Anthropogenic emissions from rapid urban sprawl, commuter/commercial traffic and industrialization along the East Coast of the United States have a profound effect on urban and regional air quality. Surface air quality over populated areas is an important issue given persuasive data linking high levels of atmospheric oxidants and particulate matter to deleterious human health effects and habitat degradation. The NARSTO Northeast Oxidant and Particle Study (NE-OPS) seeks an improved understanding of the sources, sinks, transport and photochemical transformations controlling the observed abundance of photochemical oxidants and fine particulate haze over the U.S. mid-Atlantic region. In order to: (1) place NE-OPS surface measurements into a larger-area perspective; (2) investigate meso-scale differences in PBL height and dynamics; (3) characterize synoptic-scale transport to the Philadelphia area; (4) investigate chemical/physical transformations across the urban corridor; and (5) evaluate surface in situ and remotely-sensed measurement techniques and instrument performance research flights were conducted from light aircraft measuring meteorological scalars (T, P, RH); selected trace gases (O3, CO, SO2) and aerosol optical properties (Bap, Bscat) during three NE-OPS intensive operational periods: August 1998; July-August 1999 and July 2001, concentrating around the Philadelphia, PA urban area. Intercomparisons of opportunity with a tethered balloon and surface lidar operated continuously during NE-OPS intensives showed generally good agreement. Aircraft results combined from all three intensives will be presented demonstrating substantial diurnal changes in boundary layer height and chemical character over Philadelphia during the course of a high ozone/haze episode, and sugegsting significant influence at times from synoptically-forced regional scale transport of pollution. Mesoscale effects, including the influence of low-level jets on air quality, will also be discussed.