Christiane Voigt,¹ Jochen Schreiner,¹ Andreas Kohlmann,¹ Peter Zink,¹ Konrad Mauersberger,^1* Niels Larsen,² Terry Deshler,³ Chris Kröger,³ Jim Rosen,³ Alberto Adriani,⁴ Francesco Cairo,⁴ Guido Di Donfrancesco,⁴ Maurizio Viterbini,⁴ Joelle Ovarlez,⁵ Henri Ovarlez,⁵ Christine David,⁶ Andreas Dörnbrack⁷

A comprehensive investigation of polar stratospheric clouds was performed on 25 January 2000 with instruments onboard a balloon gondola flown from Kiruna, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, containing water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatures. The presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.

¹ Max-Planck-Institut für Kernphysik, Division of Atmospheric Physics, Post Office Box 103 980, D-69029 Heidelberg, Germany.
² Division of Middle Atmosphere Research, Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark.
³ Department of Atmospheric Science, University of Wyoming, Post Office Box 3038, Laramie, WY 82071, USA.
⁴ CNR-Istituto di Fisica dell'Atmosfera, Via Fosso del Cavaliere 100, I-00133 Rome, Italy.
⁵ Laboratoire de Météorologie Dynamique du CNRS, École Polytechnique, F-91128 Palaiseau, France.
⁶ Service d'Aéronomie du CNRS, Institut Pierre-Simon Laplace, Université Paris, 4 place Jussieu, F-75252 Paris, France.
⁷ DLR Oberpfaffenhofen, Institut für Physik der Atmosphäre, D-82234 Wessling, Germany.
^*   To whom correspondence should be addressed. E-mail: Konrad.Mauersberger{at}mpi-hd.mpg.de