Some ice nucleation characteristics of Asian and Saharan desert dust

[摘要] The large (7 m×4 m cylinder, 84 m³) AIDA (Aerosol Interactionsand Dynamics in the Atmosphere) cloud chamber facility at Forschungszentrum,Karlsruhe, Germany was used to test the ice nucleating ability of two desertdust samples from the Sahara and Asia. Aerosol samples were lognormallydistributed with a mode diameter of 0.4(±0.1) μm and geometricstandard deviation of ~1.7(±0.2). At temperatures warmer than−40°C droplets were formed before ice crystals formed and there wasgenerally no deposition nucleation observed. At temperatures colder than−40°C both dust samples exhibited dual nucleation events that wereobserved during the same expansion experiment. The primary nucleation eventoccurred at ice saturation ratios of 1.1 to 1.3 and is likely to be adeposition nucleation mode. The secondary nucleation event occurred at icesaturation ratios between 1.35 and 1.5. We cannot categorically determinewhether this ice nucleation event is via a further deposition mode or acondensation mode, but the presence of some soluble material in the dustsamples leads us to favour the latter process. The activated fractions ofdesert dust ranged from ~5–10% at −20°C to 20–40% attemperatures colder than −40°C. There was no obvious differencebetween the nucleation behaviour of the two dust samples.

[学科分类] 大气科学