Five day 3D back trajectories have been calculated for air parcels arriving at different pressure levels at Uccle (Belgium, 50 480N, 4 210E, 100masl) for the time period 1969–2001 on days where an ozone sounding took place. The trajectory model used is the APTRA model, available at the European Centre for Medium-Range Weather Forecasts (ECMWF). A nonhierarchical clustering method has been applied on the obtained 3D back trajectories in function of the season for the planetary boundary layer (PBL) and the lower free troposphere (LFT) to identify natural clusters of air masses. It was possible to derive distinctive clusters of air masses for every season. In the spring season, a reduction in ozone concentrations in the PBL within slow moving air masses from the west, combined with an increase in the frequency of occurrence, seems to be compensated by a positive trend in ozone concentrations at Uccle within the other source regions, mainldy within the cluster representing slow moving air masses over the continent. More inflow from North America has been detected during the winter season, resulting in higher ozone concentrations in the PBL.