The goal of this project is to evaluate model scenarios used by the TNRCC in its SIP development. UCR will repeat the design day model simulations with new model diagnostic outputs. These new model outputs will describe the production rates and budgets of O3, odd oxygen (Ox), radical species (HOx) and odd nitrogen (NOy). UCR will first compare the performance of versions 2 and 3 of the CAMx model (ENVIRON, 2001) to determine whether improvements in the vertical dispersion in CAMx Version 3 affect O3 predictions for the Houston modeling. UCR will then select a version of the CAMx model to perform additional modeling to characterize the model hourly production rate of O3 (P(O3)), and we will compare this with the magnitude of hourly changes in ambient O3 observed in the THOE phenomenon. UCR will then identify the chemical process that contribute to P(O3) in the base case scenarios. Finally, UCR will perform model simulations for up to 4 alternative scenarios to evaluate model performance and sensitivity to assumptions used in the input data sets. This project is intended to be an investigation of model performance and does not include detailed comparisons of model to ambient data. We will, however, suggest model to data comparisons that should be performed in future work.