There are 2 accurate portable emissions measurement systems, one of which is the Semtec. My SIL is an expert on that device and has a consultancy setting up and running the machine for various customers.

The issue with the portable systems is that there are too many variables in the "real world" to run repeatable testing. Remember, emissions standards around the world are run on a dyno in a controlled environment for this reason.

I have no clue if the European NP mentioned wanted on-road emissions values or not but other then for research or development purposes any data thus generated is of limited value. Assuming that the tests are run at 68'-86*F (20-30*c) on flat pavement on a repeatable test cycle with no corners or wind, they might have some use. Either way, they won't hold up in a court of law though they might be helpful in pointing a finger at certain cars that need more study.

None of this is easy or simple.

Dan