Unimolecular Reactions Following Indoor and Outdoor Limonene Ozonolysis

Abstract

Limonene is one of the monoterpenes with the largest biogenic emissions and is also widely used as an additive in cleaning products, leading to significant indoor emissions. Studies have found that the formation of secondary organic aerosols (SOAs) from limonene oxidation has important implications for indoor air quality. Although ozonolysis is considered the major limonene oxidation pathway under most indoor conditions, little is known about the mechanisms for SOA formation from limonene ozonolysis. Here, we calculate the rate coefficients of the possible unimolecular reactions of the first-generation peroxy radicals formed by limonene ozonolysis using a high-level multiconformer transition state theory approach. We find that all of the peroxy radicals formed initially in the ozonolysis of limonene react unimolecularly with rates that are competitive both indoors and outdoors, except under highly polluted conditions. Differences in reactivity between the peroxy radicals from ozonolysis and those formed by OH, NO₃, and Cl oxidation are discussed. Finally, we sketch possible oxidation mechanisms for the different peroxy radicals under both indoor and pristine atmospheric conditions and in more polluted environments. In environments with low concentrations of HO₂ and NO, efficient autoxidation will lead to the formation of highly oxygenated organic compounds and thus likely aid in the growth of SOA.