Sensitivity of ozone to model grid resolution—I. Application of high-resolution regional acid deposition model

This paper examines the sensitivity of ozone (O3) 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 O3 formation at different grid resolutions. Horizontal grid-cell sizes of 20, 40, and 80 km were selected for this sensitivity study. Individual meteorological and chemical processes that contribute to O3 and its precursors were further separated and analyzed to determine their importance to O3 formation and the effects of grid resolution on these regulating processes. We first examined the model predictions of O3 maxima and minima at different grid resolutions over several major source areas. The results showed that the coarser-grid model tended to underpredict O3 maxima and overpredict O3 minima over the major source areas, because emission strengths were not as well resolved. Process contribution analyses of O3 over these source areas revealed that grid resolution significantly influences the magnitude of O3 formation and loss processes, especially chemistry and vertical transport. 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 with respect to grid resolution. These comparisons showed that for nonreactive species, the average transport applied to a coarse-grid cell is the same as that applied to the same area at higher resolution. For reactive species, however, the average transport is no longer the same between two different grid resolutions because the transport process interacts closely with chemistry, which is nonlinearly related to grid resolution. As a result, over the same source area, the coarser grid tended to predict more O3 but less NO2 from chemistry and to export more O3 and NO but less NO2 by vertical transport than did the finer grid.