Digging Deeper: Air Quality and Pressure SystemsRegions susceptible to air pollution problems result from the interaction of climatic and topographic conditions in the presence of pollution sources. Air pollution is especially prominent where high pressure dominates. Subsiding motions within an anticyclone suppress air trying to rise off the surface. Adiabatic warming of subsiding air creates a subsidence inversion which acts as a cap to upwardly moving air. Pollution problems dissipate when a low pressure system replaces a retreating anticyclone. Los Angeles, CA is a perfect case study for the effect of climate and topography on air pollution. Topographically, Los Angeles sits in a basin with the San Gabriel Mountains to the east of the city. During the summer the subtropical high pressure cell over the eastern Pacific has migrated to its northern most limit and expanded in size. The subsiding air of the subtropical high is compressed as it descends through the atmosphere, creating a warm layer of air aloft. In addition, adiabatic heating lowers the relative humidity, preventing the development of clouds. The cloudless skies enable much insolation to reach the surface.
Figure 6.21 Factors contributing to air
pollution in Los Angeles, CA At the surface, air cooled by the cold California current comes ashore on cool sea breezes. The dense cool air sinks into the basin as it comes onto land. The mountains to the east prevent air from moving out of the vicinity of the basin. With cool air at the surface and the warm layer aloft, inversion conditions take hold. The inversion limits the height to which pollutants generated by industrial activities and motor vehicles are moved. Under cloudless skies, insolation initiates photochemical reactions in the urban atmosphere to create the infamous smog problem that Los Angeles has.
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