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Unexpected pH-Dependent Stereochemistry in Organosulfate Formation in Aerosol
MADELINE COOKE, Yuzhi Chen, Yue Zhang, Ziying Lei, Isabel Ledskey, Jamy Lee, Nicolas Aliaga Buchenau, Andrew Lambe, Jason Surratt, Andrew Ault, University of Michigan
Abstract Number: 140
Working Group: Aerosol Chemistry
Abstract
Understanding the chemical reaction mechanisms that drive the formation of atmospheric fine aerosols is critical to predicting air quality and climate, particularly for low-volatility organic species formed from the oxidation of abundant volatile organic compounds (e.g. isoprene) known as secondary organic aerosol (SOA). Organosulfate formation from multiphase chemistry of isoprene-derived epoxides is a primary formation pathway for SOA, but our ability to predict product yields is limited due to a lack of mechanistic understanding. Herein, we find that the epoxide-ring opening mechanisms in atmospheric aerosol can occur via two pathways: SN1 or SN2. These mechanistic pathways are pH-dependent, with the SN2 mechanism increasing at higher pH and the SN1 mechanism increasing at lower pH. Both mechanisms of organosulfate formation are possible under typical ambient aerosol acidity ranges of pH 0 to 5. Our results indicate that organosulfate formation is maximized under the most acidic conditions where sulfate is still present in high concentrations (pH = 1.5). Surprisingly, both SN1 and SN2 sulfate additions lead to greater formation of specific diastereomer products as measured with hydrophilic interaction liquid chromatography (HILIC) interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (QTOFMS). The stereochemical enhancement of organosulfate compounds has important implications for the formation of SOA and degradation of air quality.