Abstract
In alpine environments, many plants, bacteria, and fungi contain ice nuclei (IN) that control freezing events, providing survival benefits. Once airborne, IN could trigger ice nucleation in cloud droplets, influencing the radiation budget and the hydrological cycle. To estimate the atmospheric relevance of alpine IN, investigations near emission sources are inevitable. In this study, we collected 14 aerosol samples over three days in August 2019 at a single site in the Austrian Alps, close to a forest of silver birches, which are known to release IN from their surface. Samples were taken during and after rainfall, as possible trigger of aerosol emission by an impactor and impinger at the ground level. In addition, we collected aerosol samples above the canopy using a rotary wing drone. Samples were analyzed for ice nucleation activity, and bioaerosols were characterized based on morphology and auto-fluorescence using microscopic techniques. We found high concentrations of IN below the canopy, with a freezing behavior similar to birch extracts. Sampled particles showed auto-fluorescent characteristics and the morphology strongly suggested the presence of cellular material. Moreover, some particles appeared to be coated with an organic film. To our knowledge, this is the first investigation of aerosol emission sources in alpine vegetation with a focus on birches.
Highlights
Many organisms commonly found in the Alps, ranging from small pathogenic bacteria e.g., Pseudomonas syringae to large vascular plants, e.g., birch trees, are known to contain ice nuclei (IN) [1,2,3]
We discovered that ice nucleating macromolecules (INMs) are extracted from the surface of birches by rain [38]
The specific objectives of our work were to: (i) investigate the ice-nucleation activity of aerosols sampled at different altitudes in an alpine environment near major sources of vegetation, (ii) examine potential associations of IN concentrations in alpine environments with meteorological parameters, and (iii) characterize biological aerosol particles according to their morphology using fluorescence and scanning electron microscopy (SEM)
Summary
Many organisms commonly found in the Alps, ranging from small pathogenic bacteria e.g., Pseudomonas syringae to large vascular plants, e.g., birch trees, are known to contain ice nuclei (IN) [1,2,3]. The main question tackled by the scientific community is whether IN from alpine vegetation especially with the presence of frost-resistant plants can be aerosolized and reach altitudes as high as the free troposphere. IN could trigger ice crystal formation of water droplets, which could lead to cloud glaciation, if respective altitudes are reached. In the absence of IN, cloud droplets would freeze homogeneously at temperatures below −38 ◦C [4]. The formed ice clouds do influence the Earth’s radiation budget by interacting with solar radiation [5,6], and play a key role in precipitation formation [7,8,9]. If precipitation reaches the land surface as rain, IN from biological surfaces can be aerosolized and transported further. Already in the 1960s, studies have shown that heavy rain events and storms are correlated with high
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