The temporal and spatial variations of ozone (O3) in polluted continental outflow over the North Atlantic were investigated during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign in July--August 2004. Our analysis utilized measurements of O3 from three smart balloons traveling at 0.5--3 km altitude in combination with simulations using the MM5/SMOKE/CMAQ air quality modeling system. Model results for over and within 300 km off North America were corroborated by comparison to a suite of measurements from ground stations, ozonesondes, and the NOAA ship Ronald H. Brown cruising in the Gulf of Maine. A prominent feature of the O3 distribution was the high mixing ratios over the North Atlantic, reaching a peak value of 171 ppbv, compared to the northeastern United States (2 days) it exhibited only -0.05--0.05 ppbv km-1. These same two balloons encountered Hurricane Alex at different times, but both measured O3 mixing ratios >100 ppbv. Our model results show clearly that polluted air from the mid-Atlantic states was channeled directly into Alex's inflow region. Overall, variations in O3 on timescales of tens of minutes to hours are attributed to its highly heterogeneous distribution in urban plumes, with variations over hours to days caused by changing source regions related to cyclonic activity.