FIGURE 2.17 Isentropic surface at 300 K (i.e., surface of constant potential temperature) over eastern North America on September 8, 1992 (from Berkowitz et al, 1995).

negative lapse rate, is known as an inversion layer. In effect, it acts as a "lid" on an air mass because the cooler air underneath it, being more dense, will not rise through it. In effect, pollutants trapped below the inversion layer are not mixed rapidly throughout the entire troposphere but are confined to the much smaller volume beneath the inversion layer; this generally results in much higher ground-level concentrations of species that are emitted at the surface.

The formation of thermal inversions is one of the most important meteorological factors contributing to air pollution problems in urban areas. There are two major sources of thermal inversions. Radiation (or

FIGURE 2.18 Variation of temperature with altitude within the troposphere: (a) normal lapse rate; (b) change in lapse rate from positive to negative, characteristic of a thermal inversion.

ground) inversions are caused by the rapid cooling of the earth's surface, along with the layer of air immediately above it, by the emission of infrared radiation immediately after sunset. On calm nights, this cooling may be sufficiently rapid that the layer of air adjacent to the surface becomes cooler than the air above; that is, an inversion forms. This can persist until sufficient heating of the surface and the air above it occurs to "break" the inversion at dawn. With this type of inversion, the inversion height—the distance from the earth's surface to the point at which the lapse rate reverses—is often quite small. For example, in the 1952 London smog episode, inversion heights as low as 150 ft were observed in some locations.

Overhead (or subsidence) inversions associated with photochemical air pollution are caused by the sinking motion of air masses as they pass over the continent. This leads to compression and heating of the air immediately below, resulting in a change in the lapse rate, that is, to the formation of an inversion layer. The inversion height is significantly higher than in the case of radiation inversions; for example, ~ 1500 ft would represent a relatively low subsidence inversion height.

Interestingly, the vertical distribution of photochemical oxidant may not be such that it falls off rapidly at the inversion layer. In fact, in a classic series of experiments, Edinger and co-workers (1972, 1973) showed that oxidant concentrations in the Los Angeles air basin could be higher within the inversion layer than at ground level. Thus, Fig. 2.19, for example, shows one temperature and oxidant profile for June 20, 1970, over Santa Monica, California, a city adjacent to the Pacific Ocean. Several "layers" of oxidant (mainly 03) exist

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