Abstract
Heterogeneous ice nucleation plays an important role in many environmental processes such as ice cloud formation, freezing of water bodies or biological freeze protection in the cryosphere. New information is needed about the seasonal availability, nature, and activity of ice nucleating particles (INPs) in alpine environments. These INPs trigger the phase transition from liquid water to solid ice at elevated subzero temperatures. We collected water samples from a series of alpine rivers and lakes (two valleys and their rivers, an artificial pond, and a natural lake system) in Obergurgl, Austria in June 2016, July 2016, November 2016, and May 2017. Each alpine river and lake was sampled multiple times across different seasons, depending on site access during different times of the year. Water samples were filtered through a 0.22 μm membrane filter to separate microbial INPs from the water, and both fractions were analyzed for ice nucleation activity (INA) by an emulsion freezing method. Microorganisms were cultured from the filters, and the cultures then analyzed for INA. Portions of the filtered samples were concentrated by lyophilization to observe potential enhancement of INA. Two sediment samples were taken as reference points for inorganic INPs. Sub-micron INPs were observed in all of the alpine water sources studied, and a seasonal shift to a higher fraction of microbial ice nucleators cultured on selective media was observed during the winter collections. Particles larger than 0.22 μm showed INA, and microbes were cultured from this fraction. Results from 60 samples gave evidence of a seasonal change in INA, presence of submicrometer INPs, and show the abundance of culturable microorganisms, with late spring and early summer showing the most active biological INPs. With additional future research on this topic ski resorts could make use of such knowledge of geographical and seasonal trends of microbial INPs in freshwater habitats in order to improve the production of artificial snow.
Highlights
Freshwater habitats are known as possible sources of ice nucleating particles (INPs) for the atmosphere
The highest and lowest freezing temperatures were observed within the river sample set
We found INPs with a broad range of activity in the investigated water sources ranging from a T50 of 233.8 K which is below the homogeneous nucleation temperature of our system up to a T50 of 250.8 K
Summary
Freshwater habitats are known as possible sources of ice nucleating particles (INPs) for the atmosphere. Wave action in lakes was shown to produce a considerable amount of aerosol (Slade et al, 2010) with inorganic (Axson et al, 2016; May et al, 2016), and biological (May et al, 2018) particles alike. Weirs have been reported to produce aerosol from rivers (Larsen et al, 2017; Knackstedt et al, 2018), and fast flow over natural steps and rocks likely produces similar results. Benson et al (2019) developed a drone water sampling system to study INPs in remote freshwater environments, and Bieber et al (2020) designed a drone air sampling system to collect airborne INPs. Weirs have been reported to produce aerosol from rivers (Larsen et al, 2017; Knackstedt et al, 2018), and fast flow over natural steps and rocks likely produces similar results. Systems such as these could help probe INPs in remote alpine environments in the future
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