Photoelectron resonance capture ionization‐aerosol mass spectrometry of the ozonolysis products of oleic acid particles: Direct measure of higher molecular weight oxygenates

The heterogeneous reaction of particle‐phase 9‐octadecenoic acid (oleic acid) and gas‐phase ozone in a flow reactor was studied by photoelectron resonance capture ionization (PERCI) mass spectrometry. This soft ionization technique facilitated one of the first simultaneous, direct observations of all four of the major products predicted for this reaction: nonanal, nonanoic acid, 9‐oxononanoic acid, and azelaic acid. In addition, a series of higher molecular weight oxygenated compounds were observed directly for the first time. The proposed structures are all cyclic oxygenates and contain the oxygen‐oxygen moiety, including secondary ozonides and cyclic geminal diperoxides. Mechanisms for the formation of these products are proposed. The mechanisms are generally 1,3‐dipolar cycloadditions that lead to five‐ and six‐member oxygen‐containing rings. The mechanisms are shown to involve short‐lived Criegee intermediates reacting with aldehydes and other Criegee intermediates. Atmospheric implications of these higher molecular weight compounds are suggested and include enhancing the fatty acid medium's capacity to act as a source of radicals due to the prominence of the peroxide moiety. The low volatility coupled with the high polarity of these compounds may alter particle phase hygroscopicity that can enhance the cloud condensation nuclei properties of these particles.