The lifetime of NO_x varies from ≈6 hours in the boundary layer to several days to a week in the upper troposphere. This leads to a large variation in its concentration: 10 ppbv (parts per billion by volume) in urban areas to 10 pptv (parts per trillion by volume) in remote regions such as the upper troposphere and remote marine boundary layers.

The following table summarizes the principal interchanges between NO, NO₂ and HNO₃ that are relevant for the chemistry of the troposphere.

Fig 1.1.3.1: NO_x reactions overview.

Image: AT2-ELS

1. NO reacts with O₃ to create NO₂ and O₂.
2. NO reacts with HO₂, the peroxy radical, to create NO₂.
3. NO₂ can be photolysed by sunlight to create NO and an oxygen atom.
4. NO₂ reacts with OH, the hydroxyl radical, to create HNO₃. HNO₃ can be photolysed by sunlight to create NO₂ and OH once more. HNO₃ can thus act as a short term sink for NO₂.
5. The oxygen atom released in equation 3 can combine with O₂ to produce O₃; hence the characterisation of NO_x as a precursor of ozone.

These reactions are discussed more fully in their context in later sections of this module. For the moment we only need to note the presence of photolytic processes in these relationships, which means that atmospheric NO_x will vary between night and day.