Q.List the three ingredients required to form photochemical smog, and describe, step by step, how they interact under sunlight to generate ozone and PAN.

Photochemical smog requires three ingredients together: oxides of nitrogen ($\text{NO}_x$) and unburnt hydrocarbons, both supplied by vehicle exhaust, and strong sunlight to drive the chemistry. The sequence runs: (1) $\text{NO}$ from exhaust is oxidised in air to $\text{NO}_2$; (2) sunlight splits $\text{NO}_2$ back into $\text{NO}$ and a free oxygen atom, $\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$; (3) that oxygen atom combines with $\text{O}_2$ to give ozone, $\text{O} + \text{O}_2 \rightarrow \text{O}_3$; (4) at the same time, unburnt hydrocarbons undergo a chain of free-radical reactions with oxygen and $\text{NO}_2$ to produce peroxyacetyl nitrate (PAN). Ozone and PAN together are the harmful oxidising end products, and the …

State the three ingredients first, then walk through the reaction sequence in order rather th …

Omitting sunlight as a required ingredient -- without it, $\text{NO}_x$ and hydrocarbons sit in t …