Mountain wave polar stratospheric clouds (PSCs) were detected on 8 February 2003 above the Scandinavian Mountains by in-situ instruments onboard the M55 Geophysica aircraft. PSC particle composition, backscatter and chlorine activation for this case are studied with a recently developed non-equilibrium microphysical box model for liquid aerosol. Results from the microphysical model, run on quasilagrangian trajectories, show that the PSC observed was composed of supercooled ternary (H2O/HNO3/H2SO4) solution (STS) particles, which are out of equilibrium with the gas phase. The measured condensed nitric acid and aerosol backscatter of the PSC can well be simulated with the model. Up to 0.15 ppbv Cl2 can be released by the PSC within 2 h, showing the propensity of these small-scale clouds for chlorine activation. Equilibrium calculations � of the sort commonly used in large scale chemistry transport models � overestimate the measured condensed nitrate by up to a factor of 3, and overestimates chlorine activation by 10%, in this mountain wave cloud.