SENSITIVITY OF OZONE TO MODEL GRID RESOLUTION .1. APPLICATION OF HIGH-RESOLUTION REGIONAL ACID DEPOSITION MODEL

This paper examines the sensitivity of ozone (O-3) predictions to grid resolution in Eulerian grid models. A high-resolution version of the regional acid deposition model (HR-RADM) was developed and applied to simulate O-3 formation at different grid resolutions. Horizontal grid- cell sizes of 20, 40, and 80 km were selected for this sensitivity stu dy. Individual meteorological and chemical processes that contribute t o O-3 and its precursors were further separated and analyzed to determ ine their importance to O-3 formation and the effects of grid resoluti on on these regulating processes. We first examined the model predicti ons of O-3 maxima and minima at different grid resolutions over severa l major source areas. The results showed that the coarser-grid model t ended to underpredict O-3 maxima and overpredict O-3 minima over the m ajor source areas, because emission strengths were not as well resolve d. Process contribution analyses of O-3 over these source areas reveal ed that grid resolution significantly influences the magnitude of O-3 formation and loss processes, especially chemistry and vertical transp ort. We also compared the process contributions between two different grid resolutions over an equal source area with nearly equal emissions to examine the nonlinearities of processes and their interactions wit h respect to grid resolution. These comparisons showed that for nonrea ctive species, the average transport applied to a coarse-grid cell is the same as that applied to the same area at higher resolution. For re active species, however, the average transport is no longer the same b etween two different grid resolutions because the transport process in teracts closely with chemistry, which is nonlinearly related to grid r esolution. As a result, over the same source area, the coarser grid te nded to predict more O-3 but less NO2 from chemistry and to export mor e O-3 and NO but less NO2 by vertical transport than did the finer gri d.