Similar freezing spectra of particles on plant canopies as in air at a high-altitude site

Abstract. Plant canopies are an important source of biological particles aerosolized into the atmosphere. Certain aerosolized microorganisms are able to efficiently freeze slightly supercooled cloud droplets and therefore affect mixed-phase cloud development. Still, spatiotemporal variability of such biological ice nucleating particles (INPs) is currently poorly understood. Here, we study this variability between late summer and leaf shedding on the scale of individual leaves collected about fortnightly from four temperate broadleaf tree species (Fagus sylvatica, Juglans regia, Prunus avium and Tilia platyphyllos) on a hillside (Gempen, 650 m a.s.l.) and in a vertical canopy profile of one Fagus sylvatica (Hölstein, 550 m a.s.l.) in north-western Switzerland. The cumulative concentration of INPs active at ≥ -10 °C (INP_-10) did not vary significantly between the investigated tree species, but as inferred from leaf mass per area and leaf carbon isotopic ratios seemed to be lower on more exposed leaves. Between August and mid-November, median INP_-10 concentration increased from 4 INPs cm^-2 leaf area to 38 cm^-2 leaf area and was positively correlated with mean relative humidity throughout 24 h prior to sampling (Spearman’s r = 0.52, p < 0.0001, n = 64). In 53 of the in total 64 samples collected at Gempen, differential INP spectra between -3 °C and -10 °C exhibited clearly discriminable patterns: in 53 % of the spectra, the amount of additionally activated INPs increased persistently with each 1 °C decrease in temperature; the remaining spectra displayed significant peaks in differential INP concentration above -9 °C, most frequently in the temperature interval between -8 °C and -9 °C (21 %), and between -7 °C and -8 °C (17 %). Interestingly, the three most frequent patterns in differential INP spectra on leaves in Gempen were also prevalent in similar fractions in air samples with clearly discriminable patterns at the high-altitude site Jungfraujoch (3580 m a.s.l., Switzerland) collected during summer in the previous year. These findings corroborate the idea that a large fraction of the airborne biological INP population above the Alps during summer originates from plant surfaces. The inquiry into which parameter, or set of parameters, could affect biological INP populations on both scales – upwind airsheds of high-altitude sites as well as individual leaves – is an intriguing question for further exploration. A first guess is that leaf wetness duration plays a role.