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t Cloud-top entrainment

Urban heat island convergence

Slope flow ^ detainment 4

C2ioud ^ venting

Slope flow ^ detainment 4

Terrain-related convergence

Urban heat island convergence

FIGURE 16.24 Schematic diagram of vertical mixing processes for pollutants (the authors thank Jerome Fast of the Pacific Northwest National Laboratory for graciously providing this diagram).

FIGURE 16.24 Schematic diagram of vertical mixing processes for pollutants (the authors thank Jerome Fast of the Pacific Northwest National Laboratory for graciously providing this diagram).

layers at higher altitudes; indeed, as seen earlier in Fig. 2.19, it has been known for a number of years that the concentrations aloft in the Los Angeles area often exceed the concentrations found simultaneously at the surface (Edinger, 1973). These observations are consistent with the results of model studies of this region (Lu and Turco, 1996). For example, Fig. 16.25 shows model-predicted ozone concentrations for one day (August 27, 1987) across this air basin and as a function of altitude and time. Figure 16.25a, representing noon PST, shows the ozone at the eastern end of the air basin held primarily in the surface layer. However, some venting up the mountain slopes is also predicted. By 4:00 p.m., transport of precursors while they react to form 03 into the eastern region has occurred, generating higher surface concentrations. Increased venting along the mountain slopes injects these high 03 levels into the free troposphere. By 8:00 p.m. (Fig. 16.25c), a stable boundary layer has formed (see Fig. 2.20), and the aged air containing high concentrations of 03 and other photochemically formed species, including particles, are found in the region of the temperature inversion; the inversion itself is enhanced by the movement of cooler marine air inland from the west.

This phenomenon of elevated layers of pollutants aloft and subsequent mixing to the surface is not restricted to areas such as Los Angeles that have large meteorological effects caused by the mountains that ring this region. For example, Berkowitz and co-workers report similar observations near Nashville, Tennessee, over the eastern United States, and over the western region of the North Atlantic Ocean (Berkowitz and Shaw, 1997; Berkowitz et ai, 1995, 1998).

These layers containing higher concentrations of pollutants provide an important mechanism for transport of ozone, particles, and their precursors to the free troposphere. In addition, in the morning when solar heating causes turbulent mixing (Fig. 2.20), these pollutants are mixed down to the surface. This not only increases the surface concentrations but also provides species that can initiate the VOC-NOx chemistry that leads to more ozone formation. As a result, there is a carryover from one day to the next, leading to smog episodes in which the pollutant concentrations increase from day to day.

Most Eulerian models assume that emissions are immediately mixed within the grid box into which they are emitted. However, depending on the size of the grid and the particular meteorological conditions, this may not be a good assumption. Stockwell (1995) has examined the effects of this assumption on model predictions and suggested how such considerations might be used in developing appropriate grid structures in models.

Finally, temperature is an important meterological parameter that affects the formation of ozone and associated species. It is known from environmental chamber studies (e.g., Hatakeyama et al., 1991), modeling studies, and air quality data (e.g., Cardelino and Chameides, 1990; Sillman and Samson, 1995) that higher temperatures are more conducive to ozone for

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