Photochemical smog formation – UGC NET Question

Photochemical smog is a secondary pollution phenomenon driven by sunlight. Nitrogen oxides and volatile hydrocarbons from vehicles and industries react under intense solar radiation to produce a mixture of oxidants including ozone and peroxyacetyl nitrate. These secondary pollutants irritate eyes, damage plant tissues and affect respiratory health. Thus, the combination of nitrogen oxides, hydrocarbons and strong sunlight described in option C correctly explains the origin of photochemical smog.

Option A:
Option A is incorrect because sulphur dioxide and soot are associated more with classical industrial smog in cool, moist climates. Photochemical smog typically occurs in sunny, warm cities and involves different chemical reactions dominated by oxidant formation.

Option B:
Option B is incorrect since heavy rainfall tends to wash pollutants out of the air and improve visibility. Water vapour and dust alone do not generate the complex oxidant mixture that characterises photochemical smog.

Option C:
Option C is correct as it highlights the key ingredients and the role of solar radiation in initiating photochemical reactions. Urban traffic emissions supply nitrogen oxides and hydrocarbons, and sunlight transforms them into harmful oxidants that accumulate under stagnant atmospheric conditions.

Option D:
Option D is incorrect because interactions between carbon dioxide and oxygen in the upper stratosphere are not responsible for urban smog near the ground. The stratosphere is connected with the ozone layer, whereas photochemical smog is a tropospheric phenomenon close to human exposure.