Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 2 2003 10.5194/acpd-3-1631-2003 http://www.atmos-chem-phys-discuss.net/3/1631/2003/ http://www.atmos-chem-phys-discuss.net/3/1631/2003/acpd-3-1631-2003.html http://www.atmos-chem-phys-discuss.net/3/1631/2003/acpd-3-1631-2003.pdf 1631 1650 2003-03-24 Rarity of upper-tropospheric low O₃ concentration events during MOZAIC flights W. A. H. Asman M. G. Lawrence C. A. M. Brenninkmeijer P. J. Crutzen J. W. M. Cuijpers P. Nédélec Max-Planck-Institute for Chemistry, Atmospheric Chemistry Division, P.O. Box 3060, 55020 Mainz, Germany Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, The Netherlands Laboratoire d’Aérologie (UMR CNRS/UPS 5560) OMP, Université Paul Sabatier, 14 Avenue Edouard Belin, 31400 Toulouse, France Only a few previous observations of very low O₃ concentrations in the upper troposphere are available. The aim of this study was to examine the rich MOZAIC data set for more. Flights with at least 25 4 s averaged concentrations less than 8 ppbv at pressures lower than 500 hPa measured using commercial aircraft within the MOZAIC project have been analysed. There are eleven flights that fulfill these conditions (excluding artefacts as discussed below), representing about 0.001% of all measurements during the analyzed period August 1994–December 1997. The low O₃ events occurred over Southeast Asia, Africa, Brazil and the sea area 200 km east of Florida (US) and were all likely to be associated with transport of air masses from tropical sea areas. These low concentration events occur in the upper troposphere during periods with generally low concentrations. They are not only found over sea, but also over land at pressure levels as low as 179 hPa.