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.