A comparison of trace gas concentrations between a ground sampling site and an instrumented aircraft was performed at the Scotia field site in central Pennsylvania. The comparison included four research flights in August 1988. The ground site was operated by the National Oceanic and Atmospheric Administration (NOAA) Aeronomy Laboratory, and the NOAA King Air instrumented aircraft was operated by the NOAA Air Resources Laboratory. The study demonstrates continuity in the values of the state parameters (wind speed and direction, temperature, and dew point) from the surface to the aircraft observations at the tines of the four flights. Considered within the context of the differing time-space scales, continuity was also demonstrated in the trace gas measurements. The aircraft and surface measurements of the trace gas concentrations were in better agreement when the aircraft measurements demonstrated uniform concentrations over the length of the flight path. The mean O3 and H2O2 values of the lowest-altitude aircraft readings were usually greater than those measured at the surface, thus indicating higher-altitude sources and removal processes at lower altitudes. The aircraft-measured values for SO2, NOy, and hydrocarbons were usually lower than those measured at the surface, as expected for primary pollutants emitted near ground level. Closer agreement was obtained in the comparisons of secondary (O3, H2O2) than of primary (SO2, NOy, hydrocarbons) pollutants. ¿ American Geophysical Union 1991