10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Experimental Study of Condensed-Phase Reaction Kinetics of Secondary Organic Aerosols from Isoprene Expoxydiols
YUZHI CHEN, Matthieu Riva, Theran P. Riedel, Havala Pye, Nicole Olson, Ziying Lei, Zhenfa Zhang, Avram Gold, William Vizuete, Barbara Turpin, Andrew Ault, Jason Surratt, University of North Carolina at Chapel Hill
Abstract Number: 1104 Working Group: Aerosol Chemistry
Abstract Isoprene epoxydiols (IEPOX), OH-initiated oxidation products of isoprene, are known to produce secondary organic aerosol (SOA) through acid-catalyzed heterogeneous reactions in the presence of sulfate-containing aerosols within the atmosphere. IEPOX-derived SOA can contribute up to 40% of summertime organic aerosol mass in isoprene-rich regions like the southeastern U.S. Among all SOA tracers identified during laboratory and field studies, 2-methyltetrols and 2-methyltetrol sulfate esters, also referred to as IEPOX-derived organosulfates (OSs), together contribute a major mass fraction (e.g. >90% at Look Rock, TN) of IEPOX-derived SOA. Regulatory models that adopt rate constants for the formation of these two species from kinetic measurements in bulk solutions, however, typically yield discrepancies between model predictions and observations suggesting a need for more accurate reaction-rate constants with sulfate-containing aerosols. Non-ideality of aerosols may affect the applicability of kinetic measurements from bulk solution studies. While computational estimates of the reaction-rate constants that took into account the non-ideality of aerosols were, on average, 2-3 orders of magnitude higher than those derived from measurements in bulk solutions (especially for IEPOX-derived OSs), and seemed promising to improve the model representation of IEPOX uptake process, supporting laboratory measurements are still lacking. We have developed an alternative approach that couples chamber experiments with offline chemical characterization and modeling to derive condensed-phase reaction-rate constants in IEPOX-derived SOA. These previous experiments, however, were conducted under only dry conditions (RH <5%) limiting our ability to extend the derived reaction-rate constants to more humid conditions. In addition, the lack of time-resolved reaction profiles of 2-methyltetrols and IEPOX-derived OSs and the potential for artifacts induced by filter collection also introduce uncertainties in the reaction-rate constants derived from the previous study.
In this work, chamber experiments were conducted using gaseous IEPOX and acidic sulfate aerosols to generate IEPOX-derived SOA in the UNC indoor chamber facility. Chamber-generated IEPOX-derived SOA samples were collected by a particle-into-liquid sampler (PILS) using a 5-minute time resolution. PILS samples were immediately analyzed by ion chromatography (IC) for inorganic constituents and ultra-performance liquid chromatography interfaced to high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization (UPLC/ESI-HR-QTOFMS) for IEPOX-derived SOA tracers. Our results show that up to 90% of the inorganic sulfate was converted to IEPOX-derived OSs and oligomers in acidified ammonium sulfate particles within one hour of reaction with gas-phase IEPOX. Our results further demonstrate that the rates of IEPOX-derived OS formation were likely underestimated by prior bulk-solution kinetic studies. New condensed-phase reaction-rate constants for the formation of the 2-methyltetrols and IEPOX-derived OSs were derived from a one-dimensional box model of our chamber experiments using characterized reaction profiles resolved by PILS-IC and PILS-UPLC/ESI-HR-QTOFMS. The impacts of new reaction-rate constants on aerosol acidity and atmospheric formation of IEPOX-derived SOA need to be further assessed with updated air quality and global models, which is critical in understanding the governing processes that lead to SOA formation and developing air-pollution mitigation strategies.