Los Angeles often experiences cool temperatures due to the onshore flow from the Pacific Ocean.
(image source : http://www.nc-climate.ncsu.edu/edu/k12/.breezes)
In Los Angeles upper air inversions develop because it is on the edge of the subtropical high pressure cell in the Pacific Ocean. Subsiding air from the high pressure cell is compressed by the increasing pressure of the surrounding air as it descends, so the air warms up as it subsides. Combined with the cool air at ground level from onshore flow, there is also a general subsidence of warm air aloft. The inversion layer acts as a lid to prevent air at ground level from rising and dispersing. The mountains inland also help trap the air.
(image source: http://www.ec.gc.ca/air/default.asp?lang=En&n=AFF4D58F-1)
Any pollutants emitted build up in the trapped air. The smog worsens in high temperatures and clears best with high winds.
(image source: http://www.huffingtonpost.com/2009/07/10/la-vows-to-be-coalfree-by_n_229832.html)
(http://daphne.palomar.edu/calenvironment/smog.htm)
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