Over at another message board, someone claims that filming crews working exterior shots in L.A. use “smog filters” to make the air here look better in the final product. I won’t claim that our air is as good as that as a mountain forest; after all, we’ve still got nearly the worst urban air in the country, if not the worst. But notwithstanding that, I did live through the smog of the 1960s and early 1970s, and remember the almost daily layer of brown haze in the warmer months of the year. At night the reflection of street lamps and other artificial lighting would result in an orange-ish sky.
And today I almost never see any of that. Whatever the actual amount of pollution, the amount of visible pollution is just a fraction of what it was in the old days. Driving around Hollywood it used to be a rare thing when you could see the Sign or Griffith Observatory in the hills above, but now it’s quite common.
So…smog filters? And how would such a thing work?
At a guess, they use stronger “haze” and UV filters in LA than in most of the country. Google around for comparative images and you’ll see how haze filters cut, well, haze - the faint white light that’s everywhere from reflections or particulates in the air. A shot without a filter is kind of washed out and lightened and the sky will be a pale bluish; filter on, and you get a much clearer image, a deeper blue sky (sometimes dramatically) etc. UV filters cut from UV down into the upper blue spectrum and have similar effects.
Given LA’s southern location, bright sun, and mix of pollution and particulates, I can see where going to stronger haze/UV filters and select use of polarized filters to cut the glare would improve the result… especially since LA is supposed to look all shiny and bright and colorful because it’s always looked that way on film. 
This would only be somewhat related to “smog” but I can see someone claiming it is.
It was only something like four years ago that I was last in LA, and yes, the smog is still bad enough to be easily visible. It might well be better than it used to be, but it still in no way resembles clean air.