Fine Dust and Nitrogen Oxide in Environmental Areas

Nowadays' environmental areas in Germany regulate the entrance of road traffic by means of environment badges / emission stickers. The problem with fine dust is supposed to be solved by these means but not yet that one of nitrogen oxides.
Up to now, primarily soot particle emissions from diesel vehicles are concerned and not a gas like nitrogen dioxide. Only vehicles achieving a clearly defined exhaust emission standard are allowed until now to enter the environmental areas. The colour of a fine dust badge (red, yellow or green) shows what kind of exhaust emission standard a vehicle observes.
Diesel vehicles with a red fine dust badge express an exhaust emission standard of Euro 2 for passenger cars and light duty commercial vehicles as well as Euro 2 for trucks and buses. Yellow badges for diesel vehicles are allied to Euro 3 and green badges to Euro 4 which normally for a diesel vehicle is in conformity with a construction year of at least 2006.
Contrarily gasoline vehicles already obtain a green badge when they dispose of a catalyst converter that is beginning of Euro 2 standard which usually means a construction year after 1993.
This makes clear that existing environmental areas in Germany have nothing to do with nitrogen oxides. Consequently a legal basis to exclude vehicles causing high nitrogen oxide emissions from environmental areas does not exist.