3.3 Horizontal Grid Cell Size

The horizontal dimension of each model grid square is based

upon (1) the sensitivity of predicted concentrations to horizontal

grid size, (2) the resolution of observed meteorological and air

quality data and/or estimated emissions data, and (3) limitations

imposed by other considerations such as a required minimum domain

size. Generally, large grid square dimensions result in smoothing

of the emission gradients, wind fields, and spatially varying

mixing heights, which in turn leads to a smoothing of the predicted

concentration field. Also, larger grid cell dimensions reduce both

computer storage space and computational time.

The following should be considered when selecting the

horizontal grid cell size:

1. The grid cells should be small enough to reflect emission

gradients and densities in urban areas, particularly

those resulting from large point sources and major

terrain or water features that may affect air flow

2. Sensitivity studies conducted by the EPA suggest that

peak ozone predictions may increase as grid size


3. Practical limitations on the grid cell size are the

resolution of the emission inventories and the density of

meteorological and air quality monitoring networks

Previous modeling studies have used horizontal grid cell sizes

generally in the range of 2 x 2 km to 8 x 8 km. A grid size of

5 x 5 km has generally been compatible with computer resource

requirements and emission inventory development.



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