ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-7517-2012 Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors Saukko E. 1 Lambe A. T. 2 3 Massoli P. 3 Koop T. 4 Wright J. P. 2 Croasdale D. R. 2 Pedernera D. A. 4 8 Onasch T. B. 2 3 Laaksonen A. 5 6 Davidovits P. 2 Worsnop D. R. 3 7 Virtanen A. 1 6 Department of Physics, Tampere University of Technology, Tampere, Finland Chemistry Department, Boston College, Chestnut Hill, MA, USA Aerodyne Research Inc., Billerica, MA, USA Faculty of Chemistry, Bielefeld University, Bielefeld, Germany Finnish Meteorological Institute, Helsinki, Finland Department of Applied Physics, University of Eastern Finland, Kuopio, Finland Division 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 17 08 2012 12 16 7517 7529 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/7517/2012/acp-12-7517-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/7517/2012/acp-12-7517-2012.pdf

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). <br><br> The main findings of the study are: (1) biogenic and anthropogenic SOA particles are found to be amorphous solid or semi-solid based on the measured bounced fraction (BF), which was typically higher than 0.6 on a 0 to 1 scale. A decrease in the BF is observed for most systems after the SOA is exposed to relative humidity of at least 80% RH, corresponding to a RH at impaction of 55%. (2) Long-chain alkanes have a low BF (indicating a "liquid-like", less viscous phase) particles at low oxidation levels (BF < 0.2 ± 0.05 for O / C = 0.1). However, BF increases substantially upon increasing oxidation. (3) Increasing the concentration of sulphuric acid (H<sub>2</sub>SO<sub>4</sub>) in solid SOA particles (here tested for longifolene SOA) causes a decrease in BF levels. (4) In the majority of cases the bounce behavior of the various SOA systems did not show correlation 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.

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