Organosulfates from Dark Aqueous Reactions of Isoprene-Derived Epoxydiols Under Cloud and Fog Conditions: Kinetics, Mechanism, and Effect of Reaction Environment on Regioselectivity of Sulfate Addition

Atmospheric oxidation of isoprene yields large quantities of highly water-soluble isoprene epoxydiols (IEPDX) that partition into fogs, clouds, and wet aerosols. In aqueous aerosols, the acid-catalyzed ring-opening of IEPDX 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 IEPDX 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 IEPDX (5-10 mM) under a range of sulfate concentrations (0.3-50 mM) and pH values (1.83-3.38) in order to better understand the fate of IEPDX in clouds and fogs. From these aqueous dark reactions of /IIEPDX isomers (trans- and cis-2-methyl-2,3-epoxybutane-1,4-diols), which are the predominant IEPDX 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). We found that the regiochemistry and stereochemistry were affected by pH, and the tertiary methyltetrol sulfate (C5H12O7S) was promoted by increasing solution acidity. Furthermore, the rate constants for the reaction of IEPDX under cloud-relevant conditions are up to 1 order of magnitude lower than reported in the literature for aerosol-relevant conditions due to a markedly different solution activities. Nevertheless, the contribution of cloud and fog water reactions to IEPDX SOA may be significant in cases of lower aqueous-phase pH (model estimate) or during droplet evaporation (not studied).

Automated summary

This study investigated the rates, product branching ratios, and stereochemistry of organosulfates from reactions of dilute IEPDX under a range of sulfate concentrations and pH values. It was found that the regiochemistry and stereochemistry of products were affected by pH, with tertiary methyltetrol sulfate promoted by increasing solution acidity. The reaction rate constants under cloud-relevant conditions were significantly lower than those reported for aerosol-relevant conditions due to different solution activities.