I suspect this has more to do with catalytic converters removing NOx. Very few cars on the road without that tech now vs the early 90's.
And the gasoline was formulated to further enhance the performance of catalytic converters at various times of the year as well as account for those vehicles that didn't have them.
The primary summer pollutant is ozone in the DC metro area, with oxygenated gasolines being used in winter to cut down on carbon monoxide, which is the primary winter pollutant.
Very few things are ever "the one, the only, THE solution!", and this is one of them. It's a multi-tiered approach. You want to see what can happen when there's little to none of this approach? Beijing.
I'm personally not a big fan of a lot of what the gub does, but in the case of gasoline enhancements, I'm pretty much all for it.