ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-2441-2004 Stratospheric aerosol measurements by dual polarisation lidar Vaughan G. 1 Wareing D. P. 1 University of Wales, Institute of Mathematical and Physical Sciences, Aberystwyth, UK 06 12 2004 4 11/12 2441 2447 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/4/2441/2004/acp-4-2441-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/2441/2004/acp-4-2441-2004.pdf

We present measurements of stratospheric aerosol made at Aberystwyth, UK (52.4&deg; N, 4.06&deg; W) during periods of background aerosol conditions. The measurements were made with a lidar system based on a 532nm laser and two polarisation channels in the receiver. When stratospheric aerosol amounts are very small, as at present, this method is, potentially, free of a number of systematic errors that bedevil more commonly-used methods. The method rests on the assumption that the aerosol consists of spherical droplets which do not depolarise the lidar signal, which is valid under most conditions. Maximum lidar ratios in background aerosol of 1.03-1.06 were measured during the period 2001-2004, with integrated backscatter in the range 2-7x10<sup>-5</sup>sr<sup>-1</sup>. In January 2003, depolarising aerosol was measured, which invalidated the dual-polarisation measurements. On 10-11&nbsp;January, the depolarising aerosol was clearly a polar stratospheric cloud (the first lidar observations of such clouds in the British Isles) but the aerosol observed on 7-8&nbsp;January was too low in altitude and too warm to be a PSC.