ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-1371-2005 Nitric Acid Trihydrate (NAT) formation at low NAT supersaturation in Polar Stratospheric Clouds (PSCs) Voigt C. 1 Schlager H. 1 Luo B. P. 2 Dörnbrack A. 1 Roiger A. 1 Stock P. 1 Curtius J. 3 Vössing H. 3 Borrmann S. 3 4 Davies S. 5 Konopka P. 6 Schiller C. 6 Shur G. 7 Peter T. 2 Institut für Physik der Atmosphäre (IPA), DLR Oberpfaffenhofen, 82234 Wessling, Germany Institut für Atmosphäre und Klima, ETH Zürich, Hönggerberg HPP, CH-8093 Zürich, Switzerland­ Institut für Physik der Atmosphäre, Universität Mainz, 55099 Mainz, Germany Max-Planck-Institut für Chemie, 55128 Mainz, Germany School of Environment, University of Leeds LS9JT, UK Forschungszentrum Jülich, ICG-I, 52425 Jülich, Germany Central aerological observatory, Moscow, reg. 141700, Russia 08 06 2005 5 5 1371 1380 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/1371/2005/acp-5-1371-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/1371/2005/acp-5-1371-2005.pdf

A PSC was detected on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (~10<sup>-4</sup>cm<sup>-3</sup>) of small nitric acid (HNO<sub>3</sub>) containing particles (<i>d</i>&lt;6&micro;m) were observed at altitudes between 18&nbsp;and 20km. Provided the temperatures remain below the NAT equilibrium temperature <i>T</i><sub>NAT</sub>, these NAT particles have the potential to grow further and to remove HNO<sub>3</sub> from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures just slightly below <i>T</i><sub>NAT</sub> (<i>T</i>&gt;<i>T</i><sub>NAT</sub>-3.1K). This unique measurement of PSC formation at extremely low NAT saturation ratios (<i>S</i><sub>NAT</sub>&le;10) constrains current NAT nucleation theories. We suggest, that the NAT particles have formed heterogeneously, but for certain not on ice. Conversely, meteoritic particles may be favorable candidates for triggering NAT nucleation at the observed low number densities.