Dear Eda. Concerning your issue about the mechanism of formation of PAHs with regard to C2 and C4 that react with those stable polycyclic compounds. A general scheme of PAH formation and sequential growth of PAH by reactions with stable and radical species, including single-ring aromatics, other PAH and acetylene, followed by the nucleation or inception of small soot particles, soot growth by coagulation and mass addition from gas phase species, and carbonization of the particulate material. Experimental and theoretical tools which have allowed the achievement of deeper insight into the corresponding chemical processes are presented. I think the following below links may help you in your analysis: