A complete list of nitrogen oxides would include…
- Nitric oxide, aka nitrogen monoxide (NO), nitrogen(II) oxide
- Nitrogen dioxide (NO2), nitrogen(IV) oxide
- Nitrous oxide (N2O), nitrogen(−I,III) oxide
- Nitrosylazide (N4O), nitrogen(−I,0,I,II) oxide
- Oxatetrazole (N4O)
- Dinitrogen_trioxide (N2O3), nitrogen(II,IV) oxide
- Dinitrogen tetroxide (N2O4), nitrogen(IV) oxide
- Dinitrogen pentoxide (N2O5), nitrogen(V) oxide
- Trinitramide (N(NO2)3 or N4O6), nitrogen(0,IV) oxides
Of primary interest in atmospheric science and environmental and occupational health are nitric oxide, nitrogen dioxide, and dinitrogen tetroxide. The term “NOx” refers to some combination of NO and NO2.
NOx is generally produced by a combustion process. At the point of emission, the composition of NOx is 90-95 percent NO, with higher temperature favoring the production of NO.
After a few hours in the atmosphere and in the presence of ozone, the NO is converted to NO2. This reaction can occur over a couple of seconds to a few hours.
NO + O3 → NO2 + O2
Under certain circumstances, that reaction can be reversed. NO2 is considered to be the more toxic of the two compounds, and is usually the one measured in ambient air studies.