A global chemical transport model is used to study the three-dimensional structure of the tropospheric ozone (O3) distribution over the North Atlantic Ocean during summer. A simplified representation of summertime O3 photochemistry appropriate for northern hemisphere midlatitudes is included in the model. The model is evaluated by comparing simulated O3 mixing ratios to summertime O3 measurements taken in and near the North Atlantic Ocean basin. The model successfully reproduces (1) the means and standard deviations of ozonesonde measurements over North America at 500 mbar; (2) the statistical characteristics of surface O3 data at Sable Island off the coast of North America and at Bermuda in the western North Atlantic; and (3) the mean midtropospheric O3 measured at Bermuda and also at the Azores in the eastern North Atlantic. The model underestimates surface O3 in the eastern North Atlantic, overestimates O3 in the lower free troposphere over the western North Atlantic, and also has difficulty simulating the upper tropospheric ozonesonde measurements over North America. An examination of the mean summertime O3 distribution simulated by the model shows a significant continental influence on boundary layer and free-tropospheric O3 over the western North Atlantic. The model has also been exercised using a preindustrial NOx emission scenario. By comparing the present-day and preindustrial simulations, we conclude that anthropogenic NOx emissions have significantly perturbed tropospheric O3 levels over most of the North Atlantic. We estimate that present-day O3 levels in the lower troposphere over the North Atlantic are at least twice as high as corresponding preindustrial O3 levels. We find that the anthropogenic impact is substantial even in the midtroposphere, where modeled present-day O3 mixing ratios are at least 1.5 times higher than preindustrial O3 levels. ¿ American Geophysical Union 1996