Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 1 2006 10.5194/acpd-6-1509-2006 http://www.atmos-chem-phys-discuss.net/6/1509/2006/ http://www.atmos-chem-phys-discuss.net/6/1509/2006/acpd-6-1509-2006.html http://www.atmos-chem-phys-discuss.net/6/1509/2006/acpd-6-1509-2006.pdf 1509 1537 2006-02-23 Some ice nucleation characteristics of Asian and Saharan desert dust P. R. Field O. Möhler P. Connolly M. Krämer R. Cotton A. J. Heymsfield H. Saathoff M. Schnaiter Met Office, OBR (Observations Based Research), UK Forschungszentrum Karlsruhe, Institute for Meteorology and Climate Research (IMK-AAF), Germany University of Manchester, School of Earth, Atmospheric and Environmental Sciences, UK Forschungszentrum Jülich, Institute of Chemistry and Dynamics of the Geosphere (ICG-I), Germany NCAR, MMM (Mesoscale and Microscale Meteorology), USA The large (7 m×4 m cylinder) AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber facility at Forschungszentrum, Karlsruhe, Germany was used to test the ice nucleating ability of two desert dust samples from the Sahara and Asia. At temperatures warmer than −40°C droplets were formed before ice crystals formed, there was no deposition nucleation observed. At temperatures colder than −40°C both dust samples exhibited dual nucleation events that were observed during the same expansion experiment. The primary nucleation event occurred at ice saturation ratios of 1.1 to 1.3 and is likely to be a deposition nucleation mode. The secondary nucleation event occurred at ice saturation ratios between 1.35 and 1.5. It is unclear whether this ice nucleation event is via a further deposition mode or a condensation mode. The activated fractions of desert dust ranged from ~5–10% at −20°C to 20–40% at temperatures colder than −40°C. There was no obvious difference between the nucleation behaviour of the two dust samples.