The Regional Atmospheric Chemistry Mechanism (RACM) is a widely used mechanism for air quality modeling. Recently a new version of the mechanism, RACM2, has been developed. RACM and RACM2 have been applied to an analysis of a number of daytime and nighttime modeling cases. This study includes an investigation of the response of forecasted ozone, nitric acid and sulfate to representative initial conditions for both mechanisms. Understanding this response is expected to be critical for the development of new chemical data assimilation methods for air quality forecasting.