Cirrus clouds are a widespread atmospheric constituent with major importance for radiative transfer, yet the mechanism of their formation is not well understood. The ice crystals of which they are composed form by nucleation of the solid phase from liquid water droplets or water vapor. Homogeneous nucleation of droplets containing sulfate solutes is thought to be the dominant process, but there is not yet a consistent explanation for the range and variability of thermodynamic conditions at which ice forms in this way. For example, nucleation of ice in droplets containing a mixture of solutes evidently occurs at warmer temperatures than required to freeze single-solute droplets. Therefore the temperature, and hence the altitude, at which cirrus clouds form may be determined by the chemical composition of aerosols dissolved within the droplets. This project is a laboratory study of the freezing of droplets composed of various aqueous solutions thought to be typical of those found in the upper troposphere. To be investigated are (1) the homogeneous freezing of solution droplets having different chemical compositions at temperatures between -35 and -65 degrees C and relative humidities between 70 and 100%, and (2) the heterogeneous freezing of solution droplets containing insoluble particles of the kind that might be present in the upper troposphere. The approach involves generation of well-characterized aerosol particles of various types, determining their ice-nucleating properties, and performing numerical modeling calculations to relate the laboratory measurements to existing theories of homogeneous and heterogeneous ice nucleation. The study will provide the data needed for predicting cirrus cloud formation in large-scale atmospheric models.
