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.
Ice nucleation in cold clouds is a decisive step in the formation of rain and snow. Observations and modelling suggest that variations in the concentrations of ice nucleating particles (INPs) affect timing, location and amount of precipitation. A quantitative description of the abundance and variability of INPs is crucial to assess and predict their influence on precipitation. Here we used the hydrological indicator δ18O to derive the fraction of water vapour lost from precipitating clouds and correlated it with the abundance of INPs in freshly fallen snow. Results show that the number of INPs active at temperatures ≥ −10 °C (INPs−10) halves for every 10% of vapour lost through precipitation. Particles of similar size (>0.5 μm) halve in number for only every 20% of vapour lost, suggesting effective microphysical processing of INPs during precipitation. We show that INPs active at moderate supercooling are rapidly depleted by precipitating clouds, limiting their impact on subsequent rainfall development in time and space.
There has been increasing interest in ice nucleation research in the last decade. To identify important gaps in our knowledge of ice nucleation processes and their impacts, two international workshops on ice nucleation were held in Vienna, Austria in 2015 and 2016. Experts from these workshops identified the following research needs: (1) uncovering the molecular identity of active sites for ice nucleation; (2) the importance of modeling for the understanding of heterogeneous ice nucleation; (3) identifying and quantifying contributions of biological ice nuclei from natural and managed environments; (4) examining the role of aging in ice nuclei; (5) conducting targeted sampling campaigns in clouds; and (6) designing lab and field experiments to increase our understanding of
Abstract. We present the laboratory results of immersion freezing efficiencies of
cellulose particles at supercooled temperature (T) conditions. Three types of
chemically homogeneous cellulose samples are used as surrogates that
represent supermicron and submicron ice-nucleating plant structural
polymers. These samples include microcrystalline cellulose (MCC), fibrous
cellulose (FC) and nanocrystalline cellulose (NCC). Our immersion freezing
dataset includes data from various ice nucleation measurement techniques
available at 17 different institutions, including nine dry dispersion
and 11 aqueous suspension techniques. With a total of 20 methods, we
performed systematic accuracy and precision analysis of measurements from
all 20 measurement techniques by evaluating T-binned (1 ∘C)
data over a wide T range (−36 ∘C <T<-4 ∘C). Specifically, we intercompared the geometric surface
area-based ice nucleation active surface site (INAS) density data derived from
our measurements as a function of T, ns,geo(T). Additionally, we also
compared the ns,geo(T) values and the freezing spectral slope parameter
(Δlog(ns,geo)/ΔT) from our measurements to previous
literature results. Results show all three cellulose materials are
reasonably ice active. The freezing efficiencies of NCC samples agree
reasonably well, whereas the diversity for the other two samples spans
≈ 10 ∘C. Despite given uncertainties within each
instrument technique, the overall trend of the ns,geo(T) spectrum traced
by the T-binned average of measurements suggests that predominantly
supermicron-sized cellulose particles (MCC and FC) generally act as more
efficient ice-nucleating particles (INPs) than NCC with about 1 order of
magnitude higher ns,geo(T).
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