A nested meteorological-chemical modeling system is used to determine the relative contribution of regional-scale transport and local photochemical production on air quality in the vicinity of Philadelphia. A regional-scale domain encompasses eastern North America with a 24-km grid spacing while nested domains centered over Philadelphia employ 8 and 4-km grid spacings. Surface and airborne meteorological and chemical measurements made during a 30-day period in July and August of 1999 as part of the Northeast Oxidant and Particulate Study (NE-OPS) are used to evaluate the performance of the modeling system. Good agreement between the simulations and observations was obtained. Layers of ozone above the convective boundary layer were measured by both research aircraft and ozonesondes during the morning. In the model, these layers developed as result of upwind vertical mixing processes the previous afternoon, subsequent horizontal transport aloft, and depletion of ozone by NO titration within the stable boundary layer at night. Ozone aloft was then entrained into the growing convective boundary, contributing to a significant fraction of the surface ozone concentrations. Through a series of sensitivity studies, most of the ozone is the result of emissions in the vicinity of Philadelphia, but up to 30-40% of the ozone during high-ozone episodes was due to transport from sources located 200 km or more upwind. Transport of precursors also contributed to the local photochemical production of ozone. Because of light winds, local emissions and meteorological conditions were largely responsible for one high-ozone episode during the 30-day period.