PAN (Peroxyacetyl nitrate) is formed as a result of following interaction :

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Correct — option (3), Nitrogen Oxide and hydrocarbons. Peroxyacetyl nitrate, written PAN, is a secondary pollutant: it is not emitted by any vehicle, chimney or factory, but is manufactured in the atmosphere out of things that are. The two ingredients are exactly those named in option (3). Hydrocarbons — the unburnt and partly burnt fuel vapours that escape from vehicle exhausts, petrol handling, solvents and industry — are attacked by sunlight and by the reactive species that sunlight generates, and are oxidised through a chain of steps to peroxyacetyl radicals. Oxides of nitrogen, produced whenever fuel burns hot enough for atmospheric nitrogen and oxygen to combine in an engine or a furnace, supply the second half of the molecule. The peroxyacetyl radical and nitrogen dioxide join, and the product is PAN. The formula itself is the memory aid: PAN carries a nitrate group, so a nitrogen source is indispensable, and it carries an acetyl group, so a hydrocarbon source is equally indispensable — no combination lacking either one can produce it. This chemistry is the defining process of photochemical smog, sometimes called Los Angeles-type smog, which forms on warm, sunny, still days over traffic-heavy cities and is an oxidising smog, quite unlike the cold, damp, sulphur-and-smoke smog of the classic London type. PAN and ground-level ozone are the two signature products of that system. PAN is a powerful eye irritant and is markedly toxic to vegetation, producing a characteristic bronzing or glazing on the undersides of leaves, and because it is more stable at low temperatures it can be carried far downwind and release its nitrogen oxides elsewhere, spreading the effects of an urban pollution problem well beyond the city that generated it.

Why the other options are wrong — (a) This supplies a nitrogen source but replaces the hydrocarbon with sulphur dioxide, printed here as 'sulfer dioxide'. Sulphur dioxide belongs to a different pollution story altogether: it is released when sulphur-bearing coal and oil are burnt, it is the central gas of the classic London-type reducing smog, and by oxidation to sulphuric acid it is a principal cause of acid rain. It contributes no carbon skeleton, and PAN's acetyl group has to come from a hydrocarbon, so no amount of sulphur dioxide can build the molecule. This is the option that catches a candidate who remembers that PAN is an air pollutant and reaches for the most familiar pollutant gases.  (b) This is the most carefully constructed distractor, because volatile organic compounds are genuinely part of photochemical smog chemistry and carbon monoxide does take part in the reaction chain that generates the oxidising radicals. What the pair lacks is nitrogen. PAN is peroxyacetyl nitrate; the nitrate group is not optional, and without oxides of nitrogen there is nothing for the peroxyacetyl radical to combine with and no PAN can form. A candidate who checks the option against the name of the compound, rather than against a general sense of which pollutants are involved in smog, will see the omission at once.  (d) This names two pollutants that are found alongside PAN rather than two that make it. Ground-level ozone is itself a secondary pollutant produced by the same photochemical system that produces PAN — the two are co-products of the reaction of nitrogen oxides with hydrocarbons in sunlight, not precursor and product. Particulate matter is a physical class of suspended solid and liquid particles and is not a reactant in this synthesis at all. Choosing this option confuses the company a pollutant keeps with the ingredients it is made from.

Concept: Air pollutants divide into primary and secondary. A primary pollutant is emitted directly from a source — carbon monoxide, sulphur dioxide, oxides of nitrogen, unburnt hydrocarbons, smoke and dust. A secondary pollutant is formed in the atmosphere by reactions among primary pollutants, often driven by sunlight, and ground-level ozone and peroxyacetyl nitrate are the two standard examples. The system that produces them is photochemical smog. Sunlight splits nitrogen dioxide, releasing an oxygen atom that forms ozone; hydrocarbons in the air are oxidised through radical chains into aldehydes and then into peroxyacetyl radicals; and those radicals combine with nitrogen dioxide to give PAN. Because the process needs strong sunlight, warm temperatures and calm air to let the reactants accumulate, photochemical smog is a hazard of sunny cities with heavy traffic, and is chemically an oxidising mixture. It is worth setting against the older London-type smog, which arose from coal smoke and sulphur dioxide in cold, damp, foggy air and was chemically reducing. The two smogs share a name and almost nothing else — different seasons, different climates, different chemistry, different remedies.

State PCS 2024 Q35 — Environment & Ecology PYQ with Answer

State PCS 2024 Q35 — Environment & Ecology PYQ with Answer