Peroxy radical chemistry and OH radical production during the NO₃-initiated oxidation of isoprene

Abstract. Peroxy radical reactions (RO₂ + RO₂) from the NO₃-initiated oxidation of isoprene are studied with both gas chromatography and a chemical ionization mass spectrometry technique that allows for more specific speciation of products than in previous studies of this system. We find high nitrate yields (~ 80%), consistent with other studies. We further see evidence of significant hydroxyl radical (OH) formation in this system, which we propose comes from RO₂ + HO₂ reactions with a yield of ~38–58%. An additional OH source is the second generation oxidation of the nitrooxyhydroperoxide, which produces OH and a dinitrooxyepoxide with a yield of ~35%. The branching ratio of the radical propagating, carbonyl- and alcohol-forming, and organic peroxide-forming channels of the RO₂ + RO₂ reaction are found to be ~18–38%, ~59–77%, and ~3–4%, respectively. HO₂ formation in this system is lower than has been previously assumed. Addition of RO₂ to isoprene is suggested as a possible route to the formation of several isoprene C₁₀-organic peroxide compounds (ROOR). The nitrooxy, allylic, and C₅ peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.