Which of the following is a powerful eye irritant in the smoke?

PAN (peroxyacetyl nitrate) is the right answer. It is a secondary pollutant formed in photochemical smog when nitrogen oxides and volatile organic compounds react under sunlight. It is one of the most potent lachrymators (tear-inducing agents) in polluted air and causes eye irritation even at very low concentrations, which is why the smog of Los Angeles was so notorious for stinging eyes.

Why the others fail:

• Ozone (A): a strong respiratory irritant that damages lung tissue, but it is not the classic eye irritant of smog. It is a secondary pollutant too, so it is often confused with PAN.
• Sulphur dioxide (B): a primary pollutant from burning sulphur-bearing coal and fuel. It irritates the upper airways and is a key ingredient of sulphurous (London-type) smog, but it is not the eye-irritant answer here.
• Carbon dioxide (C): a non-irritant gas at ambient levels. Its concern is climate change, not eye irritation.

Core concept: photochemical versus sulphurous smog. Mains connection: GS-III environment, air pollution sources, secondary pollutants and the vehicular emission control debate.

Trap: candidates pick ozone because it is the most famous smog pollutant. Remember that ozone hurts the lungs, while PAN is the eye irritant.