3-DIMENSIONAL VIEW OF THE LARGE-SCALE TROPOSPHERIC OZONE DISTRIBUTIONOVER THE NORTH-ATLANTIC OCEAN DURING SUMMER

A global chemical transport model is used to study the three-dimension al structure of the tropospheric ozone (O-3) distribution over the Nor th Atlantic Ocean during summer. A simplified representation of summer time O-3 photochemistry appropriate for northern hemisphere midlatitud es is included in the model. The model is evaluated by comparing simul ated O-3 mixing ratios to summertime O-3 measurements taken in and nea r the North Atlantic Ocean basin. The model successfully reproduces (1 ) the means and standard deviations of ozonesonde measurements over No rth America at 500 mbar; (2) the statistical characteristics of surfac e O-3 data at Sable Island off the coast of North America and at Bermu da 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 At lantic. The model underestimates surface O-3 in the eastern North Atla ntic, overestimates O-3 in the lower free troposphere over the western North Atlantic, and also has difficulty simulating the upper troposph eric ozonesonde measurements over North America. An examination of the mean summertime O-3 distribution simulated by the model shows a signi ficant 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 mo st of the North Atlantic. We estimate that present-day O-3 levels in t he lower troposphere over the North Atlantic are at least twice as hig h as corresponding preindustrial O-3 levels. We find that the anthropo genic impact is substantial even in the midtroposphere, where modeled present-day O-3 mixing ratios are at least 1.5 times higher than prein dustrial O-3 levels.