Mechanisms and kinetics studies of the atmospheric oxidation of eugenol by hydroxyl radicals and ozone molecules

Abstract Eugenol is a representative methoxyphenol derived from the pyrolysis of lignin containing a branched alkene group. Its concentration in the atmosphere is equivalent to guaiacol and syringol. In this present paper, the gas phase reaction mechanisms and kinetic parameters of eugenol with hydroxyl radicals (OH) and ozone molecules (O3) were calculated at the M06-2×/6-311+G(3df,2p)//M06-2×/6-311+G(d,p) level. There are two distinct reaction types between eugenol and OH. In particular, Path2 is most favorable in the OH additions, whereas IM16 is most advantageous in H atom abstraction pathways. OH additions have more advantages than H abstraction reactions. Thus, the comprehensive and detailed reaction schemes for the further reactions of IM2 were presented. The main products generated by IM2 are methyl (Z)-3-(2-formylpenta-1,4-dien-1-yl)-2-hydroxyoxirane-2-carboxylate (P2B-4), 2-methoxy-2-oxoacetic acid (P2B-10), 2-allylmalealdehyde (P2B-11) and other carbonyl or carboxyl compounds. As for the reaction of eugenol with O3, the cycloaddition reactions and subsequent oxidative degradation processes were also explored, which yielded the most dominant product 2-(4-hydroxy-3-methoxyphenyl) acetaldehyde (P8-1). The reaction constants of the primary reactions for eugenol with OH and O3 under the temperature range of 225– 375 K were successively calculated by POLYRATE and MESMER program. At 298 K and 1 atm, the respective rate coefficients are 5.91 × 10−11 and 5.48 × 10−16 cm3 molecule−1 s−1 and the corresponding atmospheric lifetimes are 4.70 h and 0.72 h. The short lifetimes suggest that once eugenol enters the atmosphere, it is likely to be rapidly degraded. This work aims to provide theoretical guidance for the photochemical reaction mechanisms of eugenol with OH and O3, and present a reference for more experimental researches.