Preprints
https://doi.org/10.5194/acp-2021-879
https://doi.org/10.5194/acp-2021-879

  15 Nov 2021

15 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Measurement report: Ice nucleating particles active ≥ −15 °C in free tropospheric air over western Europe

Franz Conen1, Annika Einbock1, Claudia Mignani1, and Christoph Hüglin2 Franz Conen et al.
  • 1Department of Environmental Sciences, University of Basel, CH-4056 Basel, Switzerland
  • 2Laboratory for Air Pollution / Environmental Technology, Empa, 5 CH-8600 Dübendorf, Switzerland

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.

Franz Conen et al.

Status: open (until 27 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Franz Conen et al.

Franz Conen et al.

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Short summary
Above western Europe ice typically starts to form in clouds a few kilometers above ground, if suitable aerosol particles are present. We found such particles in air masses typical for that altitude to originate most likely from bacteria and fungi living on plants. Occasional Saharan dust intrusions seem to add only little to the number concentration of particles able to freeze cloud droplets between 0 °C and −15 °C.
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