Thanks in part to tighter emissions laws, California's air is cleaner now than in the 50's.
The Los Angeles basin and San Joaquin Valley are two areas where the air just hangs for days. In fact, when the Spanish first visited what was to later become Los Angeles, they remarked how the smoke from the Indians' fires would just rise into the air and hang there.
Coastal cities, like San Diego and San Francisco, benefit from marine air exchange, which moves the air inland. Los Angeles doesn't get the same effect. What's sort of unfair is that the dope smoking hippies in San Francisco drive around in their old VW buses blowing huge clouds of smoke -- but that's all pushed toward Sacramento -- which gets dinged by the Feds because their air isn't clean.
Of course, the hippies take no responsibilty -- as they're too busy inhaling their own farts.