There are two different limits for NOx / NO2 in Europe:
1) 80 milligrams NOx per km that a diesel car or truck drives: Euro 6 . (In some cities you can't drive a diesel that emits more than that.)
2) 40 µg / m3 NO2 in the air (NO2, not NOx; averaged over 1 year): European Union legislation .
How are these two related -- how does NOx from a diesel exhaust pipe turn into NO2, then dissipate ?
Not answers, but two different approaches:
1) back-of-the-envelope calculation:
say 1000 diesel cars are driving around at 50 km/h for 1 hour, producing 80 mg NOx / km:
1000 * 50 km/h * 80 mg/km = 4 kg per hour.
But then it gets iffy: how does NOx spread, over how many cubic metres, with how much wind ? Does it dissipate / become harmless overnight ?
2) measure both in parallel, at the same times and places. (Open data welcome.)
Added: does the US have two such different kinds of limits / different laws for NOx ?
I realize that it's hard to find a middle way, say at MacKay level, between
- "it's complicated, you wouldn't understand, trust the EPA", and
- EPA NO2 Integrated Science Assessment -- 1148 pages.