ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-2-93-2002 NAT-rock formation by mother clouds: a microphysical model study Fueglistaler S. 1 Luo B.P. 1 Voigt C. 1 Carslaw K.S. 2 Peter Th. 1 Atmospheric and Climate Science, ETH Zürich, Switzerland School of the Environment, University of Leeds, Leeds, UK 15 05 2002 2 2 93 98 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/2/93/2002/acp-2-93-2002.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/2/93/2002/acp-2-93-2002.pdf

Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO<sub>3</sub> depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10<sup>-4</sup> cm<sup>-3</sup> NAT-rocks with radii around 10 <font face="Symbol" >mm</font> some kilometers below the mother cloud. The mechanism does not require selective nucleation and works even for a monodisperse particle size distribution in the mother cloud.