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

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.

Ice-nucleating particles near two major dust source regions

“…The increased n INP post heat and peroxide treatment is an unexpected result given previous studies on treated soil dust measurements (Conen et al, 2011;Hill et al, 2016;O'Sullivan et al, 2014;Tobo et al, 2014). However, increases in IN activity after heat treatment have been reported previously for airborne Saharan desert dust and aerosol collected during Saharan dust intrusions (Boose et al, 2019;Conen et al, 2022), as well as SSA and precipitation (Martin et al, 2019;McCluskey et al, 2018a), and should be further investigated in future studies. An increase in IN activity after peroxide treatment has also been reported in a Himalayan dust sample (Paramonov et al, 2018).…”

Export of ice-nucleating particles from watersheds: results from the Amazon and Tocantins river plumes

“…Droplets thus produced can be smaller than 50 µm [ 45 ] and get lofted above a canopy [ 46 ], where they evaporate in the drier air [ 47 ] and enrich it with the INPs [ 48 , 49 ] that were initially contained in the splash droplets. Whatever the mechanism, the most likely source of INPs aerosolized by raindrops impinging on the canopy would be ice-nucleation active epiphytic microorganisms, most of which are heat sensitive and only some are moderately heat tolerant [ 20 ]. Heat-sensitive INPs washed by rain from canopies will mix on the ground with a probably much larger pool of INPs that themselves are very unlikely to be transported through and above the canopy of the tropical rainforest.…”

“…Droplets thus produced can be smaller than 50 µm [45] and get lofted above a canopy [46], where they evaporate in the drier air [47] and enrich it with the INPs [48,49] that were initially contained in the splash droplets. Whatever the mechanism, the most likely source of INPs aerosolized by raindrops impinging on the canopy would be ice-nucleation active [20].…”

“…We expected differences in land cover between both watersheds to result in different INP populations in the plumes of the Amazon and Tocantins rivers. These differences should emerge primarily in INPs active at −15°C or above (INP −15 ), because most INP −15 are of biological origin and are ultimately derived from plants and associated microorganisms [18][19][20].…”

Ice-nucleating particles from multiple aerosol sources in the urban environment of Beijing under mixed-phase cloud conditions

“…Previous studies have confirmed that several kinds of atmospheric particles, including mineral dusts, carbonaceous particles, and biological species, can act as immersion INPs (Hoose and Möhler, 2012;Murray et al, 2012;Kanji et al, 2017, and references therein). When present at atmospherically relevant amounts in droplets, mineral dusts mostly catalyze supercooled aqueous droplet freezing below −15 • C (Hoose and Möhler, 2012, and references therein;Murray et al, 2012;Kanji et al, 2017, and references therein), while biological species, such as pollen, fungal spores, and viruses, generally exhibit immersion IN activ-ity below −5 • C and are fully activated below −10 to −20 • C (e.g., Chou, 2011;Conen et al, 2015;Polen et al, 2016;Kanji et al, 2017;Conen et al, 2022;Porter et al, 2022). The reported atmospheric immersion INP number concentration (N INP ) values were measured between −5 and −38 • C and were normally on the order of 10 −2 to 10 3 # L −1 (e.g., Rogers et al, 1998;DeMott et al, 2010;Chen et al, 2018;Porter et al, 2022).…”