10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Real-time Measurements of Gas/Particle Partitioning of Semivolatile Organic Compounds into Different Probe Particles in a Teflon Chamber
XIAOXI LIU, Douglas Day, Jordan Krechmer, Paul Ziemann, Jose-Luis Jimenez, University of Colorado Boulder
Abstract Number: 372 Working Group: Aerosol Chemistry
Abstract The partitioning of semivolatile organic compounds (SVOCs) into and out of particles plays an essential role in secondary organic aerosol (SOA) formation and evolution. Most atmospheric models treat gas/particle partitioning as an equilibrium between bulk gas and particle phases, despite potential kinetic limitations and differences in thermodynamics as a function of SOA and pre-existing OA composition. This study directly measures the partitioning of oxidized compounds in a Teflon chamber in the presence of single-component seeds of different phases and polarities, including oleic acid, squalane, dioctyl sebacate, pentaethylene glycol, dry/wet ammonium sulfate, dry/wet sucrose, and α-pinene SOA from ozonolysis. The oxidized compounds are generated by a 10 s OH oxidation burst of a series of alkanols under high NO conditions. A chemical ionization mass spectrometer (CIMS) was used to measure the decay of gas-phase organic nitrates, which reflects uptake by particles and chamber walls. We observed clear changes in equilibrium timescales with varying seed concentrations and in equilibrium gas-phase concentrations across different seeds. In general, the gas evolution can be reproduced by a kinetic box model that considers partitioning and evaporation to and from particles and chamber walls. The accommodation coefficient (~1) and saturation mass concentration of each compound in the presence of each seed are estimated using the model. The changes in particle size distributions and composition monitored by a scanning mobility particle sizer (SMPS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) are investigated to probe the SOA formation and evaporation. The observed SOA mass enhancements are highest with α-pinene SOA, and lowest with wet ammonium sulfate and sucrose. SOA activity coefficients (γ) were estimated for each seed with two independent methods, from equilibrium gas-phase concentrations and from SMPS-measured SOA evaporation rates. Lower γ is found when SOA and the seed have similar polarities, indicating favorable interactions. In addition, α-pinene SOA does not appear to have a diffusive limitation to the uptake of SVOCs, since it behaves like a liquid even under dry conditions. This work investigates the applicability of partitioning theory to these systems and derives the relevant quantitative parameters that were previously uncertain and rarely measured.