A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of seventeen ice nucleation measurement techniques

1 S1. Supplementary methods 1 2 This supplementary information provides additional details for the measurement 3 techniques of immersion freezing of illite NX particles with S1.1. suspension techniques and 4 S1.2. dry-dispersed particle measurement techniques (both in alphabetical order as in Table 5 1). The discussions of measurement uncertainties of temperature and n s for each measurement 6 technique are also provided. We note that the uncertainty in frozen fraction (α) used in 7 calculating n s may not be adequate, since the sensitivity of Δα (an increase or a decrease in 8 frozen fraction) is much higher at high temperatures which unexceptionally coincide with a 9 low fraction of frozen illite NX. 14 15 The BINARY setup is an optical freezing apparatus that makes use of the change in 16 droplet brightness during freezing for the automated and simultaneous detection of ice 17 nucleation in 36 microliter-sized droplets. The droplets are positioned on a hydrophobic glass 18 slide that rests on top of a Peltier cooling stage (Linkam LTS 120). The 36 droplets are 19 separated from each other by a polydimethylsiloxane (PDMS) spacer in order to prevent a 20 Wegener-Bergeron-Findeisen process. For a particular illite NX concentration (0.1, 0.5, 2, 5 21 and 10 mg/mL based on the amount of suspended mass of illite NX sample per H 2 O volume) 22 at least 3 experiments with 36 drops each were conducted, resulting in a minimum of at least 23 108 freezing events at each concentration. The droplet temperature was calibrated based on 24 phase transition temperatures of several compounds over the range from 0 to-40 °C and for 25 rates between 0.1 and 10 °C min-1. Details of the setup and its temperature calibration are 26 presented elsewhere (Budke and Koop, 2014). In addition to this temperature calibration no 27 further corrections were made to the dataset of observed individual droplet freezing 28 temperatures. However, if any droplet freezing temperatures of a particular concentration 29 were below-25 °C, this concentration was excluded from the analysis. At these temperatures, 30 the derived n s for different illite NX mass concentrations deviate from each other, indicating 31 2 that ice nucleation in these droplets was not induced by illite NX particles, but rather by ice 32 nucleating impurities contained in the water. This lower temperature limit is also in agreement 33 with the observed 25th percentile freezing temperature value …