Immersion freezing of birch pollen washing water

Birch pollen grains are known to be ice nucle- ating active biological particles. The ice nucleating activ- ity has previously been tracked down to biological macro- molecules that can be easily extracted from the pollen grains in water. In the present study, we investigated the immer- sion 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 Inter- action Simulator (LACIS). Two different birch pollen sam- ples 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 in- creased 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 deter- mination 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 re- gion 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 de- rive parameterizations for the heterogeneous nucleation rates for both INA macromolecule types, using two different meth- ods: 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.