Heterogeneous Ozonolysis of Endocyclic Unsaturated Organic Aerosol Proxies: Implications for Criegee Intermediate Dynamics and Later-Generation Reactions

Unsaturated low-volatile organic compounds have been observed in atmospheric aerosol particles (e.g., aerosols from plants, cooking, and biomass burning) and human skin. Understanding their chemical fates through reactions with ozone at the relevant interfaces is crucial to elucidating the multiphase chemical mechanisms and predicting their environmental and health impacts. In this work, heterogeneous ozonolysis of two endocyclic unsaturated organic aerosol proxies (4-cyclohexene-1,2-diacid and shikimic acid) were studied in a flow tube reactor under different O₃ exposure levels and relative humidities (30%, 60%, and 90%). Our results show strong RH dependence for the oxidation kinetics due to aerosol phase state change and secondary chemistry (e.g., parent acids reacting with the produced Criegee Intermediates). The observed multifunctional products are explained by the various Criegee Intermediates mechanisms and later-generation reactions. Three processes that lead to the evolution of first-generation carbonyl products were revealed in light of independent measurements: (1) intramolecular aldol reactions; (2) peroxide oxidation of aldehydes (i.e., Baeyer–Villiger reactions); and (3) decarboxylation induced by hydrogen peroxide. Furthermore, organic peroxides were observed to form from accretion reactions between the Criegee Intermediates and the parent compounds as well as the later-generation multifunctional products. The proposed mechanisms can be a key extension to the current understanding of heterogeneous ozonolysis of unsaturated organic aerosol.