10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Competing Effects of Water Vapor and Aerosol Liquid Water on the Yield and Molecular Composition of Secondary Organic Aerosols
LAUREN FLEMING, Julia Montoya-Aguilera, Wing-Sy DeRieux, Ying Li, Peng Lin, Alexander Laskin, Julia Laskin, Manabu Shiraiwa, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 1429 Working Group: Aerosol Chemistry
Abstract Relative humidity (RH) is known to affect the mechanism of gas-phase oxidation of volatile organic compounds (VOCs) that ultimately affects the formation and growth of secondary organic aerosol (SOA). Additionally, high RH results in aerosol liquid water (ALW) associated with an inorganic component, which is known to promote the formation of SOA by oxidizing water-soluble components in the aqueous-phase. These two effects of RH can in many cases counteract each other. For example, it has been shown that the toluene SOA yield decreases with increasing RH in experiments without hygroscopic seed particles and increases with increasing RH in those that use them. The goal of this study was to decouple the gas-phase RH effect and ALW chemistry by investigating the effect of RH and presence of aerosol liquid water (ALW) on the relative yields and chemical composition of various SOA types. In addition, we carried out experiments aimed at understanding the ALW effects on SOA organic compounds over time scales of days, comparable to aging times for SOA particles in the atmosphere. Deliquesced (liquid) or effloresced (solid) ammonium sulfate seeds were prepared in a smog chamber at 90% RH and <5% RH, respectively. The photooxidation of α-pinene or xylene occurred in the presence of conditioned ammonium sulfate seeds at the set RH. In a control set of experiments, no seed was used. A suite of online instruments monitored gas-phase composition (proton transfer reaction time-of-flight mass spectrometer), particle-phase composition (aerosol time-of-flight mass spectrometer), and particle volume concentration (Scanning Mobility Particle Sizer). Filter samples were collected for offline ESI-HRMS and/or DART-MS analysis, which provided molecular composition of SOA. SOA filter samples were aged at 85% RH or 0% RH for 24 hours and their molecular composition was subsequently probed. A “molecular corridor” analysis of the nano-DESI mass spectra resulted in the volatility distributions for SOA produced in the presence and absence of ALW for both SOA samples. Additionally, the SOA’s glass transition temperature was estimated in order to predict particle phase state under different atmospheric conditions. The results of this study underscore the importance of the effect of gaseous and liquid water on the chemical composition and key properties of SOA particles.