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Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol (IEPOX)-Derived Secondary Organic Aerosol: Identification of Highly Oxygenated Products by HILIC/ESI-HR-QTOFMS
N. CAZIMIR ARMSTRONG, Yuzhi Chen, Tianqu Cui, Yue Zhang, Jin Yan, Zhenfa Zhang, Barbara Turpin, Man Nin Chan, Andrew Ault, Avram Gold, Jason Surratt, UNC Chapel Hill
Abstract Number: 143
Working Group: Aerosol Chemistry
Abstract
Acid-driven multiphase chemistry of isoprene-derived epoxydiols (IEPOX) substantially contributes to secondary organic aerosol (SOA) formation. However, atmospheric chemical sinks of freshly-generated IEPOX-derived SOA remain unclear, and thus, are not considered in atmospheric chemistry models. We systematically examined the heterogeneous oxidation of authentic IEPOX-derived SOA particles by gas-phase hydroxyl radicals (OH) and its role as one potential atmospheric sink for IEPOX-derived SOA. Chemical changes in smog chamber-generated IEPOX-derived SOA were assessed after 1 and 4 hours of gas-phase OH exposure (107-108 molecules cm-3) using hydrophilic interaction liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), an optimal protocol for characterization of highly-oxygenated and sulfur-containing organic compounds. HILIC/ESI-HR-QTOFMS has been previously demonstrated by our group to chromatographically resolve IEPOX-derived SOA constituents. Tentative structures and formation mechanisms are proposed here for previously measured IEPOX-derived ambient fine particulate matter (PM2.5) constituents whose chemical sources were until now uncertain; this work focuses on atmospherically-relevant SOA constituents formed from the heterogeneous OH oxidation of the most abundant constituents of freshly-generated IEPOX-derived SOA, including 2-methyltetrols, 2-methyltetrol sulfates, and 2-methyltetrol sulfate dimers. Products with structures and formation mechanisms proposed here include ions at mass-to-charge ratios (m/z) 133 (C5H9O4-), 139 (C2H3O5S-), 149 (C5H9O5-), 195 (C5H7O6S-), 211 (C5H7O7S-), 213 (C5H9O7S-), 229 (C5H9O8S-), 243 (C6H11O8S-), and 273 (C7H13O9S-). We propose that subsequent to H abstraction by OH these compounds are formed predominantly via Russell reactions, Bennett-Summers reactions, and β-scission of alkoxy radicals. Understanding the formation mechanisms and structures of these atmospherically relevant SOA constituents will make it possible to quantify them, investigate reaction kinetics, and study their climate-specific properties such as ice nucleation efficiency.