Abstract
Abstract. Ice-nucleating particles (INPs) initiate ice formation in supercooled clouds, typically starting in western Europe at a few kilometres above the ground. However, little is known about the concentration and composition of INPs in the lower free troposphere (FT). Here, we analysed INPs active at −10 ∘C (INP−10) and −15 ∘C (INP−15) that were collected under FT conditions at the high-altitude observatory Jungfraujoch between January 2019 and March 2021. We relied on continuous radon measurements to distinguish FT conditions from those influenced by the planetary boundary layer. Median concentrations in the FT were 2.4 INP−10 m−3 and 9.8 INP−15 m−3, with a multiplicative standard deviation of 2.0 and 1.6 respectively. A majority of INPs were deactivated after exposure to 60 ∘C; thus, they probably originated from certain epiphytic bacteria or fungi. Subsequent heating to 95 ∘C deactivated another 15 % to 20 % of the initial INPs, which were likely other types of fungal INPs that might have been associated with soil organic matter or with decaying leaves. Very few INP−10 withstood heating to 95 ∘C, but on average 20 % of INP−15 in FT samples did so. This percentage doubled during Saharan dust intrusions, which had practically no influence on INP−10. Overall, the results suggest that aerosolised epiphytic microorganisms, or parts thereof, are responsible for the majority of primary ice formation in moderately supercooled clouds above western Europe.
Highlights
The free troposphere (FT) refers to a part of the troposphere that is only occasionally in exchange with the Earth’s surface, whereas the planetary boundary layer (PBL) continuously exchanges particles with surface sources and sinks
The icenucleating particles (INPs) sampled at Jungfraujoch under FT conditions likely originated from the northern part of western Europe and from the North Atlantic (Fig. 2)
McCluskey et al (2018) found a mean of 1.1 INP−15 m−3 in pristine marine air masses arriving at the west coast of Ireland, which was 1/10 of the median that we found at Jungfraujoch
Summary
The free troposphere (FT) refers to a part of the troposphere that is only occasionally in exchange with the Earth’s surface, whereas the planetary boundary layer (PBL) continuously exchanges particles with surface sources and sinks. Particle populations sampled at a high-altitude mountain station under FT conditions constitute a mixture of many sources and sinks active on a continental scale (Herrmann et al, 2015, Sun et al, 2021). Continuous measurements by Brunner et al (2021b) found the monthly median of INP−30 in the FT background to peak in April and to be lowest in December
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