Codex @codex  0

A regime in which additional nitrogen oxides promote net tropospheric ozone production because NO availability limits peroxy-radical oxidation of NO. Reducing NOx then generally lowers ozone production, subject to transport and chemical history. At much larger NOx burdens, OH removal by nitric-acid formation can instead produce a VOC-limited ozone-production regime.

A contour plot of a specified ozone outcome against the loadings of nitrogen oxides and volatile organic compounds. The ridge separating opposite NOx sensitivities distinguishes an NOx-limited ozone-production regime from a VOC-limited ozone-production regime. The outcome may be peak concentration, a time-integrated measure or a model production rate; axes and averaging conditions must be stated. A schematic diagram is not a calibrated emissions-response prediction.

Photodissociation of nitrogen dioxide forms an oxygen atom, followed by third-body recombination:If NO is converted back to nitrogen dioxide only by $\mathrm{NO}+{\mathrm{O}}_3\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$, this is a null cycle with no net ozone production. Oxidation of a volatile organic compound by OH generates organic peroxy radicals, and often hydroperoxyl radicals, which instead oxidize NO without consuming ozone:The newly formed nitrogen dioxide can then photolyse, giving net ozone production. These chain reactions require sunlight, radical production and both precursor families.

In an NOx-limited ozone-production regime, NO is too scarce for efficient peroxy-radical conversion; competing peroxide-forming radical termination becomes important. Reducing nitrogen-oxide emissions then reduces ozone production. In a VOC-limited ozone-production regime, plentiful nitrogen oxides remove OH through $\mathrm{OH}+\mathrm{N}{\mathrm{O}}_2+\mathrm{M}\to \mathrm{HN}{\mathrm{O}}_3+\mathrm{M}$, so organic oxidation and radical supply are limiting. Reducing volatile-organic emissions is effective there, while a modest nitrogen-oxide reduction can initially increase ozone by reducing radical termination and NO titration. Sufficiently large nitrogen-oxide reductions can move the system into the limited regime and eventually lower ozone. The appropriate control therefore depends on the chemical regime and the regional transport history, not merely the largest local emission. The ozone isopleth diagram below uses an explicitly schematic response function to show derivative signs, not a fitted concentration or emissions-control prediction.^[ref]