Chemical measurements of O3 and other trace substances in the atmosphere over southern Nova Scotia, obtained during 48 flights of the National Research Council of Canada Twin Otter aircraft, provide evidence for a variable degree of processing associated with transport times ranging from less than 1 day to 5 days or more. Effects of chemical aging and dilution are determined using photochemical age estimates derived from the ratios, ln (n-butane/propane) and ln (i-butane/propane). Age estimates are used in a qualitative sense to divide the data set into four age groups. Vertical profiles and relations between O3 and other trace substances are examined as a function of age group. We find that high-O3 events occurring in dry air are in the oldest age category. Moist high-O3 events have photochemical ages that span all four age categories, but the most polluted episodes are only observed in relatively ''new air.'' A geographically wide range of emission regions is suggested by the hydrocarbon measurements, which is consistent with back trajectory results. The relative depletion of soluble substance (i.e., aerosol particles and NOy) in dry, relative to moist, high-O3 air masses is discussed with respect to the dilution or precipitation scavenging that must accompany the transport of pollutants from the boundary layer to the dryer middle or upper troposphere. Four case studies are presented that span a wide range of conditions associated with high O3 concentrations. In a pair of these cases, samples were obtained in air masses having about the same photochemical age, altitude, and C2H2 and O3 concentration. One of the pair was from dry air and the other from moist air. The dry air mass had much lower concentrations of NOy and aerosol particles, which was interpreted as evidence for the selective removal of soluble constituents during vertical lifting. The other case studies illustrate a biomass burning plume and the effects of a stable layer over the ocean on transport. ¿ American Geophysical Union 1996