PARTICLE MICROPHYSICS AND CHEMISTRY IN REMOTELY OBSERVED MOUNTAIN POLAR STRATOSPHERIC CLOUDS

Polar stratospheric clouds (PSCs) at 22-26 km were observed over the N orwegian mountains by airborne lidar on January 15, 1995. Simulations using a mesoscale model reveal that they were caused by mountain-induc ed gravity waves. The clouds had a highly detailed filamentary structu re with bands as thin as 100 m in the vertical, and moved insignifican tly over 4 hours, suggesting them to be quasi-stationary. The aircraft flight path was parallel or close to parallel with the wind at cloud level. Such a quasi-Lagrangian observation, together with the presence of distinct aerosol layers, allows an air parcel trajectory through t he cloud to be constructed and enables the lidar images to be simulate d using a microphysical box model and light scattering calculations. T he results yield detailed information about particle evolution in PSCs and suggest that water ice nucleated directly from liquid HNO3/H2SO4/ H2O droplets as much as 4 K below the ice frost point. The observation of solid nitric acid hydrate particles downwind of the mountains show s that such mesoscale events can generate solid PSC particles that can persist on: the synoptic scale. We also draw attention to the possibl e role of mesoscale PSCs in chlorine activation and subsequent ozone d estruction.