Passive tracers are employed in a relatively high spatial and temporal resolution three-dimensional transport model to analyze a stratosphere-troposphere exchange (STE) event over the eastern United States and western North Atlantic Ocean. The model is validated against measurements taken on board the National Oceanic and Atmospheric Administration WP-3D Orion aircraft during the North Atlantic Regional Experiment study in the spring of 1996. Overall, the model reproduces the measurements well during the early part of the flight where there is indication of a small stratospheric intrusion. However, the very strong signatures of STE and mixing contained in the measurements later in the flight are not captured. Use of a finer horizontal resolution (20 km as opposed to 60 km) brings the model results closer to the aircraft measurements and yields higher values (50% at 7--8 km altitude) of ozone, O3, with a deeper penetration into the troposphere (20% at 80--120 ppbv levels).