Research flights were conducted to investigate the episodic occurrence of enhanced ozone mixing ratios over the North Atlantic Ocean (NAO) during the spring as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE). We measured meteorological variables, trace gases, and aerosol light scattering from the University of Wyoming King Air research aircraft during 16 research flights in April and early May 1996. The flights were conducted over eastern North America and over the NAO between the United States and Bermuda. On April 28, a cold front was located just west of Bermuda while a second system developed over the Midwest and central United States. In the midtroposphere, polluted layers with up to 250 ppbv CO, 150 pptv NO, and 1.8 ppbv NOy were observed within a deeper layer of enhanced O3 mixing ratios (up to 88 ppbv) over the NAO at around 32 ¿N, 64--68 ¿W. These aircraft observations, when coupled with ozone sondes, back trajectories, and satellite image loops, indicate two distinct meteorological mechanisms that combine to yield the observed chemical signature of the air. There is substantial meteorological evidence, supported by ozone sonde observations and earlier King Air flights, indicating that stratosphere/troposphere exchange associated with the upstream frontal system injects and advects dry, ozone-rich air into midtropospheric regions over the continent. These subsiding air masses have the potential to result in deep layers of enhanced ozone in the offshore postfrontal area. Convection from the developing (upwind) system appears to lift air from the continental boundary layer into the area of subsidence in the mid to upper troposphere. The result is high concentrations of gaseous and particulate pollutants along with elevated quantities of ozone. Ozone mixing ratios exceed those attributable to boundary layer venting or in-transit photochemical production. These meteorological processes lead to pollution and enhanced ozone from the stratosphere cooccurring in postfrontal air masses over the NAO. ¿ 1999 American Geophysical Union