Photochemical smog is produced by action of sunlight on hydrocarbons and nitrogen oxides. NO is converted to $\mathrm{NO}_2$ by the effect of sunlight. This $\mathrm{NO}_2$ further absorbs sunlight and disintegrates into NO and $\mathrm{O}_2$. Thus, a chain reaction begins. $\mathrm{NO}_{(g)}+\frac{1}{2}\mathrm{O}_{2(g)}$ $\xrightarrow[]{hv}$ $\mathrm{NO}_{2(g)}$ $\mathrm{NO}_2 \xrightarrow[]{hv} \mathrm{NO}_{(g)} + \mathrm{O}_{(g)}$ These oxygen atoms react with $\mathrm{O}_2$ to form ozone as : $\mathrm{O}_{2(g)} + \mathrm{O}_{(g)}$ ⇌ $\mathrm{O}_{3(g)}$ $\mathrm{O}_{3(g)} + \mathrm{NO}_{(g)}$ → $\mathrm{NO}_{2(g)} + \mathrm{O}_{2(g)}$ These two gases formed, react with hydrocarbons to produce other chemicals. $3\mathrm{CH}_4+2\mathrm{O}_3$ → $\underset{\text{formaldehyde}}{3\mathrm{CH}_2=\mathrm{O}}+3\mathrm{H}_2\mathrm{O}$ Other chemicals produced are : $\underset{\text{Acrolein}}{\mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{O}}$ $\underset{\text{Peroxyacetyl nitrate (PAN)}}{\mathrm{CH}_3-\underset{\mathrm{O}}{\underset{\parallel}{\mathrm{C}}}-\mathrm{O}-\mathrm{O}-\mathrm{NO}_2}$