This paper presents the application and initial assessment of a near-realtime air quality modeling system designed to simulate O3 and PM2.5 over the northeastern United States for July 1 – August 31, 2004. The air quality modeling system used in this study consists of operational weather forecasts from the National Weather Services ETA model at a horizontal resolution of 12 km, the PREMAQ emissions and meteorology pre-processor, and the Community Multiscale Air Quality (CMAQ) model. The simulations were performed as a pilot study between the National Oceanic and Atmospheric Administration (NOAA), the U.S. Environmental Protection Agency (EPA), and the New York State Department of Environmental Conservation (NYSDEC). The details and operational aspects of the air quality modeling system are discussed. Model predictions of 1-hr and 8-hr O3 as well as PM2.5 mass are compared against observations from EPA’s AIRNOW system using both discrete and categorical evaluation metrics. These comparisons will focus both on pollutant concentrations and the Air Quality Index (AQI), a derived quantity used to communicate ambient air quality conditions to the public. To assess the modeling system’s usefulness as a forecasting tool, we compare forecast error statistics for the model predictions against those computed for operational non-model based air quality forecasts issued by NYSDEC. Finally, to address the role of errors in the forecasted meteorology on the predictions of the AQI, we present results from the re-simulating of a high PM2.5 concentration event using meteorological fields that were closer in time to the ETA initialization time than possible under operational constraints.