Gas/Particle Partitioning of L/SVOC into Probe Particles, SOA, and tubing; and parameteterizing isoprene SOA for climate models

Description

Partitioning of low/semi-volatile organics (L/SVOCs) into particles plays an essential role in SOA formation and evolution. Most models treat gas/particle partitioning as an equilibrium between gases and particles, despite potential kinetic limitations. We used fast measurements of gas (CIMS) and particle (EESI) phases to observe the dynamic (s to min) isothermal partitioning of chemically-speciated organic compounds (from rapid bursts of OH/alkanol+NOx) in a Teflon chamber in the presence of single-component seeds of different phases and polarities and α-pinene SOA. α determined independently from gas- and particle-phase observations are consistent and average 0.88±0.33 for all studied seeds and probe gases (spanning wide range of volatility). In addition, our results imply a fast mixing timescale (~200 s) between dry α-pinene/O3 SOA and oxidized compounds. Near-unity α was also observed for individual L/SVOC and the bulk condensing compounds from toluene/OH oxidation (high NO), in contrast to previous studies that reported α of 2-3 orders magnitude lower. These results indicate that mass transfer limitations in the atmosphere may be less important than some recent studies suggest, and that low α values in some previous studies may arise from structural deficiencies in the models used. At longer timescales (min to h), changes in particle size distributions and composition (SMPS and AMS), & GECKO-A, were used to probe SOA formation and evaporation. SOA activity coefficients (γ) estimated for each seed with two independent methods showed lower γ when SOA and seed have similar polarities, indicating favorable interactions (Liu et al., submitted). Interactions with tubing can significantly affect time-resolved gas measurements. We have characterized multiple types of tubing (plastic, glass, metals), all of which show a delay that increases x10 as C* decreases x10. PFA Teflon and Silonite-coated steel are the least interactive for species tested (spanning 7 orders C*). Small polar molecules behave differently than larger organic ones, with delays predicted by Henry’s law. Glass and metal show very different phenomenology due to adsorptive partitioning to finite surface sites. Examples of impact on measurements will be shown. (Deming et al., & Liu et al, AMTD 2019) 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. (Jo et al., GMDD)