Non-Equilibrium Behavior in Fresh Isoprene Secondary Organic Aerosol

Yuzhi Chen, Rahul Zaveri, Gregory W. Vandergrift, Zezhen Cheng, Swarup China, Alla Zelenyuk, JOHN SHILLING, Pacific Northwest National Laboratory

     Abstract Number: 135
     Working Group: Aerosol Chemistry

Abstract
Recent studies have shown that instantaneous gas-particle equilibrium partitioning assumption fails to predict SOA formation, even at high relative humidity (~85%), and aging seems to be a driving factor. In this study, we probed the minimum aging timescale required to observe non-equilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas- and aerosol-phase at ~50% RH. Seed isoprene SOA was generated by photo-oxidation in the presence of effloresced ammonium sulfate seeds at <1ppbv NOx, aged photochemically or in the dark for 0.3–6 hours, and subsequently exposed to fresh isoprene SVOCs. Our results show that the equilibrium partitioning assumption is accurate for fresh isoprene SOA but breaks down after isoprene SOA has been aged for as short as 20 minutes, even in the dark. Modeling results showed that a semi-solid SOA phase state was necessary to reproduce the observed particle size distribution evolution. The observed non-equilibrium partitioning behavior and inferred semi-solid phase state were corroborated by offline mass spectrometric analysis on the bulk aerosol particles showing the formation of organosulfates and oligomers. The unexpected short timescale for phase transition within isoprene SOA has important implications for the grow­­th of atmospheric ultrafine particles to climate-relevant sizes.