Wow, this is a pretty big deal because NOx is notoriously difficult and expensive to detect at the levels where it starts to affect humans (double-digit ppb).

Wow, this is a pretty big deal because NOx is notoriously difficult and expensive to detect at the levels where it starts to affect humans (double-digit ppb). There are no cheap and reliable health-threshhold sensors for NOx like there are for particulates and VOCs. The really accurate detectors with enough sensitivity are consumables -- the NOx reacts with them and eventually you have to throw them away. The cheapest non-disposable low-ppb-rated sensors (~$500) require frequent professional recalibration. This is the main reason why we don't have a "purpleair for NOx" and aren't likely to have one anytime soon. At best you might get one or two EPA-run sensors for a given city.