Concerning the nucleation of ice by organics

The atmosphere contains a wide variety of organic compounds, which exhibit widely varying ice nucleation abilities. We report the results from a set of 3 experiments, which suggest guidelines for the treatment of ice nucleation by organics. First, we have used infrared spectroscopy to probe the process at the molecular level. These results suggest that with certain organic species as catalysts, the nucleation of ice could be preceded by changes in the structure of the surface water and the adjacent organic molecules. In a second experiment, we have repeatedly frozen drops of NaCl and (NH4)2SO4 solutions covered with organic films in order to study the effects of the organics and solutes in tandem. The results generally support the hypothesis that for any particular pair of organic and solute species, when taken over a range of solute concentrations, the freezing temperatures will be uniformly lower (i.e., a constant offset) than the melting points. Lastly, we have performed similar repeated drop freezings, yet with exposure to O3. The results suggest guidelines for determining which organic species could, by exposure to O3, have their ice nucleating abilities affected.