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Apr 5, 2004: 4 PM The physics and chemistry of phase transitions in Earth's
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Water is present in all three phases in Earth's atmosphere. However, the phase transition to either liquid or ice must be facilitated by a pre-existing aerosol particle. Aerosol particles in the atmosphere outnumber cloud drops by at least an order of magnitude, indicating that specific physical and chemical characteristics make it more likely for some aerosol particles to act as these nucleating agents. Since every cloud drop must pass through this intermediate step, understanding which physicochemical properties of the aerosol particles make them effective as either cloud condensation nuclei (CCN), in the case of the vapor-liquid transition, or ice nuclei (IN), in the case of the liquid-solid transition, is crucial to understanding how clouds are born and evolve. I will present results from the Indian Ocean Experiment which indicate that we understand the vapor-liquid transition fairly well. (I will explain why the caveat fairly is present in the previous sentence.) I will also present results from laboratory studies that show we are still far from understanding freezing.