Evidence for New Sources of NO$_x$ in the Lower Atmosphere

Laboratory studies show that the reaction of short-lived O$_2$(B$^3\Sigma_u$) molecules (lifetime ∼10 picoseconds) with N$_2$ and the photodissociation of the N$_2$:O$_2$ dimer produce NO$_x$ in the stratosphere at a rate comparable to the oxidation of N$_2$O by O($^1$D). This finding implies the existence of unidentified NO$_x$ sinks in the stratosphere. The NO$_2$ observed in this experiment is isotopically heavy with a large $^{15}$N/$^{14}$N enhancement. However, photo-dissociation of this NO$_2$ unexpectedly produced NO molecules with a low $^{15}$N/$^{14}$N ratio. The diurnal odd-nitrogen cycle in the stratosphere will be marked by a complex isotope signature that will be imprinted on the halogen and HO$_x$ catalytic cycles.