Nitrogen oxides are everywhere. There are "good" ones and "bad" ones and they're all slightly different. A collection of pretty pictures is here: link Nitrogen oxides include laughing gas (good), nitrate and nitrite salts in foods (controversial), and nitrate plant fertilizers (good). Nitrogen oxides are even important in sex (good) because the molecule NO (nitric oxide) is key in sexual arousal.* Some nitrogen oxides do more harm than good: nitrogen oxides in clouds help form acid rain and the color of "brown cloud" comes from NO2. But worst of all, nitrogen oxides come out the tail pipes of more efficient diesel vehicles. I wrote about that back here.
The reason there are so many different nitrogen oxides is because nitrogen and oxygen are Periodic Table neighbors, and being so close together, each covets its neighbor's electrons when excited. But only when provoked. Air does not react with itself under normal circumstances. But give things some radiation, a spark, or a diesel motor, and O2 and N2 do the nasty together--they recombine:
N2 + O2 ---> 2NOx
There's an "x" subscript there because NO can react further to make NO2 and other nitrogen oxides. "NOx" is so noxious that the State of California forbade the sale of new diesel vehicles beginning in 2004 (I got one of the last VW Golf models in 2003). Daimler-Benz wanted to get those sales back (leave it to the Germans to invent all the new cool diesel technology) and Daimler's BlueTec technology returns NOx emissions whence they came--back to nitrogen and water:
4NO + 4NH3 + O2 ---> 4N2 + 6H2O
That equation is mass-balanced by the way, unlike-ahem, what one finds in the Internets (FTFY). The chemistry looks like complex gas-phase chemistry but actually a metal surface mediates the rearrangements (thankfully not another precious metal catalyst). In practice, NOx abatement injects ammonia into the exhaust stream upstream of the catalyst. Ammonia is the best "blue" additive in theory, but urea, which is more easily transported and handled, can also be used in Daimler's AdBlue technology. From what I've read and heard, the technology works as well but is very expensive. So what else is new. Owners are required to periodically charge up with fresh aqueous urea, which led one owner to ask whether he couldn't just "piss in a tank instead" hahah.
A good primer on how the technology works can be found here or GIY (Google it yourself)
*Has anyone noted the irony of "NO" (nitric oxide) being a key factor in male sexual arousal?