The accuracy of predicted base case ozone and PM concentrations will be affected by

how well the model is able to characterize dilution of ozone, PM, and precursors. This, in turn,

depends in part on how precisely the model can estimate mixing heights (i.e., the PBL). The

precision of mixing height estimates is affected by the thickness of the model’s vertical layers

aloft which are near the anticipated mixing height (Dolwick, 1999). Ozone concentrations are

most heavily influenced by the rate of rise in the morning mixing height and the maximum

afternoon mixing height. Since PM is measured as a 24-hour average concentration, PM is also

influenced by the strength of nighttime mixing and the presence of a low level inversion.

Because mixing heights may vary on different days and it is necessary to simulate numerous

days and locations, model predictions can be influenced by the number of vertical layers

considered by the model.

Placement of vertical layers within the planetary boundary layer is also an important

issue. For practical reasons, it is best to have an air quality model’s vertical layers align with the

interface between layers in the meteorological model. In view of the importance of carefully

specifying the temporal variation in mixing height, we recommend high precision below and

near the anticipated maximum afternoon mixing height. In addition, specifying the vertical

extent of mixing overnight during stable conditions is also an important consideration in

determining the vertical layer structure. In this regard, we recommend that the lowest layer in

the air quality model be no more than 50 meters thick. In general, layers below the daytime

mixing height should not be too thick, or large unrealistic step increases in mixing may occur.

Layers above the boundary layer are important for characterizing clouds and precipitation, but

are less important to the daily mixing processes of pollutants. Therefore, vertical resolution

above the boundary layer is typically much coarser.


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