Investigation of\nthe Cyclohexene Oxidation Mechanism\nThrough the Direct Measurement of Organic Peroxy Radicals

Monoterpenes, the second most abundant biogenic volatile\norganic\ncompounds globally, are crucial in forming secondary organic aerosols,\nmaking their oxidation mechanisms vital for addressing climate change\nand air pollution. This study utilized cyclohexene as a surrogate\nto explore first-generation products from its ozonolysis through laboratory\nexperiments and mechanistic modeling. We employed proton transfer\nreaction mass spectrometry with NH<sub>4</sub><sup>+</sup> ion sources\n(NH<sub>4</sub><sup>+</sup>-CIMS) and a custom-built OH calibration\nsource to quantify organic peroxy radicals (RO<sub>2</sub>) and closed-shell\nspecies. Under near-real atmospheric conditions in a Potential Aerosol\nMass-Oxidation Flow Reactor, we identified 30 ozonolysis products,\nexpanding previous data sets of low-oxygen compounds. Combined with\nsimulations based on the Generator for Explicit Chemistry and Kinetics\nof Organics in the Atmosphere and relevant literature, our results\nrevealed that OH dominates over ozone in cyclohexene oxidation at\ntypical atmospheric oxidant levels with H-abstraction contributing\n30% of initial RO<sub>2</sub> radicals. Highly oxidized molecules\nprimarily arise from RO<sub>2</sub> autoxidation initiated by ozone,\nand at least 15% of ozone oxidation products follow the overlooked\nnonvinyl hydroperoxides pathway. Gaps remain especially in understanding\nRO<sub>2</sub> cross-reactions, and the structural complexity of monoterpenes\nfurther complicates research. As emissions decrease and afforestation\nincreases, understanding these mechanisms becomes increasingly critical.