Photochemical smog formed in congested metropolitan cities mainly...

Photochemical smog mainly consists of ozone, peroxyacetyl nitrate and different oxide of nitrogen and hydrocarbons released from different engines. Nitric oxide (NO) is formed by reaction between oxygen and nitrogen. This reacts with air forming nitrogen dioxide $\left(\mathrm{NO}_2\right). This gas absorbs light due$ to which nitric oxide (NO) and a molecule of nascent oxygen [O] are formed. $\mathrm{NO}_2$ \longrightarrow $\mathrm{NO}+$[$\mathrm{O}$] This atomic oxygen reacts with unburnt hydrocarbons to give rise to peroxy radicals which after reaction with oxygen form ozone or reacting with$\mathrm{NO}_2$ give rise to peroxyacetyl nitrate (PAN).