A Summary of Immersion Freezing Results from the Ice Nucleation Research Unit INUIT

Immersion freezing can be considered as one of the most important mechanisms for primary ice formation in supercooled tropospheric clouds that exist at temperatures between 0°C and -38°C. Though numerous laboratory experiments have been conducted and new parameterization concepts have been developed, it is still unclear which are the most relevant aerosol types and most appropriate formulations for heterogeneous ice nucleation processes in tropospheric clouds. Furthermore, immersion freezing results from different labs do not always agree to each other, which may partly be due to the large variety of aerosol samples from different sources and a lack of direct comparison of measurement techniques. The partners of the ice nucleation research unit INUIT funded by the Deutsche Forschungsgemeinschaft (DFG grant FOR 1525) have for the first time selected and shared with INUIT external partners several identical reference samples in order to develop high standards of laboratory ice nucleation research, to directly compare different laboratory methods and to get a comprehensive data set for evaluating the ice nucleation properties of relevant aerosols in a wide range of particle concentration, temperature, cooling rate and nucleation time. Methods involved are the cloud expansion chamber AIDA in Karlsruhe, the laminar flow cloud simulator LACIS in Leipzig, the vertical wind tunnel MWT and the acoustic droplet trap MAT in Mainz, the diffusion cell FRIDGE and the mixing chamber FINCH in Frankfurt and the cold stage experiment BINARY in Bielefeld. INUIT external partners contributed measurements with a dynamic cloud chamber at MRI in Japan, the flow diffusion chambers at CFDC (CSU and PNNL), SPIN (MIT) and PINC (ETH) as well as cold stage setups at the University of Leeds, NIPI suite (Nucleation by Immersed Particles Instrument) and the University of Colorado, Boulder. Some methods like AIDA, LACIS, FINCH, the MRI chamber and the flow diffusion instruments are based on water condensation to dry-dispersed aerosol particles followed by immersion freezing. Experiments with MWT, MAT, EDB and the cold stage setups like BINARY start with the samples already suspended in water before cooling. FRIDGE investigates the ice nucleation of dry particles on a substrate upon humidity increase. Aerosol-based experiments were performed both with polydispers and monodispers aerosols. The samples were also shared with external project partners who also investigated immersion freezing with aerosol and immersion based methods. This contribution will present and discuss the results of immersion freezing experiments with Snomax as a proxy for atmospheric bacteria, as well as illite NX from Arginotec and K-feldspar provided by the University of Darmstadt as proxies for atmospheric desert dust aerosols. The samples were extensively characterized for their physico-chemical properties before their distribution to the different labs which used identical methods for sample preparation, aerosol dispersion and aerosol characterization.