Fine particulate matter (PM2.5) in the atmosphere has been a great concern in recent years, affecting human health and contributing to reduced visibility (regional haze). In 1997 EPA revised the national ambient air quality standards, creating a new standard for fine particulate matter at 15 mg m-3 for an annual average and 65 mg m-3 for a daily average. Unlike other criteria pollutants such as ozone, PM2.5 is hard to model since PM2.5 consists of mixtures of hundreds of components over a wide size distribution range caused by various of physical and chemical processes.

In this study, we applied two widely used models MODELS-3/CMAQ and CAMx to simulate PM2.5 and ozone for the period from July 10 to 18, 1999 over the eastern United States. The models’ results will be evaluated against continuous PM2.5 and ozone measurements available from special and routine monitoring networks over the modeling domain. The differences and agreements between the models and the ambient data are discussed. Also the physical and chemical processes in the models contributing to the differences in the results will be evaluated.