Ice formation largely influences the properties of clouds and hence it has an important impact on weather and climate. Primary ice formation is a consequence of either homogeneous or heterogeneous ice nucleation. The latter process is catalyzed by a foreign substance called Ice Nucleating Particle (INP). Mineral dust particles were found to contribute to atmospheric INPs. Most types of mineral dust are ice active below -20 °C. In contrast, atmospheric observations indicate that immersion freezing as one of the most important heterogeneous ice nucleation processes can occur at temperatures higher than -15 °C. One possible explanation for cloud glaciation at high temperatures might be the presence of biological material (e.g. bacteria) inducing ice nucleation. Our fundamental process and even qualitative understanding concerning atmospheric heterogeneous ice nucleation is limited. In the framework of the present thesis, experimental and theoretical work was carried out to improve the basic understanding of the immersion freezing behavior of mineral dust and bacterial INPs. On the basis of model simulations immersion freezing experiments were designed at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). The immersion freezing behavior of mineral dust and bacterial INPs was studied in dependence of temperature and particle surface area/number at LACIS. As a results of the present thesis, it was found that the immersion freezing behavior of kaolinite being a proxy of mineral dust INPs does not depend on the droplet volume, but on the particle surface area. The kaolinite particles investigated caused freezing below -30 °C. In contrast, Ice Nucleation Active (INA) protein complexes that are attributed to bacterial INPs were found to induce freezing at -7 °C. Furthermore, it was shown that the ice nucleation activity of protein complexes is very similar regardless of whether the INA protein complex is attached to the outer cell membrane of intact bacteria or to cell membrane fragments. The immersion freezing ability depends on the number and type of INA protein complexes present in the droplet ensemble. The immersion freezing ability of mineral dust and bacterial INPs was parameterized accounting for the time effect. With this, results from literature could be reproduced for both INP types. These parameterizations can be used in e.g. cloud resolving atmospheric models.

Ice formation largely influences the properties of clouds and hence it has an important impact on weather and climate. Primary ice formation is a consequence of either homogeneous or heterogeneous ice nucleation. The latter process is catalyzed by a foreign substance called Ice Nucleating Particle (INP). Mineral dust particles were found to contribute to atmospheric INPs. Most types of mineral dust are ice active below -20 °C. In contrast, atmospheric observations indicate that immersion freezing as one of the most important heterogeneous ice nucleation processes can occur at temperatures higher than -15 °C. One possible explanation for cloud glaciation at high temperatures might be the presence of biological material (e.g. bacteria) inducing ice nucleation. Our fundamental process and even qualitative understanding concerning atmospheric heterogeneous ice nucleation is limited. In the framework of the present thesis, experimental and theoretical work was carried out to improve the basic understanding of the immersion freezing behavior of mineral dust and bacterial INPs. On the basis of model simulations immersion freezing experiments were designed at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). The immersion freezing behavior of mineral dust and bacterial INPs was studied in dependence of temperature and particle surface area/number at LACIS. As a results of the present thesis, it was found that the immersion freezing behavior of kaolinite being a proxy of mineral dust INPs does not depend on the droplet volume, but on the particle surface area. The kaolinite particles investigated caused freezing below -30 °C. In contrast, Ice Nucleation Active (INA) protein complexes that are attributed to bacterial INPs were found to induce freezing at -7 °C. Furthermore, it was shown that the ice nucleation activity of protein complexes is very similar regardless of whether the INA protein complex is attached to the outer cell membrane of intact bacteria or to cell membrane fragments. The immersion freezing ability depends on the number and type of INA protein complexes present in the droplet ensemble. The immersion freezing ability of mineral dust and bacterial INPs was parameterized accounting for the time effect. With this, results from literature could be reproduced for both INP types. These parameterizations can be used in e.g. cloud resolving atmospheric models.