Atmospheric oxidation of isoprene yields large quantities of isoprene epoxydiols (IEPOX) in regions with low NOx concentrations. Multiphase reactions of IEPOX on sulfate aerosols can contribute substantially to the organic mass fraction of fine aerosol. In aqueous fine aerosols, acid-catalyzed ring-opening reactions of IEPOX followed by nucleophilic addition of inorganic sulfate or water results in organosulfates and 2-methyltetrols, respectively. Consistent with this mechanism, the observation of isoprene-derived organosulfates in field and laboratory studies correlates well with aerosol acidity and sulfate. However, some observations suggest a mechanism of IEPOX-derived organosulfate formation in less-acidic condensed phases such as cloud droplets. Furthermore, organosulfates have been identified in cloud water. Here we investigate the formation of IEPOX-derived organosulfates in aqueous solutions using several approaches. The Gas-Aerosol Model for Mechanism Analysis (GAMMA) was used to estimate the potential for non-radical reactions of IEPOX in cloud and fog water. Aqueous dark reactions of trans-β-IEPOX in the presence of acidic sulfate (H2SO4 + (NH4)2SO4) were conducted at different concentrations and pH values and products were identified and quantified using hydrophilic interaction liquid chromatography (HILIC) coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer (ESI-HR-QTOFMS) operated in negative ion mode. Elution times and mass spectra were compared to authentic standards synthesized in-house. The rate constant for acid-catalyzed epoxide ring opening at near cloud relevant concentrations was comparable to previous studies at higher concentrations, indicating that dark reactions forming organosulfates are slow in cloud water. Findings show that formation of isomeric tertiary, as opposed to secondary, methyltetrol sulfate (C5H12O7S) may be promoted by increasing solution acidity. The isomeric secondary organosulfate formed under less acidic conditions may be more stable. This study may help provide insight on the sources of IEPOX-derived organosulfates found in the atmosphere.

Organosulfates from dark reactions of isoprene epoxydiols in cloud and fog water

2019 Fall National Meeting

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