Evidence for changes in the ozone concentrations in the free troposphere over Southern France from 1976 to 1995

Tropospheric ozone measurements were performed at the Observatoire de Haute Provence (OHP) in Southern France by three instruments during the last 10 yr: Brewer Mast (BM) ozonesondes 1985/1991, ECC ozonesondes: 1991/1995, lidar: 1990/1995. The error analysis shows an overall accuracy of 10% for the BM sondes and lidar data, and 5% for the ECC sondes. A comparison of the tropospheric seasonal means above 3 km shows that the differences between the three instruments are within the instrument accuracy. The OHP ECC data are also compared to the 1976/1983 ozone measurements performed with BM sondes at the Biscarosse station also in Southern France to estimate possible tropospheric ozone changes during the 1980s. Yearly mean values imply a maximum annual ozone increasing rate of 8 +/- 2% per decade in the 1980s, but this trend becomes insignificant if one includes the usual - 15% bias between BM and ECC. The shift of the seasonal variation from a spring maximum to a spring/summer maximum is however statistically significant during the 1980s. In the upper troposphere, the seasonal means are studied using a height scale relative to the tropopause, showing a similar seasonal variation than in the mid-troposphere, Also an increase of the ozone spring/summer maximum with the altitude is shown. A negative trend of - 10%/decade is inferred at the tropopause level using the OHP and Biscarosse data. (C) 1997. Published by Elsevier Science Ltd.