Just noticed that you're in Oregon.
I grew up in LA and remember the days when trash wasn't picked up ... we had backyard firepits. And then along came the post WW-II baby boom and population numbers went up and up. The smog wan't all that bad in WW-II days, but the smog later became bothersome. So fire-pits were banned, trast pickup started; the population boom went on; freeways arrived, and both went on and on ... leading to a massive automobile smog problem ... Solved with additives added to gasoline.
Today, auto mileage in LA is about 10% less than auto mileage using gas bought outside the LA area.
LA area geography is a good smog catcher.