Indene and naphthalene Angiotensin 1/2 (1-9) two important bicyclic PAHs, and their formation represents the starting point of PAH formation. Their model predictions are presented in the middle row of Fig. 3. According to the pathway analysis in Fig. 5, the reaction between benzyl radical/acetylene (C6H5CH2 + C2H2 = C9H8 + H, R5) dominates the formation of indene. The rate constant of R5 adopted in the present model was estimated by Blanquart et al. , which agreed well with the calculation of Vereecken and Peeters . With the increasing of the doping ratio of 2-butanol, planktonic organisms enhances the contribution of R5 (up to 40% in Flame 1) due to the increment of benzyl radical and acetylene in flames. Meanwhile, the reaction between benzene/propargyl radical (C6H6 + C3H3 = C9H8 + H, R6) proposed by Li et al. , phenyl radical/propyne (C6H5 + pC3H4 = C9H8 + H, R7) and phenyl radical/allene (C6H5 + aC3H4 = C9H8 + H, R8) calculated by Kaiser et al.  play an additional role with an overall contribution around 30% in Flame 1.