The NARSTO-NE-OPS (North East Oxidant and Particle Study) is an investigation of the coupling of the meteorological and chemical processes that control the evolution of air pollution events. The project includes four major field programs which have been carried out at a field site in northeast Philadelphia during the summers of 1998, 1999, 2001 and 2002. The activity brings together the research groups from several universities and government laboratories to apply advanced measurement techniques for understanding the physical and chemical processes contributing to air quality issues. During the 21 cumulative weeks of summer field intensives, the meteorological and chemical characteristics attending a wide range of atmospheric processes were observed. The results confirm that a three-dimensional regional scale picture of the atmosphere is essential to understanding air pollution events. Vertical profiles of atmospheric properties are needed to properly model and predict ozone and particulate matter pollution because of the significance of horizontal transport aloft and vertical mixing processes on the development and evolution of important periods of air pollution. Efforts have also focused on development and testing of several new approaches to improve measuring techniques for better understanding of the physical and chemical properties of the airborne particulate matter.