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most of the troposphere. The subsiding air is warmed adiabatically and thus tends to make the troposphere more stable and less conducive to convective mixing. Adiabatic warming of the air occurs as the air compresses while sinking; no heat is added to it.

• The subsidence of air associated with large high-pressure systems creates a pronounced inversion of the normal temperature profile (normally temperature decreases with height in the troposphere), which serves as a strong lid to contain pollutants in a shallow layer in the troposphere, as is common in the Los Angeles basin, for example. During an inversion, the temperature of the air in the lower troposphere increases with height, and the cooler air below does not mix with the warmer air above.

• Because winds associated with major high-pressure systems are generally light, there is a greater chance for pollutants to accumulate in the atmospheric boundary layer, the turbulent layer of air adjacent to the earth's surface.

• The often cloudless and warm conditions associated with large high-pressure systems also are favorable for the photochemical production of ozone (see Chapter 5).

In the eastern United States and Europe, the worst ozone pollution episodes occur when a slow-moving, high-pressure system develops in the summer, particularly around the summer solstice. This is the time with the greatest amount of daylight, when solar radiation is most direct (the sun is at a small zenith angle) and air temperatures become quite high (greater than 25ºC) (RTI, 1975; Decker et al., 1976). As the slow-moving air in the shallow boundary layer passes over major metropolitan areas, pollutant concentrations rise, and as the air slowly flows around the high-pressure system, photochemical production of ozone occurs at peak rates. Major high-pressure systems at the earth's surface are associated with ridges of high-pressure surfaces in the middle and upper troposphere. Forecasting the onset of a major episode of ozone pollution in the eastern United States involves predicting the development of ridges of high pressure at 500 millibars (mb). These ridges are generally well predicted by global numerical prediction models for periods of 3-5 days (Chen, 1989; van den Dool and Saha, 1990). High ozone episodes are often terminated by the passage of a front that brings cooler, cleaner air to the region.

The accumulation of ozone in the Los Angeles basin illustrates the importance of meteorology. The weather in that area is dominated by a persistent Pacific high, which causes air subsidence and the formation of an inversion that traps the pollutants emitted into the air mass. The local physical geography exaggerates the problem, because the prevailing flow of air in the upper atmosphere is from the northeast, which enhances the sinking motion of air

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