The Airborne Polar Experiment (APE) is an airborne mission, equipped with sufficient aerosol instrumentation to discriminate between the various composition put forward for PSCs, and with sufficient chemical instrumentation to study the chemical evolution of air passing through a PSC. The mission will specifically aim to fly in regions where lee wave clouds are likely to be found (Scandinavian Alps, Ural mountains) and in regions where no previous measurements of PCSs have been made (Siberian Arctic). The mission's combination of high frequency in situ measurements, new flight paths, meteorological analyses and detailed modeling will substantially improve the understanding of the spatial and temporal extent of PSCs and hence ozone loss. As an important spin-off, the airborne mission will provide detailed information on all forms of aerosol particles present/from the tropopause to 22 km over the European and Arctic lower stratosphere.