Chemical, microphysical and optical properties of polar stratospheric clouds

A balloon-borne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 from Kiruna/Sweden. Besides a mass spectrometer experiment for chemical particle analysis, the gondola carried optical particle counters, two backscatter sondes, a hygrometer and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated data of PSC particle properties from the different instruments were obtained. A large variability of particle types was measured in several PSC layers, and particle development was followed for more than 2 hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of solid nitric acid trihydrate (NAT) particles with a molar ratio H2O:HNO3 of 3 at temperatures near and slightly above the equilibrium temperature TNAT. The median diameter of the NAT particle size distribution was between 1 and 1.5 µm at concentrations around 0.5 cm-3. Below the NAT layers, thin cloud layers containing a few large HNO3 rich particles with diameters as large as 7 µm coexisted with smaller solid or liquid particles at temperatures as high as 2 K above TNAT.