This award supports laboratory experiments on ice nucleation, the distortion and shattering of droplets as they freeze, the light-scattering properties of ice crystals, and the chemistry of the ice-crystal surface. Specific projects include (1) determining the homogeneous freezing temperatures of solution droplets containing salts such as ammonium sulfate, thought to be typical of the droplets found in the lower stratosphere; (2) determining the heterogeneous nucleation properties of bacteria found in sea ice; (3) examining the conditions under which freezing drops become deformed and shatter; (4) measuring the scattering phase functions of ice crystals for comparison with theoretical calculations; (5) measuring the pH at the surface of ice formed by different processes and its dependence on the presence of impurities and trace gases. The work on droplet freezing employs a freezing-tube, in which the droplet slowly descends through air with accurately controlled temperature and humidity to approximate conditions in the high atmosphere. The studies of ice crystal growth and scattering employ a cloud chamber with an electrodynamic balance that permits the observation of individual, levitated crystals for long periods of time as they grow or evaporate in controlled conditions of temperature, pressure, and humidity. The work is motivated by the need for a better understanding of the microphysical characteristics of cirrus clouds - how they form, grow, and affect the transfer of solar radiation. Discrepancies currently exist between the measured and computed effects of clouds on sunlight, using different radiative-transfer models. The laboratory measurements of light scattering by crystals with well characterized sizes, shapes, and orientations will enable evaluation of the different scattering models. Results will also provide an improved characterization of the rate at which ice crystals form, as a function of droplet temperature, size, and chemical composition, which will be useful in large-scale atmospheric models.
