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Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived SOA
JIN YAN, Yue Zhang, Yuzhi Chen, N. Cazimir Armstrong, Zhenfa Zhang, Avram Gold, Andrew Lambe, Barbara Turpin, Andrew Ault, Jason Surratt, University of North Carolina at Chapel Hill
Abstract Number: 276
Working Group: Aerosol Chemistry
Abstract
In isoprene-rich regions, acid-catalyzed multiphase reactions of isoprene epoxydiols (IEPOX) with inorganic sulfate (Sulfinorg) particles form secondary organic aerosol (IEPOX-SOA), extensively converting Sulfinorg to lower-volatility particulate organosulfates (OSs), including 2-methyltetrol sulfates (2-MTSs) and their dimers. Recently, we showed that heterogeneous hydroxyl radical (OH) oxidation of particulate 2-MTSs generated multifunctional OS products. However, atmospheric models assume that OS-rich IEPOX-SOA particles remain unreactive towards heterogeneous OH oxidation, and limited laboratory studies have been conducted to examine the heterogeneous OH oxidation kinetics of full IEPOX-SOA mixtures. Hence, this study investigated the kinetics and products resulting from heterogeneous OH oxidation of freshly-generated IEPOX-SOA in order to help derive model-ready parameterizations.
First, gas-phase IEPOX was reacted with acidic Sulfinorg particles under dark conditions in order to form fresh IEPOX-SOA particles. These particles were then subsequently aged at RH of 56% in an oxidation flow reactor at OH exposures ranging from 0~15 days of equivalent atmospheric exposure. Aged IEPOX-SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) and collected onto Teflon filters for off-line molecular-level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Our results show that heterogeneous OH oxidation only caused a 7% decay of IEPOX-SOA by 10 days exposure, likely owing to the inhibition of reactive uptake of OH as fresh IEPOX-SOA particles have an inorganic core-organic shell morphology. A significantly higher fraction of IEPOX-SOA (~37%) decayed by 15 days exposure, likely due to the increasing reactive uptake of OH as IEPOX-SOA become more liquid-like with aging. Freshly-generated IEPOX-SOA constituents exhibited varying degrees of aging with 2-MTS-dimers being the most reactive, followed by 2-MTSs and 2-methyltetrols (2-MTs), respectively. Notably, extensive amounts of previously characterized particle-phase products in ambient fine aerosols were detected in our laboratory-aged IEPOX-SOA samples.