9711969 Tolbert In this project, laboratory studies of particle formation and heterogeneous chemical reactions under conditions relevant for the upper troposphere/lower stratosphere will be performed. The composition, formation mechanism and abundance of polar stratospheric clouds (PSCs) remain uncertain, and the role of heterogeneous chemistry in the upper troposphere has not been established. To probe particle formation, several types of experiments will be performed. First FTIR reflection absorption spectroscopy (RAS) will be used to make fundamental measurements of homogeneous and heterogeneous nucleation parameters for atmospheric materials. Diffusion activation energies and contact parameters will be determined to provide the necessary input for nucleation theory. Other laboratory experiments will probe the formation mechanisms of PSCs, in particular, type Ia PSCs, whose exact composition is still unknown. In addition, laboratory studies of heterogeneous chemical reactions of potential importance in the upper troposphere will be performed. The most abundant types of particles there are thought to be cirrus, subvisible cirrus, and sulfate aerosols. A Knudsen cell flow reactor coupled with FTIR RAS to probe the condensed phase will be used. Oxidation/reduction reactions that may occur between oxygenated organics and nitric acid condensed on cirrus particles will be studied, and experiments on the reactivity of deliquesced and crystalline sulfate salts will be performed.
