Sensitivity of ozone to model grid resolution — II. Detailed process analysis for ozone chemistry

Most of the current air quality models used to simulate ozone (O3) formation predict only the concentrations of O3 without the capabilities of understanding and explaining the formation processes of O3. In this paper we present a process analysis method developed to understand and quantify the chemical and physical processes that lead to formation of O3 in Eulerian grid models. In a previous study we used a high-resolution version of regional acid deposition model (HR-RADM) to simulate O3 formation at different grid resolutions. In this study we further applied this detailed process analysis method to the HR-RADM simulations to determine the roles of individual mechanistic processes contributing to O3 formation, as well as to examine the effects of grid resolution on these regulating processes. We first selected several source areas and examined the processes that lead O3 formation in these areas. The “OH-cycle” and “NO-cycle” pathways derived from the process analysis method appear as important measures that can significantly enhance our ability to quantify and explain the formation processes of O3. We also compared O3 processes between two different grid resolutions over an equal source area with nearly equal emissions. The results suggest that (1) the effects of grid resolution on the chemistry of NOx are far more important than that on the chemistry of VOC; (2) grid resolution significantly influences the competing rates of chemistry and vertical transport processes for the emitted NOx, causing the differences in O3 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 leads to a conclusion that to improve model accuracy in predicting O3 formation, it is not only necessary to have adequate horizontal grid resolution, but also necessary to have adequate vertical grid resolution and accurate representation of the vertical transport process.