Abstract
Ice nucleating particles (INP) initiate ice formation in supercooled clouds, typically starting at a few km above ground. However, little is known about the concentration and composition of INP in the lower free troposphere (FT). Here, we analysed INP active at −10 °C (INP−10) and −15 °C (INP−15) collected during FT conditions at the high-altitude observatory Jungfraujoch. 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 INP was deactivated after exposure to 60 °C, thus probably originated from certain epiphytic bacteria or fungi. Subsequent heating to 95 °C deactivated another 15 % to 20 % of the initial INP, likely other types of fungal INP that might be 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
20 Free troposphere (FT) designates a part of the troposphere that only occasionally is in exchange with Earth's surface, whereas the planetary boundary layer (PBL) continuously exchanges particles with surface sources and sinks
We relied on continuous radon measurements to distinguish free troposphere (FT) conditions from those influenced by the planetary boundary layer
The Ice nucleating particles (INP) sampled at Jungfraujoch during FT conditions likely originated from the 95 northern part of western Europe and from the North Atlantic (Fig. 2)
Summary
20 Free troposphere (FT) designates a part of the troposphere that only occasionally is in exchange with Earth's surface, whereas the planetary boundary layer (PBL) continuously exchanges particles with surface sources and sinks. A community of airborne microorganisms, for example, is in the PBL mainly composed of organisms from within a distance of several tens of kilometers (TignatPerrier et al, 2019). Particle populations sampled at a high-altitude mountain station under FT conditions. A special kind of aerosol particles, so-called ice-nucleating particles (INP), is relevant for primary ice formation in mid-level clouds (Findeisen, 1938). Ice formation starts in the midlatitudes a few kilometers above ground at around 5 °C (Kanitz et al, 2011). In FT conditions at the high-altitude observatory Jungfraujoch (3580 m a.s.l.) in the Swiss Alps, Lacher et al (2018) found similar
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