An improved approach for measuring immersion freezing in large droplets over a wide temperature range

Immersion freezing (ice nucleation by particles immersed in supercooled water) is a key process for forming ice in mixed-phase clouds. Immersion freezing experiments with particles in microliter-sized (millimeter-sized) water droplets are often applied to detecting very small numbers of ice nucleating particles (INPs). However, the application of such large droplets remains confined to the detection of INPs active at temperatures much higher than the homogeneous freezing limit, because of artifacts related to freezing of water droplets without added INPs at temperatures of -25 °C or higher on a supporting substrate. Here I report a method for measuring immersion freezing in super-microliter-sized droplets over a wide temperature range. To reduce possible artifacts, droplets are pipetted onto a thin layer of Vaseline and cooled in a clean booth. In the Cryogenic Refrigerator Applied to Freezing Test (CRAFT) system, freezing of pure (Milli-Q) water droplets are limited at temperatures above -30 °C. An intercomparison of various techniques for immersion freezing experiments with reference particles (Snomax and illite NX) demonstrates that despite the use of relatively large droplets, the CRAFT setup allows for evaluating the immersion freezing activity of the particles over almost the entire temperature range (about -30 °C to 0 °C) relevant for mixed-phase cloud formation.