Index Geophysics

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

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.

Nichman, Leonid

Fuchs, Claudia

Järvinen, Emma

Ignatius, Karoliina

Höppel, Niko Florian

Dias, Antonio

Heinritzi, Martin

Simon, Mario

Tröstl, Jasmin

Wagner, Andrea Christine

Wagner, Robert

Williamson, Christina

Yan, Chao

Connolly, Paul James

Dorsey, James Robert

Duplissy, Jonathan

Ehrhart, Sebastian

Frege, Carla

Gordon, Hamish

Hoyle, Christopher Robert

Kristensen, Thomas Bjerring

Steiner, Gerhard

McPherson Donahue, Neil

Flagan, Richard

Gallagher, Martin William

Kirkby, Jasper

Möhler, Ottmar

Saathoff, Harald

Schnaiter, Martin

Stratmann, Frank

Tomé, António

2018-09-08

Text

application/pdf

10.5194/acp-16-3651-2016

https://acp.copernicus.org/articles/16/3651/2016/

eISSN: 1680-7324

eng