Planetary boundary layer (PBL) ozone densities, retrieved from ozone soundings at Uccle (Belgium) and Payerne (Switzerland) between 1990 and 2000 are analysed. This PBL ozone is representative on a regional scale, also in highly polluted areas. At Uccle the yearly cycle of PBL ozone shows late spring and summer maxima, while in Payerne one broad summer maximum with peak in July is present. An overall and a composite multiple linear regression model has been built using meteorological and NOx data as independent variables. For Uccle and Payerne the explained variances for the composite models were 63.43% and 78.97% with an rmse of 15:95 mgm 3 and 16:50 mgm 3, respectively. Both stations have an overall significant negative trend during the time period 1997–2000. During winter and autumn there is a positive significant trend present for Payerne and Uccle, which is explained by a decrease in NOx concentrations. For some particular days dynamical effects are illustrated with 10 day 3D backwards trajectories. This shows that further improvement can onldy be achieved by including chemical and dynamical effects in the models.r 2005 Elsevier Ltd. All rights reserved.