Nitrogen oxides (${\mathrm{NO}}_x$)

In the atmospheric convention used here, ${\mathrm{NO}}_x$ denotes nitric oxide plus nitrogen dioxide. The broader reactive-nitrogen family ${\mathrm{NO}}_y$ includes additional oxidized reservoirs; the chosen nitrogen-atom counting convention must be specified.

The atmospheric nitrogen oxide $\mathrm{N}{\mathrm{O}}_2$. Daylight photodissociation forms nitrogen monoxide and an oxygen atom; subsequent third-body recombination with oxygen forms ozone. Whether this makes net ozone depends on the pathway that regenerated nitrogen dioxide.

The atmospheric nitrogen oxide $\mathrm{NO}$. It consumes ozone when converted to nitrogen dioxide by $\mathrm{NO}+{\mathrm{O}}_3\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$, but its oxidation by peroxy radicals instead enables net photochemical ozone production.