As part of the 1999 NARSTO Northeast Oxidant and Particulate Study (NE-OPS) field campaign, the DOE G-1 aircraft conducted 20 research flights in and around Philadelphia. Most flights consisted of boxes around the Philadelphia urban area. The NE side of the smallest box was located about 20 km from downtown Philadelphia and included vertical spirals and horizontal transects providing detailed coverage between 300 m and 2.5 to 3 km. Repeated transects at different times of the day gave information on vertical mixing. Trace gas measurements pertinent to understanding O₃ formation included O₃, CO, VOCs, NO, NO_y, SO₂, HCHO, H₂O₂, and organic peroxides. Measurements of these species were used as input to a constrained steady state box model that gave predictions for radical concentrations and the rate of O₃ production. We provide an overview of the G-1 data set and calculated ozone production rates. Highest O₃ concentrations in the program were observed on July 31, a day with very light wind speeds. A mid-morning and mid-afternoon flight indicate that the O₃ increase observed from the G-1 (from about 90 to 130 ppb) can be accounted for by local production. On the same day an industrial plume with extreme values of NO, CO, VOCs, and SO₂ was traversed to the SW of Philadelphia. This plume was found at the same location in the afternoon with lower concentrations of primary pollutants and with a peak O₃ of 143 ppb.