A nested meteorological-chemical modeling system is used to simulate the evolution of oxidants and aerosols in the vicinity of Philadelphia. 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. In a previous study, gas-phase chemistry on a regional 24-km grid encompassing eastern North America was employed. This study expands upon the previous work by 1) adding a 4-km grid that encompasses the urban corridor between Chesapeake Bay to New York City and 2) including aerosols. Good agreement between the simulations and observations was obtained. Details of the G-1 aircraft measurements, including urban and stack plumes, were produced by the model. Differences between the 4-km and 24-km simulations will be discussed. The 24-km simulation reproduced the diurnal variation and multi-day trends in ozone over Philadelphia; however, NOx and NOy mixing ratios were too low. The NOx and NOy mixing ratios from the 4-km simulation were much closer to the measurements, with a modest improvement in ozone as well. The relative contribution of stack, urban, and regional emission sources on local ozone concentrations in the vicinity of Philadelphia and the effect of aerosols on gas-phase chemistry will be examined.