A comprehensive investigation of polar stratospheric clouds was performed on 25 January 2000 with instruments onboard a balloon gondola flown from Kiruna, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, containing water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatures. The presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.

A balloon-borne gondola carrying a particle analysis system, a backscatter sonde, and pressure and temperature sensors was launched from Kiruna, Sweden, on 25 January 1998. Measurements within polar stratospheric cloud layers inside the Arctic polar vortex show a close correlation between large backscatter ratios and enhanced particle-related water and nitric acid signals at low temperatures. Periodic structures in the data indicate the presence of lee waves. The H2O/HNO3 molar ratios are consistently found to be above 10 at atmospheric temperatures between 189 and 192 kelvin. Such high ratios indicate ternary solution particles of H2O, HNO3, and H2SO4 rather than the presence of solid hydrates.

[1] A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, 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 PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H 2 O:HNO 3 close to 3 at temperatures near and below the equilibrium temperature T NAT . The median radius of the NAT particle size distribution was between 0.5 and 0.75 mm at concentrations around 0.5 cm À3 . Below the NAT layers and above T NAT , thin cloud layers containing a few large particles with radii up to 3.5 mm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

[1] A detailed microphysical model has been used to simulate polar stratospheric clouds (PSC) formed in mountain leewaves over northern Scandinavia and observed in a balloonborne multi-instrument flight on 25 January 2000. The measurements show cloud layers of large solid particles with nitric acid trihydrate (NAT) compositions at relatively high temperatures and layers containing liquid particles with supercooled ternary solution compositions at very low temperatures. The same PSC particle layers have been observed several times during the 2 1/2 h flight, offering a nearly Lagrangian picture of the particle evolution. The applied PSC model describes homogeneous freezing of ice below the ice frost point and diffusion-limited nonequilibrium and size-dependent growth and composition of liquid and solid-phase particles. The microphysical box model calculations are performed on two isentropic surfaces, corresponding to different observed particle layers, using temperature histories from combined high-resolution nonhydrostatic mesoscale and synoptic-scale model analyses of the meteorological conditions characterized by strong mountain leewaves. The calculated particle composition, physical phase, and particle size distributions are compared with the in situ measurements of the same particle properties. It appears that homogeneous freezing of ice in liquid solutions a few degrees below the ice frost point and subsequent release of NAT at higher temperatures might explain the characteristics of the observed solid PSC particles.

A balloon-borne mass spectrometer system has been flown successfully to determine the chemical composition of polar stratospheric aerosols over northern Scandinavia. The experiment combines an aerodynamic lens which collimates the aerosols into a narrow beam, a small sphere in which they evaporate, and a mass spectrometer for gas analysis. High-speed differential pumping by two liquid helium pumps effectively lowers the presence of ambient gases without affecting the particles of the beam. Field measurements and aerosol studies inside a large cryo-chamber have shown that the concept of particle focusing, evaporation and subsequent mass spectrometric gas analysis is a reliable technique to determine the molecular composition of aerosols especially in polar stratospheric clouds.