ABSTRACTThe vertical ozone distribution in the troposphere and stratosphere over the Atlantic Ocean has been measured in situ by shipborne ozone soundings during four RV Polarstern meridional transects in January/February 1993, October/November 1993, May/June 1994, and October/November 1996. We observed distinct differences in the vertical distribution pattern of tropospheric ozone between the northern- and the southern hemisphere: The ozone mixing ratio gradients were flat in the northern hemisphere and ozone mixing ratios in the free troposphere did never exceed 80 ppbv up to the tropopause, while the southern hemisphere exhibited a pronounced vertical gradient. Extremely dry air masses with enhanced ozone amounts up to120 ppbv have been found in the tropical free troposphere of the southern hemisphere between 0° and 20°S. The vertical ozone stratification in the troposphere of the southern hemisphere was dominated by this large scale feature. Photochemical ozone production as a consequence of the emissions of natural fires or intrusions of stratospheric air masses are the most probable sources for these ozone rich layers. Our results indicate that the boundary layer ozon distribution of the northern hemisphere is influenced by the advection of pollution derived ozone. In the tropical and mid-latitude southern hemisphere, photochemical ozone loss seems to be the dominating process.