Similar freezing spectra of particles in the phyllosphere as at mixed-phase cloud height

The phyllosphere is a major source of airborne microorganisms. Some of these microorganisms can act as ice nucleating particles (INPs) and initiate droplet freezing in supercooled clouds. Despite their role in this critical atmospheric process, little is known about the spatiotemporal variations of biological INPs at their source. We investigated this variation on the scale of single (or few) leaves about fortnightly from late summer throughout leaf senescence in two lime (Tilia platyphyllos), beech (Fagus sylvatica), cherry (Prunus avium), and walnut (Juglans regia) trees (n = 2 x 4 x 8 = 64) on a hillside (Gempen, 650 m a.s.l.) in north-western Switzerland. The overall result comprising all species shows an increasing trend in the median cumulative concentration of INPs active at -10 °C (INP_-10) from 4 INPs cm^-2 leaf area at the beginning of August to 38 INPs cm^-2 in mid-November. Further, median INP_-10 concentration was positively correlated with relative humidity throughout the 24 h prior to sampling (Spearman’s r = 0.90, p = 0.005, n = 8). Differential INP spectra between -3 °C to -10 °C displayed clearly defined patterns in 53 of the overall 64 samples. In 28 of these 53 samples (53%), the additional number of INPs activated with every 1 °C step in cooling increased steadily with decreasing temperature. In another 21% we observed a significant peak in the temperature step from -8 °C to -9 °C (i.e., around -8.5 °C), and in further 17% a peak around ‑7.5 °C. Interestingly, these types of spectra were similarly often found in air samples with clearly defined pattern (n = 53) at the high-altitude observatory Jungfraujoch (3580 m a.s.l., Switzerland) in summer 2022 (55% steady increase, 17% peak at -8.5 °C, 21% peak at -7.5 °C). This consistency in spectral pattern supports the notion that forests are a major source of biological INPs affecting atmospheric processes. It also prompts the question which parameter – perhaps leaf wetness duration? – could influence the abundance of biological INPs on both scales, on single leaves as well as in the airshed of a high-altitude observatory.