Pollution has always been a problem in the Los Angeles basin. 

The area is literally a seaside bowl formed by tectonic events (fascinating in itself) into a coastal lowland that is surrounded by steep hills and mountain ranges.

...On the north, northeast, and east, the lowland basin is bound by the Santa Monica Mountains and Puente, Elysian, and Repetto hills.^[3] To the southeast, the basin is bordered by the Santa Ana Mountains and the San Joaquin Hills