[wpasc]

I was fascinated to see how low New York was. Given the immense amount of car traffic, anyone have an idea as to why its so low?

[robotresearcher]

Because we've worked on improving it for a hundred years. A mixture of regulations and regulation-driven technology improvements.

[ssnistfajen]

Motor vehicle exhaust is tiny compared to the pollution from coal-burning. Eliminating coal has drastically improved the air quality in industrialised cities of Europe and North America.

[bigpicture]

Prevailing winds bring relatively clean air to most any city east of the Mississippi River especially in the winter time. Without nearby mountains, temperature inversions are pretty rare.

In late summer, air quality drops quite a bit as the wind drops off.

[hackeraccount]

geography plus the cars run gas instead of diesel