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FIGURE 16.23 Observed (•) and model-predicted 03 concentrations at West Point, New York, July 7, 1988, assuming (a) a spatially invariant mixing height or (b) a spatially varying mixing height. The solid line is the predicted maximum and the dashed line the predicted minimum 03 concentration (from Sistla et al, 1996).

FIGURE 16.23 Observed (•) and model-predicted 03 concentrations at West Point, New York, July 7, 1988, assuming (a) a spatially invariant mixing height or (b) a spatially varying mixing height. The solid line is the predicted maximum and the dashed line the predicted minimum 03 concentration (from Sistla et al, 1996).

It is important to note that it is not just the surface meteorological parameters that must be specified accurately, but those aloft as well. For example, many decades ago it was argued by some that high ozone levels downwind in Southern California could not result from emissions in the metropolitan Los Angeles area because the surface wind speeds were too slow to transport the pollutants in the observed time frame. However, tracer studies established that wind speeds aloft were, in fact, higher and that transport at higher elevations and mixing to the surface were, in fact, responsible for the higher observed concentrations. Accurate measurement of the vertical distribution of meteorological parameters is particularly important when there are significant emissions sources aloft such as power plants (e.g., see Al-Wali and Samson, 1996).

Figure 16.24 illustrates the vertical transport processes and complex interactions that can occur between urban areas, downwind regions, and the free troposphere (Fast, 1998). Emissions from urban regions into the surface layer can be transported into the mixed layer as well as into the free troposphere by several mechanisms, including venting up mountain slopes due to solar heating of the surface, which creates a chimney effect. As shown in Fig. f6.24, clouds also play a role in these vertical transport processes.

In many field studies, elevated concentrations of 03, particles, and other air pollutants have been found in

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