Monitoring Early-Generation Formation and Gas-to-Particle Phase Partitioning of Isoprene Oxidation Products by Ion Mobility Spectrometry Coupled to High-Resolution Chemical Ionization Mass Spectrometry

REBECCA L. RICE, Molly Frauenheim, Matthieu Riva, Peter Mettke, Hartmut Herrmann, Siddharth Iyer, Sebastian Gerber, Stephan Graf, Felipe Lopez-Hilfiker, Michael Kamrath, Jason Surratt, Zhenfa Zhang, Avram Gold, University of North Carolina at Chapel Hill

     Abstract Number: 156
     Working Group: Aerosol Chemistry

Abstract
Isoprene is the largest non-methane volatile organic species emitted into Earth's atmosphere. Isoprene oxidation by hydroxyl radicals yields low- and semivolatile oxygenated molecules that form secondary organic aerosol (SOA), which contributes to fine particulate matter. The inability of conventional mass spectrometric techniques to distinguish between isomeric species in the gas phase has posed a formidable challenge to understanding the gas phase processes in isoprene oxidation leading to SOA. In particular, the evolution of early generation isoprene oxidation products and the formation, uptake, partitioning and fate of semivolatile products remains poorly described. The recent availability of Chemical Ionization-Structures for Lossless Ion Manipulation-Ion Mobility Spectrometer-High Resolution Mass Spectrometer (CI-SLIM-IMS-HR-MS) enables separation of gas phase isomer mixtures has shown promise to advance understanding of gas phase processes and greatly improve predictive SOA modeling. We report here initial experiments coupling CI-SLIM-IMS-HR-MS to a smog chamber to monitor OH-mediated oxidation of isoprene in a low-NOx regime and to investigate partitioning and fate of semivolatile uptake products. We present real-time traces of ISOPOOH and IEPOX isomers. We have previously shown that the C5H8O3 products originally assigned as δ-HPALDs are cyclic peroxyhemiacetals and for the first time, we directly compare the relative rates of formation of these early generation gas phase products. We have shown that in-particle rearrangement of IEPOX on uptake yields the semivolatile compounds 3-methyltetrahydrofuran-2,4-diol and 3-methylenebutrane-1,2,4-triol (formerly C5-alkene triols). We monitor IEPOX uptake by (NH4)2SO42─ at pH 1.4 and 3.5 by CI-SLIM-IMS-HR-MS in the gas phase and HILIC/(-)-ESI-HR-ToF-MS in the particle phase and estimate partitioning. Furthermore, we show that the proportion of rearrangement products is highly sensitive to particle pH as well as relative humidity. The uptake experiments provide insights into the disparate results reported for relative yields of “C5-alkene triols" and 2-methyltetrols in field studies.