Stratospheric sink for nitrous oxide

In tropospheric chemistry, N2O as a relatively stable compound is of marginal importance. It gradually diffuses into the stratosphere, where the predominant sink occurs, as a result of destruction by direct photolysis by ultraviolet radiation:

and by reaction with the excited oxygen atoms, O(1D), formed in ozone photolysis:

Smaller amounts of other oxygenated nitrogen compounds (including NO2, HNO3 and N2O5) are also formed (Prather, 2007). Although the relative proportions of the two reactions vary throughout the stratosphere, direct photolysis is responsible for about 90 per cent of the total stratospheric decomposition, while the reaction with excited oxygen accounts for the remaining 10 per cent (Garcia and Solomon, 1994). The destruction of N2O, while removing a potent greenhouse gas from the atmosphere, results in a contribution to a different environmental problem: the NO formed by the reaction of N2O with O(1D) causes the decomposition of stratospheric ozone, O3 (Crutzen, 1970), thus diminishing the protective role of the ozone layer against harmful effects of UV radiation on organisms at the surface of earth. It should be remembered that the recognition of the environmental importance of this process preceded any focus on the role of N2O as a greenhouse gas, and it is now again recognized as important, as other ozone-depleting substances regulated by the Montreal Protocol (unlike N2O) are gradually removed from the stratosphere (Ravishankara et al, 2009).

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