Three-dimensional view of the large-scale tropospheric ozone distribution over the North Atlantic Ocean during summer

A global chemical transport model is used to study the three-dimensional structure of the tropospheric ozone (O-3) distribution over the North Atlantic Ocean during summer. A simplified representation of summertime O-3 photochemistry appropriate for northern hemisphere midlatitudes is included in the model. The model is evaluated by comparing simulated O-3 mixing ratios to summertime O-3 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 O-3 data at Sable Island off the coast of North America and at Bermuda in the western North Atlantic; and (3) the mean midtropospheric O-3,O- measured at Bermuda and also at the Azores in the eastern North Atlantic. The model underestimates surface O-3 in the eastern North Atlantic, overestimates O-3 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 O-3 distribution simulated by the model shows a significant continental influence on boundary layer and free-tropospheric O-3 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 O-3 levels over most of the North Atlantic. We estimate that present-day O-3 levels in the lower troposphere over the North Atlantic are at least twice as high as corresponding preindustrial O-3 levels. We find that the anthropogenic impact is substantial even in the midtroposphere, where modeled present-day O-3 mixing ratios are at least 1.5 times higher than preindustrial O-3 levels.