Why do pollution and smog often become trapped in los angeles and other similar urban areas?

Smog is usually defined as a fog that is intermixed with smoke particles and other particulate matter present in the atmosphere. This smog, along with the pollution is often trapped in the city of Los Angeles and similar urban areas, because the city is located in a basin near the coastal area, and is surrounded by the hills on all sides. There occurs a large emission of carbon due to the use of vehicles. There also occurs relatively less rainfall due to which these particles are trapped in the atmosphere.

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