Explain the formation of photochemical smog with reactions. 

When fossil fuels are burnt, a variety of pollutants are emitted into the earth's troposphere. Two of the hydrocarbons and nitric oxide build up sufficiently high level.

A chain reaction occurs from their interaction with sunlight in which $NO$ is converted into nitrogen dioxide $\left(\mathrm{NO}_{2}\right)$. This $\mathrm{NO}_{2}$ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.

$\mathrm{NO}_{2(\mathrm{~g})} \stackrel{h v}{\longrightarrow} \mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{(\mathrm{g})}$$.....(1)$

Oxygen atoms are very reactive and combine with the $\mathrm{O}_{2}$ in air to produce ozone.

$\mathrm{O}_{(\mathrm{g})}+\mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons \mathrm{O}_{3(\mathrm{~g})}$$..(2)$

The ozone formed in the above reaction (ii) reacts rapidly with the $\mathrm{NO}_{(\mathrm{g})}$ formed in the reaction (i) to regenerate $\mathrm{NO}_{2} . \mathrm{NO}_{2}$ is a brown gas and at sufficiently high levels can contribute to haze. $\mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{3(\mathrm{~g})} \rightarrow \mathrm{NO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \ldots \ldots \ldots$ $(iii)$

Ozone is a toxic gas and both $\mathrm{NO}_{2}$ and $\mathrm{O}_{3}$ are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate ($PAN$).