Write down the reactions involved during the formation of photochemical smog.

(N/A) When fossil fuels are burnt,a variety of pollutants are emitted into the earth's troposphere. Two of the most important pollutants are hydrocarbons and nitric oxide $(NO)$. When these pollutants build up to sufficiently high levels,a chain reaction occurs due to their interaction with sunlight.
$1$. $NO_2$ absorbs energy from sunlight and breaks up into nitric oxide and a free oxygen atom:
$NO_{2(g)} \stackrel{h\nu}{\longrightarrow} NO_{(g)} + O_{(g)}$
$2$. Oxygen atoms are very reactive and combine with $O_2$ in the air to produce ozone $(O_3)$:
$O_{(g)} + O_{2(g)} \rightleftharpoons O_{3(g)}$
$3$. The ozone formed in the above reaction reacts rapidly with the $NO_{(g)}$ to regenerate $NO_2$:
$NO_{(g)} + O_{3(g)} \longrightarrow NO_{2(g)} + O_{2(g)}$
$4$. Ozone is a toxic gas and both $NO_2$ and $O_3$ are strong oxidising agents. They react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde,acrolein,and peroxyacetyl nitrate $(PAN)$.