Heterogeneous ice nucleation, a process where ice nucleation is triggered by foreign particles or molecules, so called ice nuclei, plays an important role in the microphysics of clouds. Ice clouds exhibit inherently different properties when it comes to interactions with radiation, if compared to clouds consisting of liquid droplets. Therefore these clouds influence the total albedo of the Earth. This renders heterogeneous ice nucleation in clouds a process of climate relevance.
Mineral dusts have long been thought to play the major role in the underlying processes of atmospheric ice nucleation. Biological particles often show higher freezing temperatures, but tend to be rather large and therefore have a fast sinking velocity in the atmosphere. Consequently, it should be unlikely to find such particles at relevant altitudes and therefore their role in the atmosphere was only poorly examined so far. However recent field studies found diverse biological material at altitudes of 8 km and higher [1]. In this context, our working group has shown that the IN of certain pollen can be washed off and are therefore macromolecular rather than particulate [2]. Those molecules can form much smaller particles or get attached to other particles and reach much higher altitudes than e.g. the pollen itself. Furthermore, Huffman et al. [3] showed an extreme increase of IN over woodlands during and after rain events, whereby they were able to link these IN to fluorescing biological particles. This leads to the conclusion that the relevance of biological IN might has been highly underrated in the past.
In general, cold temperatures cause stress to plants. This is particularly true for the vegetation at the Northern Timberline. Here plants experience night frost even during warm seasons. Therefore those plants had to develop coping mechanisms to prevent frost damages inside their cells. This leads to special interaction mechanisms between plants and water, making them highly interesting for our research. We examined various plant materials originating from the Northern Timberline, concentrating for the most part on berries and pollen. Hereby we mostly worked with extracts and juices without visible particulate content. Therefore it is safe to assume that the regarded ice nuclei are either macromolecular or nanoparticles. Additionally, selected samples have been chemically aged in the laboratory and then again examined in regards of ice nucleation activity, which gives a closer proximity to atmospheric processes. Our results show how far the range of ice nucleation active plant samples is. Further they indicate that at least some of the plants show close relations in the chemical nature of their expressed ice nuclei.
[1] Pratt, K. A., DeMott, P. J., French, J. R., Wang, Z., Westphal, D. L., Heymsfield, A. J., Twohy, C. H., Prenni, A. J., Praether, K.A. (2009). Nat. Geosci., 2, 398 - 401.
[2] Pummer, B., Bauer, H., Bernardi, J., Bleicher, S., Grothe, H. (2012). Atmos. Chem. Phys., 12, 2541 -2550.
[3] Huffman J.A., Prenni A.J., DeMott P.J., Pöhlker C., Mason R.H., Robinson N.H., Frohlich-Nowoisky J., Tobo Y., Després V.R., Garcia E., Gochis D.J., Harris E., Müller-Germann I., Ruzene C., Schmer B., Sinha B., Day D.A., Andreae M.O., Jimenez J.L., Gallagher M., Kreidenweis S.M., Bertram A.K., Pöschl U. (2013),. Atmos. Chem. Phys. Vol. 13, 6151 - 6164.
