Organosulfates in the Midwestern United States: abundance, composition and stability

Organosulfates are components of secondary organic aerosol resulting from the oxidation of volatile organic compounds in the presence of acidic sulfate. This study characterises organosulfates in the Midwestern United States for the first time. In fine particulate matter (PM2.5) collected in Iowa City, IA, in September 2017, organosulfates were analysed using liquid chromatography coupled to high-resolution and tandem mass spectrometry (MS) to identify and quantify (or semi-quantify) major species. Among the 22 identified species, methyltetrol sulfate (m/z 215; C5H11SO7-) had the largest contribution to the bisulfate (m/z 97) product ion, as determined by precursor-ion MS/MS (59.5% of signal), followed by ten other isoprene-derived organosulfates (15.2 %), seven monoterpene-derived organosulfates (5.6 %), three anthropogenic organosulfates (4.3 %) and one species of unknown origin (0.6 %). Among the quantified species were hydroxyacetone sulfate (4.8 +/- 1.1 ng m(-3)), glycolic acid sulfate (21.0 +/- 1.5 ng m(-3)), 2-methylgyceric acid sulfate (15.1 +/- 0.8 ng m(-3)), C5H7SO7- (m/z 211; 17.9 +/- 0.9 ng m(-3)), C5H9SO7- (m/z 213; 16.0 +/- 1.0 ng m(-3)), and methyltetrol sulfate (214 +/- 8 ng m(-3)); together, these species accounted for 4.4% of organic carbon. To further validate the measurement of organic species in PM using filter samples, the stability of organosulfates on filters frozen at -20 degrees C was evaluated over the course of 1 year. The stored samples revealed no degradation of organosulfates, indicating their stability on filters stored frozen for extended periods of time. This study provides new insight into the abundance and identity of organosulfates in the Midwestern US and demonstrates that isoprene-derived organosulfates, in particular, are a significant contributor to PM2.5 organic carbon.