Alot of factors "Koast, there are less production factories in LA than there was when I was growing up in the '60's, Uniroyal Tire shut down and its factory is now the Citadel outlets as just one example, and LA and Orange County clamped down very hard on fatories that remained open and were spewing tons of garbage into the basin with almost no regulation prior to the mid-seventies. The oil refieries in the Torrence/Wilmington/El Segundo area have dramatically cut back on pollution.
Ari quality improved as vehicle emissions became more stringent, especially in California, despite there being more cars per capita in LA than anywhere else in the world.
The worst place in the world to drive by IMO is the Farmer John pork rendering plant in Maywood on Vernon and the Kal Kan plant next door to it, I worked in Maywood for about 7 years and when these places were running hot the smell was unbearable for blocks around.