10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Understanding Missing Sources of Fine Particulate Organosulfur Compounds in the Atmosphere: Implications from Ambient Measurements and Laboratory Experiments
Abstract Number: 1308 Working Group: Aerosol Chemistry
Abstract Organosulfur compounds are ubiquitous in Earth’s atmosphere. These compounds are non-negligible contributors to fine organic aerosol mass. Among them, sulfate esters or organosulfates (OSs) contribute the greatest mass fraction of particulate organosulfur compounds. These aerosol constituents are formed from multiphase chemical processes, such as the acid-catalyzed reactive uptake of gas-phase products generated from the atmospheric oxidation of biogenic and anthropogenic volatile organic compounds (VOCs) in the presence of aqueous sulfate aerosols. While the presence of OSs has been reported in fine aerosol collected from many locations, especially in biogenic-rich areas, the impacts of such products on the physicochemical properties of and the sulfate distribution within secondary organic aerosol (SOA) remain unclear. As sulfate over continental regions is generally of anthropogenic origin or from the oxidation of dimethyl sulfide, understanding the physical and chemical processes associated with SOA formation and properties, and how sulfate impacts them is crucial to properly representing SOA in models and evaluating their impacts on climate and human health.
SOA samples collected from the southeastern U.S. and the Amazon forest (isoprene and monoterpene-dominated atmospheres), were chemically characterized to evaluate the distribution of fine particulate organosulfur compounds. OSs and methanesulfonic acid (MSA) were quantified by ultra-performance liquid chromatography interfaced with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS). Total organosulfur mass was determined by isotope ratio inductively coupled plasma mass spectrometry. OSs were found to contribute significantly to the organic aerosol (OA) mass in both areas. Total quantified OSs plus MSA, however, only accounted for about 50% of total organosulfur, suggesting the presence of a significant amount of unidentified organosulfur chemicals of different functionality, and unidentified processes that form them. We hypothesize that the missing mass is comprised of sulfate-containing oligomers formed from multiphase chemistry of BVOC oxidation products, especially the isomeric isoprene epoxydiols (IEPOX), with acidic sulfate aerosols. Targeted atmospheric simulation chamber experiments re-investigating the multiphase chemistry of IEPOX revealed that formation of OSs cannot fully account for the amount of sulfate consumed, suggesting that the missing source of organosulfur compounds can arise from previously uncharacterized IEPOX chemistry. Overall, our findings provide a comprehensive picture of particle-phase processing governing isoprene-derived SOA composition and suggest the role of IEPOX-OSs in SOA evolution has previously been underestimated.