Long-term ozonesonde data (1970--1999) from Hohenpeissenberg, Germany (48¿N, 11¿E) and Payerne, Switzerland (47¿N, 7¿E) have been studied for the boundary layer (from 900 to 750 hPa) and lower troposphere (from 750 to 550 hPa) using back air trajectory analysis. The residence times of air masses over central Europe have been estimated for each day utilizing the trajectory data and are tagged with the corresponding mixing ratios of ozone. It is shown that mixing ratios increase with increasing residence time in summer at a rate of about 2 ppbv/day, but stabilize after about 6 days. Mixing ratios corresponding to central European residence times of 4--6 days are defined as photochemically aged ozone. Correlation and slope analysis is made between ozone and residence time (1--6 days) using a statistical regression model, and ozone mixing ratios extrapolated to zero day are defined as background ozone. The maximum in photochemically aged ozone occurs in summer. Background ozone shows lower values and generally a broad maximum extending from spring to summer. Absolute values and variations of ozone in photochemically aged and background air are quite similar at Hohenpeissenberg and Payerne, indicating their good regional representativeness for central Europe. Regional monthly averaged ozone build-up (above the background value) over central Europe is about 10 ppbv in summer. Central Europe is found to be a net source of ozone throughout the year, except in winter. However, an increasing trend of ozone in background and photochemically aged air has been observed in winter during the 1980s. Only the boundary layer photochemically aged ozone showed a decreasing trend (-1.6 ¿ 0.2 ppbv/year), consistent with the NOx emissions trend over central Europe during the 1990s. Long-term changes in photochemically aged and background ozone suggest that intercontinental transport has a significant effect on ozone mixing ratios in the lower troposphere as well as in background ozone in the boundary layer over central Europe. The present analysis ignores transport of ozone from the stratosphere, which might also contribute to interannual variations and changes.