Irritation in the eyes is characteristically caused by which of the following…

Concept — how air pollutants are classified by the tissue they attack. In sunlit polluted air, nitrogen oxides and unburnt hydrocarbons are driven through a chain of photochemical reactions. Their products are secondary pollutants — species never emitted from a chimney or exhaust pipe but assembled in the atmosphere itself. Pollutants are then grouped by the tissue their characteristic action falls on: some are principally upper-airway irritants, some principally deep-lung irritants, and one family of these secondary products — the peroxyacyl nitrates — is classed as a lachrymator, that is, a substance whose defining effect on people is smarting and watering of the eyes.

Application — placing the four offered pollutants in that scheme. Peroxyacetyl nitrate, PAN, formula CH3COOONO2, is the leading member of the lachrymator family. It is built when the acetyl peroxy radical formed by hydrocarbon photo-oxidation couples with nitrogen dioxide: CH3C(O)OO· + NO2 → CH3COOONO2. PAN is the pollutant conventionally held responsible for the eye irritation reported during photochemical smog episodes; controlled-exposure work reports eye irritation from PAN at concentrations of the order of 0.6 mg/m³ (roughly 130 ppb). Of the four pollutants offered, it is the only one whose characteristic, defining effect is on the eye.

Cross-check — testing the other three against the same scheme.

• Sulphur di-oxide, SO2, is a primary pollutant emitted directly from sulphur-bearing coal and fuel oil. Because it is extremely soluble in water it is captured largely in the upper respiratory tract, so it is classed as an upper-airway irritant causing bronchial constriction, and after oxidation to sulphuric acid it drives acid rain. It can sting the eyes too, but only at concentrations well above those met in ambient smog, so eye irritation is not its defining effect.

• Ozone, O3, is also a photochemical secondary oxidant, but it is only sparingly soluble in water and therefore passes the upper airway to reach the deep lung; it is classed as a deep-lung irritant acting on the bronchioles and alveoli. Ozone adds to the general discomfort of a smog episode, yet within photochemical smog the eye irritation itself is attributed to the peroxyacyl nitrates.

• Nitrous oxide, N2O (laughing gas), is a chemically stable anaesthetic and long-lived greenhouse gas, not an irritant at all. It must not be confused with nitrogen dioxide, NO2, the reactive brown gas that supplies the nitrogen atom for PAN formation.

Result. The pollutant characteristically responsible for irritation of the eyes is PAN — peroxyacetyl nitrate. This is also the option marked correct in the official UGC NET June 2012 Paper I answer key for this question.