The ice nucleation activity of pollen from silver birch (Betula pendula), pines (e.g. Pinus sylvestris) and other trees has been assigned not only to pollen grains but also to subpollen particles (SPP) and extractable macromolecules, i.e. ice-nucleating macromolecules (INMs) (Pummer et al., 2012). The number concentration of pollen in comparison to other ice-nucleating particles suggests a minor impact to atmospheric cloud glaciation (Hoose et al., 2010). When focusing on macromolecules, the importance of INMs from vegetation, however, needs to be re-evaluated in respect to atmospheric ice nucleation. It has been shown that INMs are present in nearly every tissue of birches (Felgitsch et al., 2018) and furthermore, that the macromolecules are extracted from the surface, when they come into contact with water (Seifried et al., 2020). We hypothesize that extractable INMs from tree surfaces are emitted during rainfall by splash induced emissions and field experiments were performed to evaluate the amount of INMs extracted by rain-droplets. Sampled rainwater, which was splashed off from birch surfaces, revealed INMs in high number concentration (108 cm-2) and can be attributed to the vegetation surface (Seifried et al., 2020). To further investigate emission sources an aerosol sampling tool (including an impinger and an impactor) has been developed and mounted on two rotary-wing drones (Bieber et al., 2020). Aerosol samples were collected in an alpine environment on ground level and above the canopy of birches and pines. We found that the bioaerosol concentration increased after rainfall and collected INMs show a similar onset freezing temperature as birch surface extracts (around -20°C). Microscopic images revealed a fluorescent organic film on aerosol particles, which might be linked to extractable material from bio-surfaces. We suggest splash induced aerosolization of INMs during rainfall to be an underestimated source for atmospheric cloud glaciation, since INMs can easily be carried on larger aerosol particles, e.g. on SPP or on mineral dust particles.
Pummer, B. G., Bauer, H., Bernardi, J., Bleicher, S., and Grothe, H.: Suspendable macromolecules are responsible for ice nucleation activity of birch and conifer pollen, Atmos. Chem. Phys., 12, 2541–2550, https://doi.org/10.5194/acp-12-2541-2012, 2012.
Hoose, C., J. E. Kristjánsson, and S. M. Burrows.: How important is biological ice nucleation in clouds on a global scale?, Environ. Res. Lett., 5, https://doi.org/10.1088/1748-9326/5/2/024009, 2010.
Felgitsch, L., Baloh, P., Burkart, J., Mayr, M., Momken, M. E., Seifried, T. M., Winkler, P., Schmale III, D. G., and Grothe, H.: Birch leaves and branches as a source of ice-nucleating macromolecules, Atmos. Chem. Phys., 18, 16063–16079, https://doi.org/10.5194/acp-18-16063-2018, 2018.
Seifried, T. M., Bieber, P., Felgitsch, L., Vlasich, J., Reyzek, F., Schmale III, D. G., and Grothe, H.: Surfaces of silver birch (Betula pendula) are sources of biological ice nuclei: in vivo and in situ investigations, Biogeosciences, 17, 5655–5667, https://doi.org/10.5194/bg-17-5655-2020, 2020.
Bieber, P.; Seifried, T.M.; Burkart, J.; Gratzl, J.; Kasper-Giebl, A.; Schmale, D.G., III; Grothe, H. A Drone-Based Bioaerosol Sampling System to Monitor Ice Nucleation Particles in the Lower Atmosphere. Remote Sens., 12, 552, 2020.