In the presence of sunlight, \(\mathrm{NO}_2\) undergoes photolysis to form \(\mathrm{NO}\) and atomic oxygen. Atomic oxygen then combines with the molecular oxygen in the presence of some molecule which acts as a source of transfer of energy to form ozone, \(\mathrm{O}_3\). The ozone formed reacts with \(\mathrm{NO}\) to form \(\mathrm{NO}_2\) and \(\mathrm{O}_2\). Thus \(\mathrm{NO}_2\) cycle is completed.
Reactions: \(-\mathrm{NO}_2 \stackrel{\mathrm{hv}}{\longrightarrow} \mathrm{NO}+\mathrm{O}\),
\(\mathrm{O}+\mathrm{O}_2 \longrightarrow \mathrm{O}_3, \mathrm{O}_3+\mathrm{NO} \longrightarrow \mathrm{NO}_2+\mathrm{O}_2\)
When hydrocarbons are present, they combine with oxygen atom, produced in the first step during photolysis of \(\mathrm{NO}_2\), to form highly reactive free radicals as intermediates. These free radicals initiate a variety of reactions:
\(\mathrm{RCO}+\mathrm{O}_2 \longrightarrow \mathrm{RCO}_3, \mathrm{RCO}_3+\mathrm{NO} \longrightarrow\)
\(\mathrm{RCO}_2+\mathrm{NO}_2\),
\(\mathrm{RCO}_3+\mathrm{O}_2 \longrightarrow \mathrm{O}_3+\mathrm{RCO}_2, \mathrm{RCO}_3+\mathrm{NO}_2\)
\(\longrightarrow \underset{\text{(peroxyacetylnitrate - PAN)}}{\mathrm{RCO}_3 \mathrm{NO}_2}\)
As a result concentration of ozone, PAN and aldehydes and ketones is a build up in the atmosphere.