Rarity of upper-tropospheric low O 3 mixing ratio events during MOZAIC flights

Only a few previous observations of very low O 3 mixing ratios 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 mixing ratios 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 fulfil these conditions (excluding artefacts as discussed below), representing about 0.001% of all measurements during the analysed period August 1994-December 1997. The low O 3 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 mixing ratio events occur in the upper troposphere during periods with generally low mixing ratios. They are not only found over sea, but also over land at pressure levels as low as 179 hPa. It could well be that some of the low O 3 mixing ratio events measured during two or more flights belong to the same bigger low O 3 mixing ratio area.