Aqueous-Phase Autoxidation of Organic Compounds: Study on Fate and Detection of Organic Peroxides

TANIA GAUTAM, Lisa Ng, Ran Zhao, University of Alberta

     Abstract Number: 89
     Working Group: Aerosol Chemistry

Abstract
Autoxidation is known to cause food rancidity, cellular degradation and deterioration of petroleum fuels. Gas-phase autoxidation mechanisms occur via successive H-abstractions by peroxy radicals and subsequent O₂ addition. Organic peroxides are a major fraction of secondary organic aerosols (SOA) and serve as reservoirs of HO_x and RO_x radicals. They are also key reaction intermediates arising from autooxidation. Currently, there is a lack of evidence on autoxidation in atmospheric aqueous phases (e.g., cloudwater). Organic peroxides can help resolve the intricacies behind aqueous-phase autoxidation mechanisms. Yet, the detection of peroxides remains elusive due to their labile nature and a lack of selective analytical technique. Our goal is to study novel aqueous-phase autooxidation mechanisms using advanced analytical techniques and probe the stability and chemistry of intermediate organic peroxides. In particular, aqueous-phase autoxidation mechanism was investigated for the first time by evidencing the formation of products containing multiple hydroperoxide groups. Photooxidation experiments were performed on laboratory simulated cloudwater constituting model compounds: 7-octenoic acid (7-OE), 3-octenoic acid (3-OE), limononic acid (LA) and benzoic acid (BNA). The C=C bond in olefins provides molecular specificity to OH initiated mechanisms, assisting our elucidation of mechanisms. To monitor peroxides, offline and online analyses were performed with iodometry assisted (-)ESI/HPLC-MS and (+)APCI-MS respectively. Successful identification of peroxides and their corresponding alcohol products was achieved via redox chemistry of iodometry. Time-resolved evolution of second generation peroxide species was achieved via direct-injection MS. In particular, (+)APCI-MS² on ammonium-peroxide adducts was utilized. Systematically designed experiments revealed susceptibility of peroxides towards elevated temperatures (25, 50 and 80℃), photolytic (254 nm) and time-dependent (0-24 hr) measurements. Our results provide an intuitive approach for the first ever verification of aqueous phase autoxidation mechanism. The fundamental knowledge gleaned from our study can assist in understanding rapid transformations of organic peroxides in the aqueous phase.