In support of the NARSTO-NE-OPS 1998, Millersville University deployed two tethered balloons to an altitude of 300 meters near Philadelphia, PA in order to document the structure and evolution of the urban boundary layer.  From 14-22 August 1998, sensors carried aloft on a 5 m3 tethered balloon produced 176 vertical profiles of O3 concentration and meteorological variables (T, p, RH, wind speed and direction).  A 100 m3 balloon was used to suspend particle samplers at altitudes of 75, 150, 225, and 300 m AGL in order to obtain integrated PM2.5 "dry" mass aloft in conjunction with surface PM measurements obtained over the same period.  This dataset is combined with the NOAA network of surface and upper air measurements, radar and satellite imagery, and models, as well as measurements obtained by other participating institutions, to investigate the atmospheric conditions leading to the observed concentrations of O3 and fine particles in the urban plume.  Measurements obtained by the tethered instruments were also useful for intercomparisons with surface, lidar and aircraft measurements, and filled the data void between the surface and the lowest levels accessible to these platforms.  The results of this pilot study are discussed with special emphasis on the case study of 20-22 August 1998 when the propagation of an aloft thermal ridge produced a moderate episode with O3 concentrations reaching 100 ppbv and PM2.5 mass increasing by a factor of two (25 – 50 mg m-3).  Results indicate that the episode developed concurrently with the increased stability brought about by the advection of warm air aloft, subsidence, and light surface winds.  Of interest is the fact that moderate-to-high concentrations of pollutants can be achieved without the extensive stagnation commonly associated with northward movement of a subtropical air mass, and is important because of the frequency with which short-wave propagating systems affect the northeast corridor.