These gases are then used to clean up smog-producing nitrogen oxides, or NOx.

FORBES: Clearing the Air

These include the exhaust gases such as nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOCs), and carbon monoxide (CO).

BBC: London's clue to stubborn ozone levels

Also, to comply with prevailing U.S. emissions standards on nitrogen oxides, or NOx, these big diesels require fairly complicated systems of postcombustion treatment with a water-based urea solution, called AdBlue.

WSJ: 2013 Porsche Cayenne Diesel Review: Only Geeks Need Apply | Rumble Seat by Dan Neil