[Piotrsko]

Afaik, nox produces brown visible smog and since most of California has valley areas that capture this brown n stinky stuff, you get this huge base wanting to remove the brown stinky component.
If you cant see the pollution, it must not really be there.

I see that as the main nox reduction requirement. Not sure how harmful it is after X many years.

First law of social science: some do, some don't, some will, some won't.