10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effect of Temperature on Evaporation of α-Pinene Secondary Organic Aerosol
ZIJUN LI, Angela Buchholz, Olli-Pekka Tikkanen, Eetu Kari, Liqing Hao, Taina Yli-Juuti, Annele Virtanen, University of Eastern Finland
Abstract Number: 507 Working Group: Aerosol Physics
Abstract Volatility of secondary organic aerosol (SOA) constituents governs their partitioning between gas and particulate phase. Current studies have reported slower SOA evaporation than expected from Volatility Basis Set (VBS) parametrizations, suggesting considerable impacts from particle-phase diffusion and from condensed phase reactions (e.g. oligomer degradation). Aerosol measurements have revealed that lower temperature can significantly hinder molecular diffusion and suppress oligomer decomposition. Given the importance of these processes on SOA evaporation, investigations on SOA evaporation at low temperature are needed.
Here, evaporation of α-pinene photooxidation (αpinOH) and ozonolysis (αpinO3) SOA was examined under a range of relative humidity (RH), both at warm (20 °C) and cold temperature (10 °C). Briefly, αpinOH and αpinO3 SOA was generated using an oxidation flow reactor and then selected by two parallel Nano-DMAs with open loop sheath flow. After that, a sample flow of 80-nm SOA was either led directly to the instruments via a bypass line or fed into two 100-L Residence Time Chambers (RTCs). Short residence time data was obtained by varying the bypass tube length, while intermediate and long residence time data were collected from the RTCs during the filling period and in ~1h intervals after SOA filling, respectively. Aerosol characterization was conducted by a Scanning Mobility Particle Sizer (SMPS) and a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). Size selection, evaporation and size change measurement were conducted in the temperature and RH controlled environment.
Evaporation of α-pinene SOA was hindered at lower temperature. This is consistent with the temperature dependence of saturation vapor pressures, but it may additionally suggest a temperature effect in the diffusion limitations in SOA particle bulk and the degradation of oligomers. Different evaporation behavior was observed for αpinOH and αpinO3 SOA. This different volatility distribution most likely stems from a change in the particle chemical composition which is not represented by the very similar O:C ratio derived for the two SOA types. In addition, a kinetic-based evaporation model, coupled with Clausius–Clapeyron relation, was employed to reproduce the observed evaporation and derive the enthalpy of vaporization. These simulations showed that the derived enthalpy of vaporization was higher than the typical values assumed (30–40 kJ/mol) in global aerosol models.