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M. Kohn:
"Laboratory and field measurements of immersion freezing utilizing a newly developed cloud chamber";
Betreuer/in(nen), Begutachter/in(nen): U. Lohmann, H. Grothe, Z. Kanji; Institute for Atmospheric and Climate Science, 2016; Rigorosum: 21.03.2016.

The occurrence and properties of clouds in the atmosphere significantly influence the Earth´s energy budget, thus influencing the climate. Aside from the effect on the radiative properties, ice formation is a crucial mechanism leading to precipitation. Therefore, a comprehensive understanding about the constraints effecting atmospheric ice formation is required to predict weather and climate correctly. Mechanisms that lead to the formation of ice crystals are not yet fully understood. In the atmosphere, ice crystals can form homogeneously or heterogeneously. For the latter, an ice nucleating particle (INP) provides a surface for ice to nucleate upon. Different modes of heterogeneous ice nucleation exist, which can be investigated in the laboratory or by deploying instrumentation in the field to measure ambient INP concentrations. In mixed-phase clouds, where ice crystals and supercooled cloud droplets co-exist, freezing mechanisms involving the liquid phase such as immersion freezing are particularly relevant.

During this study, the newly developed Portable Immersion Mode Cooling chAmber (PIMCA), which is a vertical extension of the Portable Ice Nucleation Chamber (PINC), is introduced. The PIMCA-PINC setup is currently the only available portable instrument measuring immersion mode INPs in-situ. It is deployable in the field and can be used for inter-comparison studies in other laboratories. It allows for the measurement of the ice nucleation (IN) properties of individual aerosol particles. Computational flow dynamics simulations, homogeneous and heterogeneous freezing experiments were used to validate the performance of the chamber.

The first field study investigating ambient immersion freezing took place in spring 2014 during the Zurich AMBient Immersion freezing Study (ZAMBIS). Ice nucleation was observed and has been able to be quantified manly close to homogeneous freezing temperatures with PIMCAPINC. Two additional drop freezing methods using bulk sampling allowed for immersion mode ice nucleation measurements over a wide temperature range between 233 K and 268 K. Due to differences in the sampled aerosol particles sizes, utilizing these three independent ice nucleation techniques allowed interpretation on particle size effects of ambient INPs. Parallel measurements of bioaerosols and meteorological conditions were used to investigate which factors influence the ambient INP concentration. The pollen season of birch was chosen to estimate the atmospheric relevance of pollen due to their abundance and known IN abilities. During the peak of the pollen season, an increase in the INP concentration was observed.

The portability of the new PIMCA-PINC setup made a direct inter-comparisons with other state of the art IN devices for measurements in the immersion mode possible. PIMCA-PINC was compared to LACIS (Leipzig Aerosol Cloud Interaction Simulator) in parallel, both instruments measuring single immersed aerosol particles. This comparison yielded excellent agreement between the instruments. Our inter-comparisons indicated that differences in the particle generation and size selection can lead to significant variations in immersion freezing temperatures. Lastly, two continuous flow diffusion chambers (CFDCs) PINC and SPIN (the Spectrometer for Ice Nuclei) were used in order to better understand which IN mechanisms are measured and how they differ. PINC and SPIN agreed well when taking residence time effects and ice crystal growth rates into account. A comparison of all four instruments showed a discrepancy between the immersion freezing devices and the CFDCs measuring in the condensation mode. A clear offset was found, between the frozen fraction in immersion mode and activated fraction in condensation mode that was dependent on both temperature and aerosol type investigated. Future work is needed to separate between differences in instrumentation and ice nucleation mechanisms.

http://e-collection.library.ethz.ch/eserv/eth:50024/eth-50024-01.pdf