Nitrogen oxides (NOX) is shorthand for NO and NO2. NO is generated for example by lightnings and in combustion processes but can be oxidised relatively quickly to NO2 by reaction with ozone in the ambient air. On the other hand, UV light from the sun can decompose NO2 in the atmosphere into NO and a free oxygen radical. In some situations, it is practical to speak of “NOX” rather than the individual constituents. The reactions are important parts of atmospheric chemistry and the formation of smog and acid rain.