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Effect of Solution Activity on Regioselectivity of Sulfate Addition in Acid-Catalysed Aqueous Reactions of IEPOX
SARAH SUDA PETTERS, Tianqu Cui, Zhenfa Zhang, Avram Gold, V. Faye McNeill, Jason Surratt, Barbara Turpin, University of North Carolina at Chapel Hill
Abstract Number: 662
Working Group: Aerosol Chemistry
Abstract
Atmospheric oxidation of isoprene yields large quantities of highly water-soluble isoprene epoxydiols (IEPOX) that partition into fogs, clouds, and wet aerosols. In aqueous aerosols, acid-catalyzed ring-opening of IEPOX followed by nucleophilic addition of inorganic sulfate or water forms organosulfates and 2-methyltetrols, respectively, contributing substantially to secondary organic aerosol (SOA). However, the fate of IEPOX in clouds, fogs and evaporating hydrometeors is not well understood. Here we investigate the rates, product branching ratios, and stereochemistry of organosulfates from reactions of dilute IEPOX (5 to 10 mM) under a range of sulfate concentrations (0.3 to 50 mM) and pH values (1.83-3.38) in order to better understand the fate of IEPOX in clouds and fogs. From these aqueous dark reactions of β-IEPOX isomers (trans- and cis-2-methyl-2,3-epoxybutane-1,4-diols), which are the predominant IEPOX isomers, products were identified and quantified using hydrophilic interaction liquid chromatography coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer operated in negative ion mode (HILIC/(–)ESI-HR-QTOFMS). The rate constants for the reaction of IEPOX under cloud-relevant conditions are up to one order of magnitude lower than reported in the literature for aerosol-relevant conditions due to markedly different solution activity. We found that regiochemistry and stereochemistry were affected by pH and the tertiary methyltetrol sulfate (C5H12O7S; MTS) was promoted by increasing solution acidity and provide insight into a plausible mechanism for oxirane ring opening of trans-β-IEPOX that could prove helpful in understanding the processing of IEPOX to isomeric MTSs over the broad pH ranges observed in aerosols, fogs, and cloud droplets. The contribution of cloud and fog water reactions to IEPOX SOA may be significant in cases of lower aqueous-phase pH or during droplet evaporation. Process-level insight into the mechanistic controls on observed stereoisomers will likely be helpful in constraining the contribution of cloud and fog conditions to the breakdown of IEPOX in the atmosphere.