As part of the North Atlantic Regional Experiment, the National Research Council of Canada Twin Otter aircraft was used to measure the concentration of O3 and related compounds in the atmosphere over southern Nova Scotia. Forty-eight flights were conducted, primarily over the surface sampling site in Chebogue Point, Nova Scotia, or over the adjoining Atlantic Ocean. A typical flight included one or more vertical profiles from 30 m above the surface to an altitude of 3 or 5 km. We present here O3 measurements and supporting chemical and meteorological data including NOy, CO, accumulation mode aerosol particles, winds, temperature, and dew point. Data are presented in a format which illustrates day-to-day variability and vertical structure. We find that Nova Scotia is impacted by a wide variety of air masses with varying chemical content depending on flow conditions relative to the locations of upwind emission regions. As an aid to understanding the chemical composition of the air, we characterize four types of events: (1) moist continental boundary layer air with high concentrations of O3 and other anthropogenic pollutants which is advected to Nova Scotia in relatively thin vertical layers, usually with a base altitude of several hundred meters; (2) ''background'' air with concentrations of anthropogenic ingredients much lower than experienced in continental pollution episodes but higher than observed in more remote regions of Canada, suggesting a dilute anthropogenic or biomass burning influence; (3) near-surface air which because of a strong temperature inversion over the Atlantic Ocean, is decoupled from air aloft, with the consequence that near-surface measurements do not give a representative view of the eastward transport of the North American plume; and (4) dry air masses with high O3 concentration in which we have to distinguish between boundary layer and upper atmosphere source regions. ¿ American Geophysical Union 1996