The contributions of di erent reaction pathways to the formation of polycyclic aromatic hydrocarbons (PAHs), the actual soot precursors, were investigated for the shock tube pyrolysis of acetylene, benzene and an acetyleneÈbenzene (1 : 1) mixture. This study provides information on the principal reaction steps leading to the fast formation of high molecular weight PAHs before soot inception. It is found that reaction pathways to high molecular weight PAHs and soot precursors including only successive growth steps of aliphatic hydrocarbons are in disagreement with the measured induction time of soot formation taken from the literature. The limiting, most time-consuming step in this reaction sequence is shown to be the formation of " " outer rings ÏÏ, that is, PAHs consisting of four-carbon bay structures. In the model developed in the present study, these difficulties are overcome by introducing combinative reaction steps of aryls.
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