Browsing by Author "Tomsche, L."
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- ItemImmersion freezing of birch pollen washing water(München : European Geopyhsical Union, 2013) Augustin, S.; Wex, H.; Niedermeier, D.; Pummer, B.; Grothe, H.; Hartmann, S.; Tomsche, L.; Clauss, T.; Voigtländer, J.; Ignatius, K.; Stratmann, F.Birch pollen grains are known to be ice nucleating active biological particles. The ice nucleating activity has previously been tracked down to biological macromolecules that can be easily extracted from the pollen grains in water. In the present study, we investigated the immersion freezing behavior of these ice nucleating active (INA) macromolecules. Therefore we measured the frozen fractions of particles generated from birch pollen washing water as a function of temperature at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). Two different birch pollen samples were considered, with one originating from Sweden and one from the Czech Republic. For the Czech and Swedish birch pollen samples, freezing was observed to start at −19 and −17 °C, respectively. The fraction of frozen droplets increased for both samples down to −24 °C. Further cooling did not increase the frozen fractions any more. Instead, a plateau formed at frozen fractions below 1. This fact could be used to determine the amount of INA macromolecules in the droplets examined here, which in turn allowed for the determination of nucleation rates for single INA macromolecules. The main differences between the Swedish birch pollen and the Czech birch pollen were obvious in the temperature range between −17 and −24 °C. In this range, a second plateau region could be seen for Swedish birch pollen. As we assume INA macromolecules to be the reason for the ice nucleation, we concluded that birch pollen is able to produce at least two different types of INA macromolecules. We were able to derive parameterizations for the heterogeneous nucleation rates for both INA macromolecule types, using two different methods: a simple exponential fit and the Soccer ball model. With these parameterization methods we were able to describe the ice nucleation behavior of single INA macromolecules from both the Czech and the Swedish birch pollen.
- ItemKaolinite particles as ice nuclei: Learning from the use of different kaolinite samples and different coatings(Göttingen : Copernicus, 2014) Wex, H.; Demott, P.J.; Tobo, Y.; Hartmann, S.; Rösch, M.; Clauss, T.; Tomsche, L.; Niedermeier, D.; Stratmann, F.Kaolinite particles from two different sources (Fluka and Clay Minerals Society (CMS)) were examined with respect to their ability to act as ice nuclei (IN). This was done in the water-subsaturated regime where often deposition ice nucleation is assumed to occur, and for water-supersaturated conditions, i.e., in the immersion freezing mode. Measurements were done using a flow tube (the Leipzig Aerosol Cloud Interaction Simulator, LACIS) and a continuous-flow diffusion chamber (CFDC). Pure and coated particles were used, with coating thicknesses of a few nanometers or less, where the coating consisted of levoglucosan, succinic acid or sulfuric acid. In general, it was found that the coatings strongly reduced deposition ice nucleation. Remaining ice formation in the water-subsaturated regime could be attributed to immersion freezing, with particles immersed in concentrated solutions formed by the coatings. In the immersion freezing mode, ice nucleation rate coefficients het from both instruments agreed well with each other, particularly when the residence times in the instruments were accounted for. Fluka kaolinite particles coated with either levoglucosan or succinic acid showed the same IN activity as pure Fluka kaolinite particles; i.e., it can be assumed that these two types of coating did not alter the ice-active surface chemically, and that the coatings were diluted enough in the droplets that were formed prior to the ice nucleation, so that freezing point depression was negligible. However, Fluka kaolinite particles, which were either coated with pure sulfuric acid or were first coated with the acid and then exposed to additional water vapor, both showed a reduced ability to nucleate ice compared to the pure particles. For the CMS kaolinite particles, the ability to nucleate ice in the immersion freezing mode was similar for all examined particles, i.e., for the pure ones and the ones with the different types of coating. Moreover, het derived for the CMS kaolinite particles was comparable to het derived for Fluka kaolinite particles coated with sulfuric acid. This is suggestive for the Fluka kaolinite possessing a type of ice-nucleating surface feature which is not present on the CMS kaolinite, and which can be destroyed by reaction with sulfuric acid. This might be potassium feldspar.