Is There a Relationship between the Immersion Freezing Activity and the Mineralogical Composition of Natural Dust Aerosols?

Primary ice formation in tropospheric clouds, followed by a cascade of secondary ice formation and ice multiplication processes, very often initiates the formation of precipitation, in particular over continents where a variety of aerosols from different sources are present. A major mechanism of primary ice formation in so-called mixed-phase clouds, which can exist at temperatures between 0°C and about -35°C, is immersion freezing, which happens when the proper particle, a so-called ice-nucleating particle (INP), is immersed in a supercooled cloud droplet that is cooled to the proper temperature. Various aerosol particle types have been identified to potentially act as INP, but only a minor fraction of their total number in the atmosphere is active in this freezing mode, in particular at higher temperatures.

This contribution addresses the question, what actually is the proper particle and the proper temperature in cases, where desert dust aerosols are present in the atmosphere and contribute to the abundance of INPs. Eleven dust samples from different source regions and with different mineralogical compositions were used for a series of aerosol and cloud experiments at the AIDA facility of the Karlsruhe Institute of Technology (KIT). The dust samples were characterized for their elemental and mineralogical composition with a number of analytical techniques like EDX and XRD.

Part of the samples were dispersed into the AIDA cloud chamber to measure the immersion freezing ability in cloud expansion experiments at temperatures between about -20°C and -30°C. Another part of the samples was dispersed at higher number concentrations into a smaller aerosol vessel and sampled with an impinger into ultrapure water, which was then shared with the Universities of Frankfurt, Mainz and Bielefeld for measuring the immersion freezing efficiency of the different samples with the FRIDGE, BINARY and MAL experiments, respectively. The aerosol from both the cloud and the aerosol chambers was also sampled on filters for more offline measurements of the immersion freezing activities with the WISDOM experiment of the Weizmann Institute and the INSEKT experiment of the Karlsruhe Institute of Technology. The aerosol was also sampled with the single particle mass spectrometer LAAP-TOF of The University of Manchester, which facilitates the identification of both the elemental and mineralogical composition of single aerosol particles.

This contribution will give an overview of the results for both the mineralogical composition of the samples and their immersion freezing efficiency in a wide range of temperatures as measured by the various methods.