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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castellano: DISPER CUSTIC DESCAR RADIA italiano:
deutsch: DIS CUS DES RAD
castellano: DIS CUS DES RAD english: DIS CUS DES RAD
português: DIS CUS DES RAD italiano: DIS CUS DES RAD
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