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In the winter of 1994/95 the TRANSALL research aircraft performed several flights in the region of the Arctic vortex during the period oflow stratospheric temperatures. The results of simultaneous measurements of HNO3 column amounts by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and of aerosol backscatterprofiles by the Ozone Lidar EXperiment (OLEX) are presented for two typical flight scenarios across the polar vortex boundary on December17, 1994 and January 11/12, 1995. On December 17 and January 12, thecolumn amounts of gaseous HNO3 decreased significantly inregions with low stratospheric temperatures. This decrease was correlated with the extent of the polar stratospheric clouds. Depolarisation measurements showed that type 1b PSCs were observed primarily, butequilibrium calculations for H2SO4/HNO3/H 2O aerosols seem to underestimate the observed HNO3 sequestering. [ABSTRACT FROM AUTHOR]