= Solution
<Photodissociation> of <nitrogen dioxide> forms an <oxygen> atom, followed by <third-body recombination>:
$$
\mathrm{NO_2+h\nu\to NO+O},\qquad
\mathrm{O+O_2+M\to O_3+M}.
$$
If NO is converted back to <nitrogen dioxide> only by $\mathrm{NO+O_3\to NO_2+O_2}$, 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>:
$$
\mathrm{RO_2+NO\to RO+NO_2},\qquad
\mathrm{HO_2+NO\to OH+NO_2}.
$$
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 $\mathrm{OH+NO_2+M\to HNO_3+M}$, 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.<The sensitivity of ozone to nitrogen oxides and hydrocarbons in regional ozone episodes by Sanford Sillman and others>[Journal of Geophysical Research 95 (1990), pp. 1837–1851, model comparison of rural and urban precursor sensitivity]{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-id-ozone-regimes.png]
{title=Original schematic showing opposite local ozone responses to nitrogen-oxide controls in two chemical regimes}
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