SENSITIVITY OF OZONE TO MODEL GRID RESOLUTION .2. DETAILED PROCESS ANALYSIS FOR OZONE CHEMISTRY

Most of the current air quality models used to simulate ozone (O-3) fo rmation predict only the concentrations of O-3 without the capabilitie s of understanding and explaining the formation processes of O-3. In t his paper we present a process analysis method developed to understand and quantify the chemical and physical processes that lead to formati on of O-3 in Eulerian grid models. In a previous study we used a high- resolution version of regional acid deposition model (HR-RADM) to simu late O-3 formation at different grid resolutions. In this study we fur ther applied this detailed process analysis method to the HR-RADM simu lations to determine the roles of individual mechanistic processes con tributing to O-3 formation, as well as to examine the effects of grid resolution on these regulating processes. We first selected several so urce areas and examined the processes that lead O-3 formation in these areas. The ''OH-cycle'' and ''NO-cycle'' pathways derived from the pr ocess analysis method appear as important measures that can significan tly enhance our ability to quantify and explain the formation processe s of O-3. We also compared O-3 processes between two different grid re solutions over an equal source area with nearly equal emissions. The r esults suggest that (1) the effects of grid resolution on the chemistr y of NOx are far more important than that on the chemistry of VOC; (2) grid resolution significantly influences the competing rates of chemi stry and vertical transport processes for the emitted NOx, causing the differences in O-3-predictions between two different grid resolutions . Because the balance of chemistry and vertical transport controls the model predictions, correct representation is needed for both. This le ads to a conclusion that to improve model accuracy in predicting O-3 f ormation, it is not only necessary to have adequate horizontal grid re solution, but also necessary to have adequate vertical grid resolution and accurate representation of the vertical transport process.