a potential inception mechanism.