An optical ice microscope apparatus equipped with a sealed cooling stage and CCD camera was used to examine contact freezing events between a water droplet and ice nucleating particles (INP) containing either fresh or oxidized organic hydrocarbons. Samples were characterized using Fourier Transfer Infrared Spectroscopy and Raman Microspectroscopy. The organic hydrocarbons considered were octacosane, squalane, and squalene. All of the samples proved to be moderately efficient ice nuclei that induced freezing between -23 to -26 ?C regardless of whether the INP was solid or liquid. Oxidation was shown to affect each INP differently, unlike what is seen in previous group results (Fornea et al., 2009; Brooks et al., 2014). Additionally, phase differences between compounds appear to correspond to whether oxidation improves ice nucleation efficiency. Oxidation of solid samples from previous work and new results indicate an improvement in ice nucleation efficiency by 1 to 2 ?C. Ice nucleation results for liquid samples suggest varied effects on ice nucleation efficiency with oxidation depending on molecular process. Changes in sample viscosity as a result of oxidation of liquids appear to affect the ice nucleation ability. Exposure of liquid INP to oxidation resulted in an increase in viscosity for squalene and a decrease for squalane. A decrease in viscosity after oxidation corresponds to a decrease in ice nucleation ability of 1 to 2 ?C. An increase in viscosity after oxidation is associated with an increase in ice nucleation ability by about 2-3 ?C. Our results suggest that liquids can act as ice nuclei, plausibly due to a decrease in the energy barrier brought about by the flexible nature of the molecules.
