Q.Which of the following gases emitted by motor vehicles is responsible for the formation of photochemical smog?

Smog comes in two chemically distinct forms. Classical (London-type) smog forms in cold, humid,

still winter weather over coal-burning industrial cities: smoke, fog and sulfur dioxide combine, the

fog's moisture converting $\text{SO}_2$ into a fine sulfuric-acid mist. It is chemically

reducing, because $\text{SO}_2$ itself is readily oxidised rather than being an oxidant.

Photochemical smog, by contrast, forms over sunny, traffic-heavy cities and needs no fog at all:

its ingredients are $\text{NO}_x$ and unburnt hydrocarbons from vehicle exhaust, activated by strong

sunlight. Sunlight splits $\text{NO}_2$ into $\text{NO}$ and a free oxygen atom

($\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$), which combines with $\text{O}_2$ to give

ozone; hydrocarbon radicals separately react with oxygen and $\text{NO}_2$ to give peroxyacetyl …