Photochemical smog is a resultant of the reaction among

Answer: (a) NO₂, O₃ and peroxyacetyl nitrate in the presence of sunlight

In sunlight, NO₂ breaks up and forms ozone; hydrocarbons form PAN — together they make photochemical smog.

The idea

There are two kinds of smog. Classical (London) smog is smoke and sulphur dioxide mixed with fog; it forms in cold, damp weather. Photochemical (Los Angeles) smog is made by sunlight acting on vehicle exhaust; it forms on warm, sunny days and has no fog in it.

Vehicles give off nitric oxide and hydrocarbons → Nitric oxide becomes nitrogen dioxide (brown) → Sunlight splits nitrogen dioxide → The free oxygen atom joins O₂ → ozone → Hydrocarbons + nitrogen dioxide → PAN → Brown haze that stings the eyes

Why the others are wrong

(b) CO and O₂ are not the main ingredients.

(c) Photochemical smog needs sunlight and warmth, not low temperatures.

(d) It forms in bright daytime, not in the evening.

The trap

(c) describes the other kind of smog — sulphurous “London smog” forms in cold, damp air.

Also know

PAN is peroxyacetyl nitrate, a strong eye irritant that also damages plants. The London smog of December 1952 killed several thousand people and led to Britain’s Clean Air Act. Delhi’s winter smog is a mix of both kinds, held down by a temperature inversion.