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We present ozone measurements of the Global Atmospheric Sampling Program(GASP) performed from four commercial and one research aircraft in the late1970s to compare them with respective measurements of the ongoing MOZAICproject. Multi-annual averages of UT/LS ozone were built using the aircraft datasets (1975–1979 and 1994–2001), and long-term changes between the 1970s and1990s were derived by comparison. The data were binned relative to thedynamical tropopause to separate between UT and LS air masses. LS data wereanalysed using equivalent latitudes. In the UT, pronounced increases of 20–40%are found over the Middle East and South Asia in the spring and summer seasons.Increases are also found over Japan, Europe, and the eastern parts of theUnited States depending on season. LS ozone over northern mid- and highlatitudes was found to be lower in the 1990s compared to the 1970s in allseasons of the year. In addition, a comparison with long-term changesdeduced from ozonesondes is presented. The early 1970s European Brewer-Mast (BM)sonde data agree with GASP within the range of uncertainty (UT) or measuredslightly less ozone (LS). In contrast, the 1990s BM sensors show consistentlyand significantly higher UT/LS ozone values than MOZAIC. This unequal behaviourof aircraft/sonde comparisons in the 1970s and 1990s leads to differences inthe estimated long-term changesover Europe: while the comparison between GASP and MOZAIC indicates ozonechanges of −5% to 10% over Europe, the sondes suggest a much largerincrease of 10%–35% depending on station and season, althoughstatistical significance is not conclusive due to data sample limitations.In contrast to the BM sondes, the Electrochemical Cell (ECC) sonde atWallops Island, USA, measured higher UT ozone than both GASP and MOZAIC.Hence, long-term changes from GASP/MOZAIC agree within the range ofuncertainty with the changes deduced from Wallops Island

Long-term changes in UT/LS ozone between the late 1970s and the 1990s deduced from the GASP and MOZAIC aircraft programs and from ozonesondes