California Air Basins.  Air pollution tends to stagnate within these air basins due to natural barriers, such as mountains, unless prevailing winds are strong enough to disperse it into other areas.
California is divided into 15 Air Basins to better manage air pollution. Air basin boundaries were determined by grouping together areas with similar geographical and meteorological features. Political boundaries were also considered in determining the air basin boundaries. Some air basins are relatively small, while others are quite large.  Furthermore, some are very much like a basin, consisting of valleys surrounded by mountains, while others are more open. While air pollution can move freely within an air basin, it can also sometimes be transported from one basin to another. (http://www.arb.ca.gov/desig/airbasins/airbasins.htm)
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