The main difference is that Europe has separate regulations for Diesel and Gasoline cars, while the US has regulations independent of engine type. You are right

The main difference is that Europe has separate regulations for Diesel and Gasoline cars, while the US has regulations independent of engine type. You are right that this leads to lax regulations on NOx. That said, the new Euro 6 regulations for Diesel cars should bring Europe closer to US levels. This is actually an area where TTIP might improve things a lot by aligning different emission standards.