Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing.

An optical ice microscope apparatus equipped with a sealed cooling stage and a CCD camera was used to examine contact freezing events between a water droplet and ice nucleating particles (INP) containing organic hydrocarbons including octacosane, squalane, and squalene. Sample viscosities were measured with a capillary viscometer and compositions were characterized using Fourier transfer infrared spectroscopy with horizontal attenuated total reflectance and Raman microspectroscopy. All of the samples proved to be moderately efficient ice nuclei that induced freezing between -23 and -26 °C, regardless of whether the INP was solid or liquid. At their ice nucleating temperatures, the viscosity of the liquid samples (squalane and squalene) was 0.6 P or greater. Oxidation increased the viscosity of squalene to over 1330 P, but decreased the viscosity of squalane to 0.07 P at room temperature. Most importantly, our results demonstrate that even moderately viscous liquids in contact with water droplets can act to catalyze freezing, plausibly by providing a flexible template which decreases the energy barrier to ice nucleation. The simple soccer ball model of nucleation theory was used to derive the probability of freezing and nucleation rate coefficients as a function of temperature for each type of INP.