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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.

Methane consumes hydroxyl radicals during its initial oxidation and therefore influences atmospheric oxidative capacity and the removal of other trace gases. Subsequent oxidation generates peroxy radicals. In the presence of sufficient nitrogen monoxide, these convert nitrogen monoxide to nitrogen dioxide without consuming ozone, allowing net photochemical ozone production. At low nitrogen monoxide, peroxide formation and ozone-destroying radical reactions compete. Methane oxidation ultimately produces carbon monoxide and carbon dioxide; the sign and rate of the associated ozone response depend on the radical/nitrogen oxides regime, not methane alone.

Lower carbon monoxide and VOC emissions generally reduce the supply of peroxy radicals from their oxidation and therefore reduce photochemical ozone production where nitrogen monoxide is available. Lower nitrogen oxides abundance suppresses the nitrogen monoxide-to-nitrogen dioxide pathway in NOx-limited ozone production regions. These are reasons for a regional/background ozone decrease when fossil-fuel precursor emissions decline.

The response can have the opposite local sign in a nitrogen monoxide-rich urban core: reducing nitrogen monoxide weakens ozone titration, and reducing nitrogen oxides in a VOC-limited ozone production regime can initially lessen radical termination and raise ozone. Less carbon monoxide can also leave more hydroxyl radical available to remove methane, partly opposing the hydroxyl radical demand from leaked hydrogen. Changes in precursor transport, chemistry and sunlight matter. Reduced emissions usually reduce net regional ozone production, but do not guarantee a lower ozone concentration at every monitoring site.

An organic compound readily present in the gas phase under the conditions of interest. In polluted-air chemistry, its oxidation can generate peroxy radicals which convert nitrogen monoxide to nitrogen dioxide without consuming ozone. Volatility alone does not determine reactivity or ozone yield.