June 29, 2000

1:00 p.m.

Fisher 138

Dr. William H. Cantrell II

Department of Chemistry

Indiana University, Bloomington

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 article. 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.

How much more information is needed to accurately predict the CCN or IN distribution once the number distribution of aerosol particles is known? Is the bulk chemical composition enough, or is a complete chemical and morphological characterization of every aerosol particle required? I will discuss some recent measurements from the Indian Ocean Experiment which clarify those questions as they apply to the vapor-liquid transition, and will present results from laboratory experiments pertaining to the liquid-solid transition.