# How is NOx mg/km from diesel cars and trucks related to µg/m3 in the air?

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

• Having sat in on a few lectures from air quality folks, the relationship is extremely complicated. Involves humidity, wind, temperature, precipitation, speed of vehicles, etc, etc. My understanding is that the best you can do is figure out averages (per 1000 vehicle miles over 1000 km2).
– LShaver
Commented Mar 29, 2017 at 16:13

It's phenomenally complicated, poorly understood, and very difficult to model.

It depends on the layout of buildings, where and when emissions happen, what the local winds are, and what's happening upwind.

It's all a bunch of numerical approximations (so much turbulence, so no analytic closed-form solution), and very expensive to get a snapshot of the entire system at any one time. Also, we can only roughly estimate what the actual emissions are.

Even in one city, the relationship is complex and hard to bottom out. Across cities in the same country, many different relationships. And more complexity again in different climatic zones.

Sean Beevers has written a lot about it and related subjects, as have his colleagues at Kings College London Environmental Research Group

For somewhere like London, the NOx emissions will get emitted in the city and hang around, as well as getting blown in from elsewhere.

For a city in New Zealand, winds might blow pretty much all the local pollution away pretty frequently.