The primary pollutant that leads to photochemical smog is:

We begin by recalling what scientists mean by the term “photochemical smog.” This phenomenon, first studied over the city of Los Angeles, is a brownish haze that forms when primary pollutants emitted directly from sources react under the influence of strong sunlight to generate a host of secondary pollutants, such as $$\text{O}_3$$ (ozone), PAN (peroxyacetyl nitrate), acrolein, and formaldehyde. The phrase “photochemical” itself tells us that light (photo) drives the chemical reactions involved.

Now, which substances act as the principal—or primary—reactants that start this chain of sunlight-induced reactions? The major exhaust components released from automobiles are hydrocarbons (unburnt fuel vapours) and a group of gases collectively called “nitrogen oxides,” abbreviated as $$\text{NO}_x$$, where the subscript $$x$$ denotes either $$\text{NO}$$ (nitric oxide) or $$\text{NO}_2$$ (nitrogen dioxide). When bright sunlight strikes these $$\text{NO}_x$$ molecules, they absorb energy and enter excited states. In that excited state, $$\text{NO}_2$$ splits:

$$\text{NO}_2 + h\nu \longrightarrow \text{NO} + \text{O}$$

The atomic oxygen $$\text{O}$$ thus produced then combines with molecular oxygen $$\text{O}_2$$ already present in air to yield ozone $$\text{O}_3$$, one of the most irritating secondary pollutants of photochemical smog:

$$\text{O} + \text{O}_2 \longrightarrow \text{O}_3$$

So we see that without the initial presence of nitrogen oxides, this entire reaction sequence would not begin. Hydrocarbons are indeed also needed, because they react with $$\text{NO}$$ to form radicals that perpetuate the cycle; however, among the options provided, nitrogen oxides play the primary triggering role.

Option B lists ozone, but ozone is a secondary pollutant formed later in the mechanism, not the original substance emitted from vehicle exhaust. Option C gives acrolein, which again is a secondary aldehyde produced during the photochemical reactions, and Option D gives sulphur dioxide, which is associated more with classical (“London type”) smog and acid rain rather than photochemical smog.

Therefore, the pollutant that leads to photochemical smog—meaning the chief primary pollutant initiating the reaction chain—is the nitrogen oxides group.

Hence, the correct answer is Option A.