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A balloon-borne experiment was successfully flown on January 25, 1998 to provide the first chemical analysis of polar stratospheric cloud particles (PSCs). The experiment was developed at the Max Planck Institute of Nuclear Physics (Max-Planck-Institut für Kernphysik), Heidelberg, Division of Atmospheric Physics. The results show that water and nitric acid condense to form PSCs (Schreiner et al., Science, Vol 283, 12 February 1999), which subsequently will provide the surfaces for chlorine activation. PSCs are a crucial element in the development of the large scale ozone hole above Antarctica.
Atmospheric water and nitric acid are thought to condense aerosols onto the small sulfate background to form liquid or solid particles. Although less frequently observed in the Northern hemisphere, stratospheric cloud layers in altitude ranges from 15 to 24 km have been monitored above Scandinavia. The ESRANGE launch facilities near Kiruna, Sweden, provide the unique opportunity to study the polar atmosphere in the midst of winter conditions.
Laboratory studies and model calculations have advanced the knowledge on PSC formation, composition, and reactivity. However, a detailed chemical analysis within a cloud layer has never been achieved. It is difficult to transfer the fragile particles into an analysis instrument without altering their phase and composition. Experimental verification of theoretical models describing PSC formation and processing has remained an important research goal in atmospheric science.
During the last years, a PSC analysis instrument has been developed in the Division of Atmospheric Physics at the Max Planck Institute of Nuclear Physics. An important component of the experiment is an aerodynamic lens that focuses atmospheric aerosols into a narrow beam followed by particle evaporation within an accommodation sphere and mass analysis. During the balloon flight on January 25, 1998, the gondola with the Heidelberg experiment encountered between 20 and 23 km two PSC layers formed in mountain lee waves over Scandinavia. The results show water-rich particles with a ratio of H2O/nitric acid above 10. Early models of PSC's assumed solid particles with much higher nitric acid contents. In future flights the sensitivity of the analysis experiment will be increased to extend the measurements to trace gases dissolved in cloud particles.
Journal
Science