The impact of the variability of the persistence of tropospheric layers on the distribution of observed ozone layers is quantified by means of Lagrangian modeling. More than 3000 ozone anomalies were detected above Europe in vertical profiles in 1997 and 1998. The origin of the layers is identified with backward Lagrangian particle dispersion computations. Forward simulations are used to evaluate the growth rate of the plume in order to discuss the lifetime of the layers. Stratospheric intrusions are most sensitive to the horizontal strain, especially in winter, and the mixing of layers coming from the planetary boundary layer is enhanced by the summertime convection. The respective role of the origin of the layers and the seasonality is discussed, as well as the formation of streamers. These results underline the need to account for the variability of the tropospheric mixing in investigations of transport processes based on climatologies of tropospheric layers.