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On thing that makes the smog worse on the west coast is the mountains just to the east. They tend to hold the smog in. I've been to Mexico City many times and it sits in kind of bowl. Crazy hazy smog days.
It's called inversion and the air just can't escape the basin. I lived in LA in the early 80's and sometime around 80-81 we had the worst smog inversion since 1955. The mountains were just a few miles from our place in Pasadena, and on summer days all you could see in the distance was a brown haze. Might as well have been Kansas on a dusty day. That rarely, if ever, occurs today.
That being said, the legal situation with aftermarket equipment is ridiculous. Typical legalistic narrow-minded government overreach. In other words, even if you could demonstrate that your modified H/C/I/E car ran as clean or cleaner than stock, you have no standing.