Real-Time Measurements of Gas/Particle Partitioning of Semivolatile Organic Compounds into Different Probe Particles in a Teflon Chamber

Description

The partitioning of semivolatile organics (SVOCs) into particles plays an essential role in SOA formation and evolution. Most models treat gas/particle partitioning as an equilibrium between gas and particle phases, despite potential kinetic limitations and differences in thermodynamics as a function of SOA and pre-existing OA composition. We directly measure 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 a-pinene SOA. Oxidized compounds are generated by 10 s oxidation of a series of alkanols, and CIMS is used to measure the decay of gas-phase organic nitrates. We observed clear changes in equilibrium timescales with varying seed concentrations and in equilibrium gas-phase concentrations across different seeds, which can be reproduced by a kinetic box model. The accommodation coeff. (~1) and saturation mass concentration (C*) of each species in the presence of each seed are derived using the model. Changes in particle size distributions and composition monitored by SMPS and HR-ToF-AMS are used to probe SOA formation and evaporation. SOA activity coefficients (γ) can be estimated for each seed with two independent methods. Lower γ is found when SOA and seed have similar polarities, indicating favorable interactions. a-pinene SOA does not appear to have a diffusive limitation to uptake of SVOCs, since it behaves like a liquid even under dry conditions. The applicability of partitioning theory to these systems and the relevant quantitative parameters, including the dependencies on seed particle composition, will be discussed. The interaction of gases with tubings and instruments can significantly affect time-resolved gas-phase measurements. We have characterized multiple types of tubing, all of which show a delay that increases x10 as C* decreases x10. PFA Teflon is the least interactive tubing for the species tested. Examples of impact on real-time measurements of gases and SOA will be shown. In addition, a simplified parameterization for IEPOX-SOA has been developed for climate models, which performs similarly to an explicit model, at a much lower computational cost. Finally, we discuss work investigating molecular decomposition of SOA when using thermal desorption analytical methods and its impact on inferred volatility and gas-particle partitioning