A relatively simple balloonborne device for measuring the local aerosol backscatter at multiple wavelengths has been developed and field tested. The instrument produces detailed profiles which are inherently similar to those generated from lidar soundings. It is also sensitive, being able to measure the 20-km stratospheric aerosol background layer with a signal-to-noise ratio of approximately 100:1. An important feature of this device is its ability to be calibrated in an absolute sense. Theoretical considerations show that the measurements can be accurately converted to aerosol mass loading in the stratosphere for conditions ranging from background to heavy volcanic influence. The instrument is not expected to replace other observational techniques; rather it provides highly complementary information as well as furnishes a cost-effective alternative to measurement systems available to only a few scientists.

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[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.

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