Los Angeles in particular is really bad. You've got like 20 million people (a number higher than the individual populations of 45 US states!) all living in flat

Los Angeles in particular is really bad. You've got like 20 million people (a number higher than the individual populations of 45 US states!) all living in flat area mostly hemmed on 3 sides by mountains and on the fourth side by the ocean. Winds tend to blow in from the ocean, and don't tend to pass over the mountains. This tends to trap a pocket of air over the city. This wouldn't be so bad if car usage wasn't so necessary to get around there, however car ownership in the LA metro is something like 80-90 percent, with around 1.6 cars per household on average. (Compare to about 32% and 0.45 per household in Tokyo, a very similar place in terms of geography and population) EVs represent about 1% of those cars, so, conservatively you've got about 15 million cars filling up that one air pocket overhead with combustion byproducts. It gets worse! Traffic in Los Angeles is so bad that a lot of those cars are in stop and go hours-long commutes every day which is a particularly inefficient regime for fuel use and emissions. On top of that this bad traffic has an interesting side effect: over half of all car trips in Los Angeles are less than 3 miles. This is also bad for emissions because the catalytic converter may not get to heat up and achieve full efficiency.