Articles | Volume 16, issue 5
Atmos. Chem. Phys., 16, 3651–3664, 2016

Special issue: The CERN CLOUD experiment (ACP/AMT inter-journal SI)

Atmos. Chem. Phys., 16, 3651–3664, 2016

Research article 17 Mar 2016

Research article | 17 Mar 2016

Phase transition observations and discrimination of small cloud particles by light polarization in expansion chamber experiments

Leonid Nichman1, Claudia Fuchs2, Emma Järvinen3, Karoliina Ignatius4, Niko Florian Höppel3, Antonio Dias5, Martin Heinritzi6, Mario Simon6, Jasmin Tröstl2, Andrea Christine Wagner6, Robert Wagner7, Christina Williamson6,a,b, Chao Yan7, Paul James Connolly1, James Robert Dorsey1,8, Jonathan Duplissy9, Sebastian Ehrhart5, Carla Frege2, Hamish Gordon5, Christopher Robert Hoyle2,10, Thomas Bjerring Kristensen4, Gerhard Steiner7,11,c, Neil McPherson Donahue12, Richard Flagan13, Martin William Gallagher1, Jasper Kirkby5,6, Ottmar Möhler3, Harald Saathoff3, Martin Schnaiter3, Frank Stratmann4, and António Tomé14 Leonid Nichman et al.
  • 1School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
  • 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland
  • 3Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 4Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany
  • 5PH Department, CERN, Geneva, Switzerland
  • 6Institute for Atmospheric and Environmental Sciences, Goethe-University Frankfurt, Frankfurt am Main, Germany
  • 7Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Helsinki, Finland
  • 8National Centre for Atmospheric Science, Manchester, UK
  • 9Helsinki Institute of Physics, Helsinki, Finland
  • 10WSL Institute for Snow and Avalanche Research SLF Davos, Davos, Switzerland
  • 11Ion Molecule Reactions & Environmental Physics Institute of Ion Physics and Applied Physics Leopold-Franzens University, Innsbruck, Austria
  • 12Centre for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA 15213, USA
  • 13Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, 91125, USA
  • 14CENTRA-SIM, University of Lisbon and University of Beira Interior, 1749-016 Lisbon, Portugal
  • anow at: Chemical Sciences Division NOAA Earth System Research Laboratory, Boulder, CO, USA
  • balso at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • cnow at: Aerosol Physics and Environmental Physics Faculty of Physics, University of Vienna, Vienna, Austria

Abstract. Cloud microphysical processes involving the ice phase in tropospheric clouds are among the major uncertainties in cloud formation, weather, and general circulation models. The detection of aerosol particles, liquid droplets, and ice crystals, especially in the small cloud particle-size range below 50 μm, remains challenging in mixed phase, often unstable environments. The Cloud Aerosol Spectrometer with Polarization (CASPOL) is an airborne instrument that has the ability to detect such small cloud particles and measure the variability in polarization state of their backscattered light. Here we operate the versatile Cosmics Leaving OUtdoor Droplets (CLOUD) chamber facility at the European Organization for Nuclear Research (CERN) to produce controlled mixed phase and other clouds by adiabatic expansions in an ultraclean environment, and use the CASPOL to discriminate between different aerosols, water, and ice particles. In this paper, optical property measurements of mixed-phase clouds and viscous secondary organic aerosol (SOA) are presented. We report observations of significant liquid–viscous SOA particle polarization transitions under dry conditions using CASPOL. Cluster analysis techniques were subsequently used to classify different types of particles according to their polarization ratios during phase transition. A classification map is presented for water droplets, organic aerosol (e.g., SOA and oxalic acid), crystalline substances such as ammonium sulfate, and volcanic ash. Finally, we discuss the benefits and limitations of this classification approach for atmospherically relevant concentrations and mixtures with respect to the CLOUD 8–9 campaigns and its potential contribution to tropical troposphere layer analysis.

Short summary
Processes in the atmosphere are often governed by the physical and chemical properties of small cloud particles. Ice, water, and mixed clouds, as well as viscous aerosols, were formed under controlled conditions at the CLOUD-CERN facility. The experimental results show a link between cloud particle properties and their unique optical fingerprints. The classification map presented here allows easier discrimination between various particles such as viscous organic aerosol, salt, ice, and liquid.
Final-revised paper