Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors
1Department of Physics, Tampere University of Technology, Tampere, Finland
2Chemistry Department, Boston College, Chestnut Hill, MA, USA
3Aerodyne Research Inc., Billerica, MA, USA
4Faculty of Chemistry, Bielefeld University, Bielefeld, Germany
5Finnish Meteorological Institute, Helsinki, Finland
6Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
7Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Helsinki, Finland
*now at: Faculty of Mathematics, Astronomy and Physics, National University of Córdoba, Córdoba, Argentina
**now at: Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
Abstract. The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA particles spanning a wide range of oxygen to carbon ratios (O/C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O/C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS).
The main findings of the study are: (1) Biogenic and anthropogenic SOA particles are found to be solid or semi-solid until a relative humidity of at least 50 % RH at impaction is reached. (2) Long-chain alkanes produce liquid SOA particles when generated at low oxidation level of O/C<0.2, but at higher oxidation levels they solidify. (3) Increasing sulphuric acid (H2SO4) within the SOA particles reduces the threshold of humidity-induced phase changes. (4) The bounce behavior of the various SOA systems did not show a consistent linear relationship with the particle O/C. Rather, the molar mass of the gas-phase VOC precursor showed a positive correlation with the resistance to the RH-induced phase change of the formed SOA particles.