How Ozone is Formed in the Atmosphere?

Ozone, like several other photochemical oxidants (substances that are formed in air from nitrogen oxides and hydrocarbons under the influence of sunlight) is formed primarily from nitrogen dioxide (NO₂). Ozone like other oxidants such as PAN (peroxyacetyl-nitrate) harms plants.

Ozone can also react with nitrogen monoxide (NO), whereby the ozone is used up and nitrogen dioxide forms anew. If only these compounds are present, the ozone levels will not be very high. It is when hydrocarbons enter into the picture that really high levels of ozone are reached. That is because the hydrocarbons can transform nitro­gen monoxide to nitrogen dioxide without using up the ozone. In this way the nitrogen dioxide can again become the starting material for ozone.

Miller et al. (1994) have proposed a mechanism for atmospheric ozone production: reaction between highly vibrationally excited O₂ and ground- state O₂.

Image Source: eoimages.gsfc.nasa.gov

The standard view is that solar radiation at wavelengths shorter than 242 nm photo-dissociates oxygen, and the resultant atoms form ozone by O + O₂ reaction. But the atmosphere is a stew, albeit dilute, of high-energy particles. This is particularly true for oxygen. The products of direct ozone photo- dissociation are not necessarily distinguishable from those originating with O₂.

Thus, ozone photo-dissociation, usually viewed as a do-nothing process in terms of changing the ozone balance, can have a significant effect in this regard, particularly because ozone photo-absorption rates are much greater than those for oxygen (Slanger, 1994).

O(‘D) is quenched rapidly by all collision partners and is the single most important intermediate in the atmosphere as it generates several reactive molecules (OH, NO, and CH₃) important in stratospheric chemistry. Till recently, the reactions shown in 1b, giving ground-state products have been largely ignored. But the active O₂ molecules appear to have much impact on atmospheric chemistry in that they are copiously produced and unlike O(‘D), are not rapidly quenched by O₂ and N₂ (Slanger, 1994).