Important details concerning polar stratospheric ozone depletion are still required for a more complete quantitative understanding. This is particularly true in the Arctic stratosphere where the loss of ozone is not as severe as in the Antarctic. The microphysics of the nucleation, growth, composition, sedimentation of particles in polar stratospheric clouds (PSCs) is one of the critical areas for understanding Arctic ozone loss. PSC particles provide sites for the heterogeneous chemistry required to activate chlorine, which then destroys ozone. The pivotal role played by PSC particles is most evident in the Arctic when ozone loss is essentially modulated by temperature, a surrogate for PSCs, from year to year. The frequency, duration, and timing of PSC occurrence determines to a large extent the amount of ozone loss. Removing some of the present uncertainties concerning PSC formation temperatures, particle composition, and particle phase will permit more careful modeling of the amount of chlorine processed by PSCs. This project is to collaborate on joint measurements of PSC characteristics during the Third European Stratospheric Experiment on Ozone. The instruments will be flown onto one balloon gondola which will simultaneously measure, in situ, aerosol concentration, size, composition, depolarization, backscattering, and gas phase concentrations of nitric acid and water. The investigation will focus on methods to infer particle index of refraction, which is a function of composition. These joint measurements will be the first direct measurements of the complete thermodynamic environment in which PSC particles exist, coincident with a characterization of aerosol size, concentration, composition, phase, and optical properties. These measurements should reveal the extent to which the measured PSCs are in thermodynamic equilibrium with their environment, which hydrates of HNO3 are formed, and the abundance of condensed HCI, an important species for chlorine activation.
