Peroxy radical chemistry and OH radical production during the NO_3-initiated oxidation of isoprene

Peroxy radical reactions (RO_2 + RO_2) 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 RO_2 + HO_2 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 RO_2 + RO_2 reaction are found to be ~18–38%, ~59–77%, and ~3–4%, respectively. HO_2 formation in this system is lower than has been previously assumed. Addition of RO_2 to isoprene is suggested as a possible route to the formation of several isoprene C_(10)-organic peroxide compounds (ROOR). The nitrooxy, allylic, and C_5 peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.