Measurement report: The ice-nucleating activity of lichen sampled in a northern European boreal forest

Abstract. Ice nucleating particles (INPs) facilitate heterogeneous freezing of cloud droplets and thus modify cloud properties. Hence, it is important to understand the sources of INPs. During the HyICE-2018 campaign, which took place in the boreal forest of Hyytiälä, substantial concentrations of biological INPs were observed despite many potential biological sources of INPs being snow covered. A potential source of INP that were not covered in snow were lichens that grow on trees, hence we investigated these lichens as a potential source of biological INPs in this boreal forest environment. INPs derived from lichen sampled during HyICE-2018 are shown to nucleate ice at temperatures as warm as -5 °C in aqueous solutions extracted from 0.03 g ml^-1 lichen. In particular, we identify two species of INPs active at -16 and -18 °C and smaller than 2 µm in size, which makes them good candidates for atmospheric INPs. These particles are smaller than whole lichen spores. In species specific tests we show that the four investigated lichen species harbour differently sized INPs, which might suggest that some species of lichen are more important as a source of INPs than others. This study shows that lichen from a European Boreal forest harbour INPs, which may be especially important in habitats where few other biological INP sources are present, such as in a snow covered environment. The great terrestrial abundance of lichen INPs calls for further research to combine their ice nucleating ability with dispersal studies to evaluate the flux of lichenous INPs into the atmosphere as well as to what extent these particles reach heights and locations where they might influence cloud properties.