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Initial pH Governs Secondary Organic Aerosol Viscosity and Morphology after Uptake of Isoprene Epoxydiols (IEPOX)
ZIYING LEI, Yuzhi Chen, Yue Zhang, Madeline Cooke, Isabel Ledsky, N. Cazimir Armstrong, Nicole Olson, Zhenfa Zhang, Avram Gold, Jason Surratt, Andrew Ault, University of Michigan
Abstract Number: 203
Working Group: Aerosol Chemistry
Abstract
Aerosol acidity increases secondary organic aerosol (SOA) formation by facilitating reactions that increase the amount of organic material in the condensed phase. Isoprene-derived SOA is largely formed through acid-catalyzed reactive uptake of gaseous species, such as isoprene-derived epoxydiols (IEPOX) onto acidic, sulfate-containing particles. However, the resulting physicochemical properties of mixed inorganic-organic particles after reactive uptake of IEPOX to particles with acidities covering the range of atmospheric pH values (0-5) are not well understood. This study investigated morphology, phase state, and chemical composition of individual organic-inorganic particles with different initial acidities (pH = 1, 3, and 5) after IEPOX uptake using atomic force microscopy coupled with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Enhanced IEPOX reactive uptake to the most acidic seed particles (pH = 1) resulted in 23% more formation of organosulfates compared to less acidic seed particles (pH 3 and 5). Distinct phase separations (i.e., core-shell morphologies) primarily occurred for initial pH values < 3. Increased aerosol acidity (lower pH) also led to more viscous organic components of SOA particles and more irregularly shaped morphologies as the organic phase transitioned to semi-solid or solid. Conversion of inorganic sulfate to organosulfates corresponded with the transition to the higher viscosity of the organic phase and more complex structures. This study highlights that aerosol acidity controls key multiphase chemical reactions and the subsequent modification of aerosol physicochemical properties, such as viscosity and morphology, which can be used to improve predictions of SOA formation, as well as subsequent climate and health impacts.