New Theoretical Insights into the Atmospheric Chemistry of Methyl Chavicol Initiated by OH and NO3 Radicals

Methyl chavicol, as a volatile oxygenated aromatic biogenic volatile organic compound (BVOC), is emitted in large quantities from a variety of plants. Currently, the high second organic aerosol (SOA) yield (40%) from photochemical oxidation of methyl chavicol and its unclear formation mechanism have attracted enormous research interest. In this study, we conducted an in-depth theoretical investigation on the atmospheric oxidation processes of methyl chavicol initiated by OH and NO3 radicals using quantum chemistry methods. The formation mechanisms for its highly oxidized multifunctional products, such as 2-hydroxy-3-(4-methoxyphenyl)propanal, 2-hydroxy-3-(3-hydroxy-4-methoxyphenyl)propanal, 1-hydroxy-3-(4-methoxyphenyl)propan-2-one, and 1-hydroxy-3-(3-hydroxy-4-methoxyphenyl)propan-2-one, have been clarified for the first time, which were detected in the SOA components in the chamber experiment of photo-oxidation of methyl chavicol. Hence, gas-phase atmospheric oxidation of methyl chavicol is a probable major contributor to SOA formation arising from biogenic emissions of methyl chavicol. The calculated rate coefficient for the gas-phase reaction of methyl chavicol with OH radicals is 3.06 x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 1 atm, and the total branching ratios of the formation of IM2, IM3, IM8, IM9, IM10, and IM11 account for the largest proportion (95.18%) of overall branching ratios. The lifetime of methyl chavicol with respect to the OH radical is determined to be 4.54 h, indicating a potential effect of methyl chavicol photo-oxidation on atmospheric photochemistry and SOA formation relatively far from where it is emitted.