Atmospheric oxidation mechanism of camphene initiated by hydroxyl radicals.

The mechanism of the atmospheric oxidation of camphene initiated by the OH radical has been studied using quantum chemistry and reaction kinetic calculations. The calculations show that the initial additions are mediated by pre-reactive complexes. The adduct R8 formed by addition to the terminal CH2 combines with O2 in great proportions, while the adduct R3 formed via addition to the other double bond undergoes rapid ring scission. Radicals R5 and R6 resulting from H-abstraction at the C5- and C6-site stand out among the H-abstractions with relatively higher branching ratios. We suggest that the initial additions and H-abstractions form the radicals R8, R3, R5, and R6 in fractions of ∼0.86, ∼0.06, ∼0.03, and ∼0.03, respectively, at 298 K. For reactants with multiple conformers, the multi-conformer transition state (MCTST) method is used for the determination of the effective unimolecular rate coefficients. The fates of the R8, R3, R5, and R6 radicals are investigated. The gas-phase oxidation products under high/low NO conditions, mainly the highly oxygenated molecules, which further contribute to the formation of secondary organic aerosol (SOA), are determined. The formation of acetone is found as well, with an estimated branching ratio of ∼0.27 due to the high ratio of the R8 radical in the initial steps, in close agreement with the experimentally measured value of ∼0.39.