AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Synergistic O3 + OH Oxidation Pathway to High Molecular Weight, Extremely Low-Volatility Organic Compounds Revealed in Beta-Pinene Secondary Organic Aerosol
CHRISTOPHER KENSETH, Yuanlong Huang, Ran Zhao, Nathan Dalleska, Caleb Hethcox, Brian Stoltz, John Seinfeld, California Institute of Technology
Abstract Number: 153 Working Group: Aerosol Chemistry
Abstract High molecular weight, extremely low-volatility dimeric compounds have been identified as significant components of both ambient and laboratory-derived monoterpene secondary organic aerosol (SOA), and have been implicated as key players in new particle formation and growth. Recent studies have shown that these compounds are formed only through O3- and not OH-initiated oxidation, despite the apparent monomeric building blocks being present in the OH system. This result, together with prompt formation of the dimers observed at the onset of O3 oxidation experiments, strongly suggests that traditionally cited particle-phase formation pathways are not significant in dimer production, pointing instead toward formation through gas-phase reactions involving early-stage oxidation products and/or reactive intermediates, e.g., stabilized Criegee intermediates (SCI) or organic peroxy radicals (RO2). However, unambiguous elucidation of the mechanisms underlying dimer formation and their relative importance remains sparse. In this study, we present a comprehensive dataset on the identity, abundance, and kinetics of molecular products in SOA formed from the O3- and OH-initiated oxidation of beta-pinene, the second most abundant monoterpene in the atmosphere (global emissions estimated at 19 Tg yr-1). Through a combination of ultra performance liquid chromatography/electrospray ionization mass spectrometry (UPLC/ESI-MS), hydrogen/deuterium exchange (HDX) UPLC/ESI-MS, 13C isotopic labeling, and OH/SCI scavenging, we identify a new reactive pathway to extremely low-volatility dimeric compounds in SOA formed from O3 oxidation involving gas- and particle-phase reaction of O3-derived products/intermediates with those generated from oxidation by OH produced in situ via vinyl hydroperoxide (VHP) decomposition. This novel feature of monoterpene SOA formation represents a potentially significant source of high molecular weight, extremely low-volatility organic compounds to the atmosphere, and also highlights the importance of understanding and accounting for the likely role of oxidative synergism in ambient aerosol formation, where SOA precursors are susceptible to concurrent oxidation by O3 and OH.