The vertical distribution of ozone over the Grenoble area is investigated for two summer smog episodes in 1999 and 2003. The effects of the interactions of local dynamical processes with large-scale circulations were estimated using numerical modelling. The results suggest that a terrain-induced shear layer above the boundary layer confines pollutants within the valley. Vertical exchanges through this shear layer have a considerable influence on the near-surface ozone concentration from day to day.