Abstract
Abstract. For decades, drop-freezing instruments have contributed to a better understanding of biological ice nucleation and its likely implications for cloud and precipitation development. Yet, current instruments have limitations. Drops analysed on a cold stage are subject to evaporation and potential contamination. The use of closed tubes provides a partial solution to these problems, but freezing events are still difficult to be clearly detected. Here, we present a new apparatus where freezing in closed tubes is detected automatically by a change in light transmission upon ice development, caused by the formation of air bubbles and crystal facets that scatter light. Risks of contamination and introduction of biases linked to detecting the freezing temperature of a sample are then minimized. To illustrate the performance of the new apparatus we show initial results of two assays with snow samples. In one, we repeatedly analysed the sample (208 tubes) over the course of a month with storage at +4 °C, during which evidence for biological ice nucleation activity emerged through an increase in the number of ice nucleators active around −4 °C. In the second assay, we indicate the possibility of increasingly isolating a single ice nucleator from a precipitation sample, potentially determining the nature of a particle responsible for a nucleation activity measured directly in the sample. These two seminal approaches highlight the relevance of this handy apparatus for providing new points of view in biological ice nucleation research.
Highlights
Certain particles suspended in the atmosphere provide surfaces for nucleating ice in rising and cooling air masses
We present the example of an assay conducted with this new apparatus that would not have been possible to achieve with previous instruments and, in addition, we describe another interesting application that could be realized with it in the future
We have developed the traditional immersion freezing nucleation method further by detecting the phase change from liquid to ice in closed test tubes through the reduction of light transmission upon freezing
Summary
Certain particles suspended in the atmosphere provide surfaces for nucleating ice in rising and cooling air masses. A sample (melted snow, rain or cloud water, impinger liquid with trapped aerosol) is divided into aliquots in the form of small drops on plates or larger aliquots in tubes These are allocated in a cooling bath where temperature decreases over time. Pure substances with a known freezing temperature range were tested to assess the repeatability in the detection of nucleation events For this purpose we repeated five times the analysis of the same array of 52 tubes containing 200 μL of sample each. Samples tested were montmorillonite (at the concentration of 50 μg mL−1) and SNOMAX® (0.1 μg mL−1), showing a freezing temperature range from −7.1 ◦C to −13.0 ◦C (median −11.9 ◦C) and from −4.3 ◦C to −5.4 ◦C (median −5.0 ◦C), respectively For both substances, the median value of the standard deviation in freezing temperatures of repeatedly frozen individual tubes was 0.20 ◦C, comparable to the value reported in Vali (2008) for a soil suspension. We can say that the precision of the apparatus (1 standard deviation) must be smaller than 0.2 ◦C
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