High ozone concentrations are observed in late spring and summer during anticyclonic situations. Much of the ozone is the result of a photochemical production on a large scale due to the presence of precursors NOx, CO and HC, while the air mass is moving over polluted areas. Daily concentration minima of radon, a radioactive gas emitted from the ground, can be considered as a good tracer of the continental character of the air mass. Correlation between daily maxima of ozone and daily minima of radon is analyzed. Positive correlations are observed in summer, strengthening the hypothesis of ozone formation on a large scale (the half-life of radon is 3.8 days). On the other hand, in winter, daily minima of radon and daily maxima of ozone are negatively correlated. During this season, deposition and chemical destruction of ozone seem to be more significant than production while the air mass is moving over the continent.