Explain the formation of photochemical smog with reactions.

(N/A) When fossil fuels are burnt,a variety of pollutants are emitted into the earth's troposphere. Two of the primary pollutants,hydrocarbons and nitric oxide $(NO)$,build up to sufficiently high levels.
$A$ chain reaction occurs from their interaction with sunlight in which $NO$ is converted into nitrogen dioxide $(NO_2)$. This $NO_2$ in turn absorbs energy from sunlight and breaks up into nitric oxide and a free oxygen atom.
$NO_{2(g)} \stackrel{h\nu}{\longrightarrow} NO_{(g)} + O_{(g)}$ $(I)$
Oxygen atoms are very reactive and combine with the $O_2$ in air to produce ozone.
$O_{(g)} + O_{2(g)} \rightleftharpoons O_{3(g)}$ $(II)$
The ozone formed in the above reaction $(II)$ reacts rapidly with the $NO_{(g)}$ formed in the reaction $(I)$ to regenerate $NO_2$. $NO_2$ is a brown gas and at sufficiently high levels can contribute to haze.
$NO_{(g)} + O_{3(g)} \rightarrow NO_{2(g)} + O_{2(g)}$ $(III)$
Ozone is a toxic gas and both $NO_2$ and $O_3$ are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde $(HCHO)$,acrolein $(CH_2=CHCHO)$,and peroxyacetyl nitrate $(PAN)$.