The Surface of Tree Tissues as Source of Extractable Ice Nucleating Macromolecules during Rainfall Events

Several biological particles are able to trigger heterogeneous ice nucleation at subzero temperatures above -38°C. Many plants species such as winter rye [1], certain berries [2], pines and birches [3, 4] are known to contain biological ice-nucleating particles (BINPs) or rather ice-nucleating macromolecules (INMs). However, the influence of these BINPs on atmospheric processes including cloud glaciation and precipitation formation, as well as transport mechanisms of BINPs from the land surface into the atmosphere remain uncertain. If those INMs are easily available on the surfaces of a plant, they could be washed down by heavy rain events and could add an important new source for BINPs in the atmosphere, which has not received enough attention in the past.

In this study, we have focused on alpine trees, which form INMs extractable from their surfaces. We examined ice nucleation activity of samples from different birches (Betula pendula) and pines (Pinus sylvestris) growing in the Alps in Austria, Europe. Filtered aqueous extracts of leaves, needles, bark and wood were analyzed in the laboratory in terms of heterogeneous ice nucleation using VODCA (Vienna Optical Droplet Crystallization Analyzer), a cryo-microscope  for  emulsion  samples.  All plant tissues contained INMs in the submicron size range. Furthermore, we conducted a field experiment, in which we investigated the possibility of INMs to be released from the surface of the trees into the atmosphere during rain showers.

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