a New Horizontal Gradient, Continuous Flow, Ice Thermal Diffusion Chamber and Detailed Observation of Condensation - and Deposition Nucleations.

It is well recognized that conditions in the environment surrounding ice nucleus (IN) particles must be accurately controlled in order to understand their nucleation behavior. For the condensation-freezing and deposition mechanisms of ice nucleation, the main factors representing the environment are supersaturation and supercooling (temperature). Starting from the concept of the wedge-shaped ice thermal diffusion chamber of Schaller and Fukuta, and that of the Fukuta -Saxena cloud condensation nucleus (CCN) spectrometer, and by further considering operational efficiency and accuracy, a continuous-flow, horizontal gradient, ice thermal diffusion chamber has been developed. The chamber consists of preprocessing, main activation and ice crystal settling sections. A common top plate is used for all three sections and is held isothermal while a temperature gradient is maintained across the bottom plate of the main section. In the preprocessing and main sections, both plates are covered with ice. A new method is developed to smoothly coat ice on the plates. This configuration of the main section results in a nearly constant temperature horizontally in the direction across the sample flow, and produces a range of supersaturations. Heat pipes are utilized on the sides of the bottom plate to insure temperature uniformity along the direction of the flow. The sample air is sandwiched in the region of maximum supersaturation between layers of filtered and predried air so that the sample achieves a nearly constant supersaturation in the main section without transient supersaturations. The preprocessing section is held isothermal at the top plate temperature. The design and flow of the main section permit the nucleated ice crystals to be carried into the ice crystal settling section without loss. The bottom plate in the settling section is maintained at a temperature slightly lower than that of the top plate. Formation of transient supersaturations at the entrance of the settling section has also been avoided by delaying vapor diffusion while allowing thermal diffusion to proceed. The problem of transient supersaturation development has been examined for shear (Poiseuille) flows using a numerical method. Wall effects have also been estimated for the supersaturation distribution. The stability of the sample flow throughout the entire chamber has been confirmed with smoke tests. A new method has been developed and is employed for ice crystal detection. Mylar copy film (carbon paper) holding condensed water droplets is placed in the settling section. Ice crystals nucleated in the main section fall on the film and grow there to visible sizes. The positions of the ice crystals across the flow direction give the supersaturations and temperatures at which they nucleated. The optimum design of the chamber has been determined. Two numerical models have been developed to compute the temperature and supersaturation fields within the chamber. The first provides the steady state profiles in vertical planes perpendicular to the flow and includes the wall effects. The second gives the steady state profiles in vertical planes parallel to the flow. The use of the newly developed chamber has resulted in following findings. For all the temperature and supersaturation ranges studied, the rate of deposition nucleation is much lower than that for condensation-freezing. For silver iodide (AgI) particles of average diameter 0.25 (mu)m, only a small fraction are active below 1% water supersaturation, while for 1,5 - dihydroxynaphthalene (DN) of average diameter 0.45 (mu)m, a much larger fraction are active. The total number of active nuclei for these samples was approximately three times greater for DN than for AgI at high supersaturations. For temperatures warmer than -10(DEGREES)C, the condensation -freezing mechanism is not effective for AgI. Natural nuclei and kaolinite show some supersaturation dependence with large variances. Additionally, high natural nuclei counts were observed in cloud downdrafts and low counts during precipitation.