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Abstract

Liquid and solid particles in polar stratospheric clouds are of central importance for the depletion of stratospheric ozone. Surface-catalyzed reactions on these particles convert halogenes, which are of anthropogenic origin, from relatively inert reservoir species into forms that e ciently destroy ozone. The knowledge about this particles has improved considerably during the last years, but still lacks in many respects, in particular concerning the nucleation of solid nitric acid hydrate particles. In early December 2001 balloon-borne in-situ measurements of aerosol composition, size, number, phase, and backscatter were completed in an Arctic stratospheric cloud developed in a mountain-wave event. Particle composition was measured with an improved aerosol mass spectrometer. The principle of the aerosol mass spectrometer is based on the separation of aerosols from ambient air by the combination of an aerodynamic lens and a di erentially pumped vacuum system. The composition of the particles is then analyzed with a magnetic mass spectrometer. Other instruments included four optical particle counters, a laser- and white light backscattersonde. The atmospheric environment was characterized with accurate pressure, temperature, and water vapor measurement. These measurements represent comprehensive in-situ observations of all phases of polar stratospheric cloud particles that are liquid ternary solution droplets, nitric acid trihydrate, and ice particles. Atmospheric temperatures tightly control the type of particles that form. At cloud top, there was a thin layer of solid particles narrowly distributed around a radius of 2.0 m at concentrations of 0.001 cm