An Overlooked Kinetic Relaxation in the Formation of Sesquiterpene-Derived Criegee Intermediates

Abstract. Criegee intermediates (CIs) from sesquiterpene ozonolysis contribute to secondary organic aerosol (SOA) formation. The ozonolysis rate constants of typical sesquiterpenes have been measured by prior experiments, but whether these values can represent the formation rate constants of sesquiterpene-derived CIs remains questionable. This study reports the overlooked kinetic relaxation in the CI formation from representative sesquiterpenes, including α-cedrene, α-copaene, β-caryophyllene, α-farnesene and β-farnesene. We found that the apparent formation rate constant of sesquiterpene-derived CIs is initially unstable but gradually approaches a plateau, which equals the rate constant of sesquiterpene ozonolysis. Such behavior arises because CI formation cannot instantaneously respond to sesquiterpene ozonolysis due to the finite time for the production of primary ozonides (POZs). Our results also reveal that the kinetic relaxation is sensitive to temperature variations, as cold surges extend the relaxation timescale from minutes to hours (or hours to days) for most sesquiterpenes while heatwaves reversely diminish the relaxation timescale. Exceptionally, β-caryophyllene exhibits the longest relaxation timescale (at least 2.6 days) even during the heatwaves. Our findings demonstrate that neglecting this kinetic relaxation may substantially overestimate the rate of sesquiterpene-derived CI formation and thus the CI-driven atmospheric oxidation capacity under extreme temperature conditions.