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Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Methyltetrol Sulfates: Plausible Formation Mechanisms of Previously Unexplained Organosulfates in Ambient Fine Aerosols
YUZHI CHEN, Yue Zhang, Andrew Lambe, Rongshuang Xu, Ziying Lei, Nicole Olson, Zhenfa Zhang, Tessa Szalkowski, Tianqu Cui, William Vizuete, Avram Gold, Barbara Turpin, Andrew Ault, Man Nin Chan, Jason Surratt, UNC-Chapel Hill
Abstract Number: 109
Working Group: Aerosol Chemistry
Abstract
Methyltetrol sulfates are unique tracers for secondary organic aerosol (SOA) formed from acid-driven multiphase chemistry of isoprene epoxydiols. Ambient measurements demonstrate that methyltetrol sulfates are the single most abundant SOA tracer, up to 13% of organic carbon (OC) mass and up to 80 % of organosulfate (OS) mass, in PM2.5 collected from the Southeast US and Amazon rainforest. Thanks to recent developments in liquid chromatography rendering excellent separations of OSs, 2-methyltetrol sulfate diastereomers (2-MTSs) have been identified as the dominant isomers of methyltetrol sulfates, but their atmospheric sinks remain unknown. We investigated the oxidative aging of authentic 2-MTS aerosols by gas-phase hydroxyl radical (•OH) at a relative humidity of 61 ± 1 % using an oxidation flow reactor. The effective rate constant for this heterogeneous reaction was determined as 4.9 ± 0.6 × 10−13 cm3 molecule−1 s−1, corresponding to an atmospheric lifetime of 16 ± 2 days (assuming •OH concentration of 1.5 × 106 molecule cm−3). Chemical changes to 2-MTSs were monitored by hydrophilic interaction liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Previously unknown OSs detected in atmospheric PM2.5 at mass-to-charge ratios (m/z) 139 (C2H3O5S−), 155 (C2H3O6S−), 169 (C3H5O6S−), 171 (C3H7O6S−), 185 (C3H5O6S−), 199 (C4H7O7S−), 211 (C5H7O7S−), 213 (C5H9O7S−), 227 (C5H7O8S−), 229 (C5H9O8S−) and 231 (C5H11O8S−) were observed as oxidation products of 2-MTSs and plausible reaction mechanisms were proposed. This work highlights that heterogeneous •OH oxidation could be a competitive sink for particulate 2-MTSs and a source of more oxygenated/functionalized OSs detected in atmospheric PM2.5, likely modifying air quality- and climate-relevant aerosol physicochemical properties of SOA containing 2-MTSs. Heterogeneous oxidation chemistry is currently lacking and should be considered in large-scale models to better understand the transformation and abundance of atmospheric particulate OSs.