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Temperature (°C)

FIGURE 2.19 Temperature and oxidant profiles at 1:28 p.m. over Santa Monica, California, on June 20, 1970 (from Edinger, 1973).

within the inversion layer, reaching concentrations as high as 0.2 ppm, compared to a ground-level concentration of ~0.1 ppm.

The reason for this phenomenon is that the mountain slopes surrounding the Los Angeles air basin become heated by the sun; the layer of air in contact with the slopes is also heated and moves up the slopes. When the inversion layer is deep and strong, much of this rising polluted air does not get sufficiently warm to penetrate the inversion completely, so it moves out and away from the slopes and remains within the inversion layer; multiple pollution peaks within the inversion layer result.

Because of such meteorological phenomena, pollutants can be trapped aloft and transported over large distances, undergoing essentially no deposition at the surfaces. Clearly, surface measurements are not adequate to document such high-level transport.

The boundary layer is the lowest part of the atmosphere, closest to the earth's surface. Stull (1988) defines the boundary layer as "that part of the troposphere that is directly influenced by the presence of the earth's surface, and responds to surface forcings [such as heat transfer, pollutant emissions, evaporation etc.] with a timescale of about an hour or less." Typical boundary layer heights range from ~ 100 to 3000 m in altitude. The rest of the overlying troposphere is called the free troposphere.

Figure 2.20 summarizes the role of inversions and the boundary layer in terms of typical changes in mixing of the atmosphere close to the earth's surface at various times of the day (Stull, 1988). At midday, there is generally a reasonably well-mixed convective layer lying above the surface layer into which the direct emissions are injected. As the sun goes down, radiative cooling results in the formation of a stable nocturnal boundary layer, corresponding to a radiation inversion. Above this is a residual layer that contains the species that were well-mixed in the boundary layer during the day but that do not mix rapidly during the night with either the nocturnal boundary layer below or the free troposphere above. At sunrise, heating of the earth's surface results in mixing of the contents of the nocturnal boundary layer and the residual layer above it. Clearly, such meteorological changes can have significant impacts on the spatial distribution of pollutants emitted at the earth's surface, leading to chemistry that varies both spatially (in 3 dimensions) and diurnally. Coupling between the transport processes and chemistry on local to global scales is discussed by Kley (1997).

In summary, as we shall see throughout this book, meteorological parameters are extremely important, not only in determining the dispersion and transport of pollutants but also in determining their chemistry. The reader is encouraged to consult meteorology texts for a much more detailed treatment of this subject.

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