A time series analyses of planetary boundary layer ozone data is presented from 1990 to 2001. The ozone data is derived from the Uccle (50º48'N, 4º21'E) balloon sounding time series. The climatology of the 1990-2001 time series shows a spring and a summer maximum. To explain the ozone behaviour in the lower troposphere by meteorological parameters, 5 multiple linear regression models (one composite model of four climatological seasons and one overall model) have been designed, to better elucidate the dependence of ozone on meteorology. To choose the best model, a stepwise selection criterion has been applied. During the winter season, there is an offset present between the calibration and the validation period. This offset can be explained by the differences in the post flight procedures applied to the BM and ECC data. This offset hampers trend analysis when using the BM and ECC data in the lower troposphere where the winter season is included.