OH Radical Oxidation\nof Organosulfates in the Atmospheric\nAqueous Phase

Organosulfates (OS, ROSO<sub>3</sub><sup>–</sup>), ubiquitous\nconstituents of atmospheric particulate matter (PM), influence both\nthe physicochemical and climatic properties of PM. Although the formation\npathways of OS have been extensively researched, only a few studies\nhave investigated the atmospheric fate of this class of compounds.\nHere, to better understand the reactivity and transformation of OS\nunder cloudwater- and aerosol-relevant conditions, we investigate\nthe hydroxyl radical (OH) oxidation bimolecular rate constants (<i>k</i><sub>OS+OH</sub><sup>II</sup>) and products of five atmospherically\nrelevant OS as a function of pH and ionic strength: methyl sulfate\n(MeS), ethyl sulfate (EtS), propyl sulfate (PrS), hydroxyacetone sulfate\n(HaS) and phenyl sulfate (PhS). Our results show that OS are oxidized\nby OH with <i>k</i><sub>OS+OH</sub><sup>II</sup> between\n10<sup>8</sup> – 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>, which corresponds to atmospheric lifetimes of minutes\nin aqueous aerosol to days in cloudwater. We find that <i>k</i><sub>OS+OH</sub><sup>II</sup> increases with carbon chain length\n(MeS < EtS < PrS) and aromaticity (PrS < PhS), but does not\ndepend on solution pH (2, 9). In addition, we find that whereas the\nOH reactivity of the aliphatic OS studied here decreases by ∼2×\nwith increasing ionic strength (0–15 M), the reactivity of\nPhS decreases by ∼10×. The oxidation of EtS and PrS produced\norganic peroxides (ROOH) as first-generation oxidation products, which\nsubsequently photolyzed; the oxidation of PhS resulted in hydroxylated\naromatic products. These results highlight the need for inclusion\nof OS loss pathways in atmospheric models, and suggest caution in\nusing ambient OS concentration measurements alone to estimate their\nproduction rates.