The oxidation regime and SOA composition in limonene ozonolysis: roles of different double bonds, radicals, and water

Abstract. Volatile organic compounds play an important role in air quality and climate change, largely because they contribute to the formation of oxidizing compounds and secondary organic aerosol (SOA). In this study, a series of products, including peroxides and carbonyl compounds in both gaseous and particulate phases, were simultaneously detected to investigate the oxidation regime and SOA composition in limonene ozonolysis. The roles of different double bonds (DBs), radicals, and water were also examined. In our first investigation, we focused on representative oxidizing compounds produced in limonene ozonolysis, including stabilized Criegee intermediates (SCIs), OH radicals, and peroxides. The dependence of H2O2 and hydroxymethyl hydroperoxide (HMHP) formation on RH demonstrates that the reaction with water is an important reaction pathway for limonene SCIs, and the lower limit SCI yields of endocyclic and exocyclic DBs were estimated to be ∼0.24 and ∼0.43, respectively. The OH yield was determined by adding sufficient amounts of an OH scavenger, and the OH yields of endocyclic and exocyclic DBs were ∼0.65 and ∼0.24, respectively. These results indicate that in limonene ozonolysis the endocyclic DB is inclined to generate OH radicals through the hydroperoxide channel, while the exocyclic DB has a higher fraction of forming SCIs. Additionally, other gas-phase and particle-phase peroxides were also studied in this work. The formation of performic acid (PFA) and peracetic acid (PAA) was promoted significantly by increasing RH and the degree of oxidation, and the discrepancy between the experimental and model results suggested some missing formation pathways. Considerable generation of H2O2 from SOA in the aqueous phase was observed, especially at a high [O3] ∕ [limonene] ratio, which was mainly attributed to the hydration and decomposition of unstable peroxides in SOA such as peroxycarboxylic acids and peroxyhemiacetals. Different DBs and OH scavengers had a large impact on the particulate peroxides, and their stability indicated that the types of peroxides in SOA changed under different conditions. As for the contribution of peroxides to SOA, the results demonstrated that the mass fraction of particulate peroxides in limonene SOA was less than 0.2 at a low [O3] ∕ [limonene] ratio, while the mass fraction was 0.4–0.6 at a high [O3] ∕ [limonene] ratio. The partitioning behavior of peroxides showed that multi-generation oxidation helped produce more low-volatility peroxides, which partially explained the higher SOA yield. The partitioning behavior of carbonyls was also examined and the experimental partitioning coefficients (Kp) were found to be typically several orders of magnitude higher than the theoretical values. This study provided new insights into the oxidation regime and SOA composition in limonene ozonolysis, and limonene showed its specificity in many aspects when both endocyclic and exocyclic DBs were ozonated. We suggest that the atmospheric implications of terpenes containing more than one DB and the SOA composition, especially particulate peroxides, need further study.