Particle-phase Diffusion Limitations in Fresh Isoprene Secondary Organic Aerosol
YUZHI CHEN, Rahul Zaveri, Zezhen Cheng, Gregory Vandergrift, Swarup China, Alla Zelenyuk, John Shilling,
Pacific Northwest National Laboratory Abstract Number: 605
Working Group: Aerosol Chemistry
AbstractEnvironmental chambers have been conventionally used to investigate secondary organic aerosol (SOA) formation and generate model-ready parameterizations based on semivolatile equilibrium partitioning theory, which have successfully improved the prediction of SOA formation and evolution in atmospheric models. However, recent studies have shown that traditional equilibrium partitioning theory fails to predict SOA mass generated from oxidation of binary or more complex precursor VOC mixtures. Our previous experimental work has shown that fresh isoprene SOA would not rapidly partition into α-pinene SOA and isoprene SOA that are aged for 15−18 hours (at an OH concentration of ~ 2 × 10
6 molecules/cm
3) as equilibrium partitioning theory would predict, even under humid condition (RH ~ 70%). In this study, we probed the aging time scale required to trigger mass transfer limitation of semivolatile compounds partitioning between the gas- and aerosol-phase. Isoprene SOA seed was prepared by irradiating gas-phase isoprene and H
2O
2 in the presence of dry ammonium sulfate seed in the PNNL environmental chamber at ~ 23 °C and RH ~50%. Once the initial isoprene SOA growth peak was attained, isoprene SOA was aged under either light or dark for 0 – 6 hours, followed by a second photochemical growth period with injection of additional isoprene vapor. Surprisingly, equilibrium partitioning overpredicted the observed SOA yield when we assumed the fresh SOA is soluble in the aged SOA seed for the second photochemical growth period regardless of the aging time and irradiation condition. A box model was used to further investigate the cause of observed mass transfer limitation and showed that a semi-solid phase had to be assumed by the model to reproduce the observed evolution of particle size distribution. Filter samples were also collected and analyzed offline by nanospray desorption electrospray ionization mass spectrometry (nanoDESI-MS) to elucidate the changes in molecular-level chemical composition pertaining to the changed aerosol phase state.