MPSC 2024 Prelims Paper-I · Question 35 of 100

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.

• (1)Nitrogen oxide and sulfer dioxide — 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.
• (2)Carbon monoxide and volatile organic compounds — 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.
• (4)Ozone and particulate matter — 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.

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.

MPSC's environment questions on pollution reliably test the primary-versus-secondary distinction and the precursor chemistry of the named secondary pollutants, because these are precise, checkable facts that reward study over general awareness. This item can be answered from the compound's own name if the candidate reads it: peroxyacetyl nitrate needs nitrogen for the nitrate and carbon for the acetyl, which eliminates the sulphur dioxide option and the carbon monoxide option at once. Building the habit of decomposing a chemical name into the fragments it demands is worth more than memorising a reaction, and it transfers to other items in the same family. Note also that the paper prints 'sulfer dioxide' in option (1); the misspelling is the Commission's and does not change what the option asserts. A further habit worth building on this topic is to learn each pollutant as a triple — source, formation and effect — because MPSC asks all three. For PAN that means vehicle-derived hydrocarbons and oxides of nitrogen as the source, photochemical reaction in sunlight as the formation, and eye irritation and leaf bronzing as the effect; a question can be set on any one of the three.

• Peroxyacetyl nitrate, or PAN, is a secondary pollutant formed in the atmosphere by the reaction of oxides of nitrogen with hydrocarbons in the presence of sunlight; it is not emitted directly by any source.
• PAN and ground-level ozone are the two signature products of photochemical or Los Angeles-type smog, an oxidising smog that forms on warm, sunny, still days over traffic-heavy cities.
• PAN is a strong eye irritant and is toxic to vegetation, producing a characteristic bronzing or glazing on the undersides of leaves.
• PAN is more stable at low temperature and can be transported long distances, releasing oxides of nitrogen far downwind of the city that produced its precursors.
• London-type smog is chemically the opposite case: a reducing smog of smoke and sulphur dioxide from coal burning, forming in cold, damp, foggy conditions rather than in sunlight.

• Reaching for sulphur dioxide because it is the most familiar air pollutant; it belongs to reducing smog and acid rain, not to the synthesis of PAN
• Accepting an option that names volatile organic compounds without any nitrogen source, when PAN's nitrate group requires oxides of nitrogen
• Treating ozone as a precursor of PAN when the two are co-products of the same photochemical system
• Forgetting that sunlight is a necessary condition, which is why photochemical smog is a warm-season, sunny-city phenomenon and not a winter fog problem

Pollution chemistry appears in MPSC papers as one-line questions about how a named pollutant is formed, which pollutants are primary and which secondary, and which pollutant is associated with which type of smog or which effect on health or vegetation. The distractors are always genuine pollutants, so recognition is no help; the discriminating knowledge is the reaction each compound requires. Expect PAN, ground-level ozone, sulphur dioxide, oxides of nitrogen and particulate matter to circulate through these questions in various combinations, and expect the paper to print at least one of the chemical names with an unconventional spelling.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following pairs of pollutants are both secondary pollutants ?

  • (a)Carbon monoxide and sulphur dioxide
  • (b)Ground-level ozone and peroxyacetyl nitrate
  • (c)Oxides of nitrogen and unburnt hydrocarbons
  • (d)Smoke and suspended particulate matter

  Answer(b) Ground-level ozone and peroxyacetyl nitrate — both are formed in the atmosphere by reactions among primary pollutants in the presence of sunlight, and neither is emitted directly by any source. Carbon monoxide, sulphur dioxide, oxides of nitrogen, unburnt hydrocarbons, smoke and particulate matter are all emitted directly and are therefore primary pollutants.

• practice — not a real PYQ

  Photochemical smog differs from classic London-type smog chiefly in which of the following respects ?

  • (a)It is an oxidising mixture forming in warm sunlight, whereas London-type smog is a reducing mixture forming in cold damp air
  • (b)It is a reducing mixture forming in cold air, whereas London-type smog is oxidising
  • (c)It is caused by coal smoke, whereas London-type smog is caused by vehicle exhaust
  • (d)It contains no nitrogen compounds, whereas London-type smog is rich in them

  Answer(a) It is an oxidising mixture forming in warm sunlight, whereas London-type smog is a reducing mixture forming in cold damp air — photochemical smog arises from vehicle-derived oxides of nitrogen and hydrocarbons reacting in sunlight to give ozone and PAN, while London-type smog arose from coal smoke and sulphur dioxide in cold, foggy conditions.