Abstract
Abstract. Birch pollen are known to release ice-nucleating macromolecules (INM), but little is known about the production and release of INM from other parts of the tree. We examined the ice nucleation activity of samples from 10 different birch trees (Betula spp.). Samples were taken from nine birch trees in Tyrol, Austria, and from one tree in a small urban park in Vienna, Austria. Filtered aqueous extracts of 30 samples of leaves, primary wood (new branch wood, green in colour, photosynthetically active), and secondary wood (older branch wood, brown in colour, with no photosynthetic activity) were analysed in terms of ice nucleation activity using VODCA (Vienna Optical Droplet Crystallization Analyser), a cryo microscope for emulsion samples. All samples contained ice-nucleating particles in the submicron size range. Concentrations of ice nuclei ranged from 6.7×104 to 6.1×109 mg−1 sample. Mean freezing temperatures varied between −15.6 and −31.3 ∘C; the range of temperatures where washes of birch pollen and dilutions thereof typically freeze. The freezing behaviour of three concentrations of birch pollen washing water (initial wash, 1 : 100, and 1 : 10 000) were significantly associated with more than a quarter of our samples, including some of the samples with highest and lowest activity. This indicates a relationship between the INM of wood, leaves, and pollen. Extracts derived from secondary wood showed the highest concentrations of INM and the highest freezing temperatures. Extracts from the leaves exhibited the highest variation in INM and freezing temperatures. Infrared spectra of the extracts and tested birch samples show qualitative similarity, suggesting the chemical components may be broadly similar.
Highlights
Pure water can typically be supercooled to temperatures below the melting point of ice (0 ◦C at atmospheric pressure) without freezing (Cantrell and Heymsfield, 2005; Hegg and Baker, 2009; Murray et al, 2010)
For most secondary wood extracts, we found slightly higher mean freezing temperature (MFT) than for the primary wood samples
The spectra of the different extracts of TBA (Fig. 7) show a strong resemblance to each other, but we find three main differences. (a) The intensity at 1510 cm−1: while the band is strongly visible in the spectrum of secondary wood extracts, it is much less pronounced in the spectra of primary wood and leaf extracts. (b) The band at 1070 cm−1 is strongest visible for the leaf extract, where it nearly swallows its neighbour at 1110 cm−1, while it is only present as a slight shoulder for the wood extracts. (c) The region of 920 cm−1 and below increases in intensity from leaf extract over primary wood extract to secondary wood extract
Summary
Pure water can typically be supercooled to temperatures below the melting point of ice (0 ◦C at atmospheric pressure) without freezing (Cantrell and Heymsfield, 2005; Hegg and Baker, 2009; Murray et al, 2010). Water molecules have to be arranged in an ice-like pattern and overcome a critical cluster size (Turnball and Fisher, 1949; Cantrell and Heymsfield, 2005). This freezing mechanism, if happening as a stochastic process from the pure liquid, and in the absence of catalysing substances, is called homogeneous ice nucleation (Cantrell and Heymsfield, 2005).
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