While we're at it, I'll point out that high density (along with mixed-use zoning) allows decreased use of cars rather than an increase, because not only is everything packed closer together, but the greater population density can support better public mass transit - NYC has the lowest rate of car ownership (and also the lowest per capita energy use) of any city in the country because of this. The problem in greater LA is that it's medium density, not just in LA proper but in the surrounding suburbs, or as I've seen it described, "LA surrounded by LA".
The rest of the problems with air pollution, as has been pointed out, are due to a high population, high car ownership, and unfavorable topography combined with the climate, and would be far worse if California hadn't taken the steps we have over the years thanks to laws which allowed us to set higher standards for ourselves, as well as other actions. Any other location dealing with the same issues would be worse off than us, if they lacked the authority we have to take more rigorous measures.