Pollen have been discussed as effective ice nuclei in the immersion freezing mode [Diehl]. Only recently, we have shown that water which had been in contact with pollen turned out to contain as good ice nuclei as the pollen grains themselves [Pummer]. We found macromolecules of 100-300 kDa being responsivble for the observed effect. Currently, we intensify our investigation on the identification of these IN by applying different separation techniques (e.g. solid phase extraction) and spectrometric methods (e.g. MS, NMR). Furthermore, we determined the impact of different environmental stresses (ultraviolet light, nitrogen dioxide, ozone) on the IN activity. While ozone showed no significant impact, nitrogen dioxide abd UV light denaturated most pollen IN. Only birch IN showed extraordinary stability towards this treatment. In an additional experiment, we measured the IN activity at lower pollen concentrations. We found out that birch IN concentrations are several orders of magnitude higher than those of other species. Diehl K et al.: Atmos. Res. 61, 125-133, 2002. Pummer BG et al.: Atmos. Chem. Phys. 12, 2541-2550, 2012