[1] Nucleation of ice crystals in the atmosphere often occurs through heterogeneous freezing processes in which an atmospheric aerosol acts as the ice nuclei (IN). Depending on the ambient conditions and composition of the available IN, heterogeneous nucleation may occur through one of several freezing mechanisms, including contact and immersion. In this study, an optical microscope apparatus equipped with a cooling stage and a digital camera was used to observe the freezing events of individual droplet-IN samples. For each experiment, a particular IN was either placed in contact with the surface or immersed in the bulk of an ultra pure water droplet. Using volcanic ash as the IN, we observed that inside-out contact freezing occurred at warmer temperatures than immersion freezing. We also conducted contact freezing experiments using three representative aerosol types as the potential IN. The most effective contact freezing IN was Pahokee Peat soil with an average freezing temperature of À10.5°C, followed by volcanic ash (À11.2°C), and finally soot (À25.6°C). In addition, we have used classical nucleation theory to derive the heterogeneous nucleation rates for the IN compositions explored.

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