Measurements of vertical profiles and level averages of ozone over the north central Atlantic near the Azores are presented. Lagrangian experiments were carried out in clean air and polluted air, and differences between the cases are discussed. Significant vertical structure was found in the profiles from both cases. Trajectory analysis indicates that the source regions for ozone in the marine boundary layer and in the air above the inversion capping the boundary layer were different. No significant production or destruction of ozone in the boundary layer was observed in the clean case. No substantial differences in ozone concentration between cloudy and clear air were observed, although cloud processes in the polluted case may have been partially responsible for an observed overall reduction in boundary layer ozone concentration.