A recent study of the Atlanta area (Cohan, 2006) used the Decoupled Direct Method
(DDM) to examine the sensitivity of ozone production efficiency to horizontal resolution. The
efficiency of ozone production should be closely related to model response to emisions controls.
The analysis found that 12km resolution is sufficient (in Atlanta) to predict ozone production
efficiency. However, 4km resolution is needed to identify small areas of local NOx inhibition.
They also found that model performance for ozone at 4km and 12km resolution was similar.
Horizontal resolution will also have an impact on primary particulate matter. Because
there is no chemistry, we expect spatial gradients in concentrations to be much higher for
primary PM components compared to secondary components. The model’s ability to capture
these gradients will depend in large part upon the horizontal resolution of the model. Therefore,
emissions density is an important factor in considering grid resolution. Additionally, terrain
features (mountains, water) should be considered when choosing grid cell size.
Intuitively, one would expect to get more accurate results in urban applications with
smaller grid cells (e.g., 4 km) provided the spatial details in the emissions and meteorological
inputs support making such predictions. Thus, using 4 km grid cells for urban or fine portions of
nested regional grids and 12 km cells in coarse portions of regional grids are desirable goals.
However, model performance at 4 km resolution may not be any better than at 12 km resolution.
Additionally, extensive use of urban grids with 4 vs. 12 km grid cells and regional grids with 12
vs. 36 km grid cells greatly increases computer costs, run times and data base management
needs. Further, elsewhere in this guidance we identify needs to model large domains, many
days, and several emission control scenarios. We also identify a number of diagnostic tests
which would be desirable and suggest using more vertical layers than has commonly been done
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