A three-dimensional regional chemistry model is used to simulate distributions of a number of tropospheric species over eastern North America and the western North Atlantic for August of 1993. The model is tested by comparisons of the results with observations obtained at two ground stations in Nova Scotia, Canada, and on board two aircraft. The modeled mixing ratios of CO and O3 as well as the ratio of ΔO3/ΔCO at the two ground sites are in satisfactory agreement with the observations. More than 90% of the aircraft-observed O3 and CO and 75% of NOy can be reproduced by the model with relative differences less than 50 and 100%, respectively. The model-simulated correlations among O3, CO, and NOy are close to the aircraft observations. Most of the model-calculated vertical distributions of these species have the same pattern as the observations. The calculated profiles coincide with the observations with mean discrepancies of about 25, 30, and 100% for O3, CO, and NOy, respectively. The model-calculated wind speeds are in good agreement with the observations. Continental plumes within the model may be spatially off by a few hundred kilometers or temporally off by several hours. Possible inaccuracy in side boundary conditions and some difficulties in capturing small scale processes like small scale intrusion from the stratosphere and subgrid scale convective mixing are possible explanations for discrepancies between chemical data and model results. ¿ 1998 American Geophysical Union