UPPSC 2019 Prelims GS Paper-I · Question 101 of 150

Correct — D, oxide of nitrogen and sulphur. The word 'smog' is a portmanteau of smoke and fog, coined in the early twentieth century for the sooty winter fogs of coal-burning London, and it now covers two distinct chemical phenomena — both of which are driven by the oxides named in this option. The classical or London type, also called sulphurous or reducing smog, forms when coal and heavy fuel oil release sulphur dioxide (SO2) and smoke particles into cold, damp, stagnant air held down by a temperature inversion; the SO2 dissolves in fog droplets and oxidises to sulphuric acid, producing the choking grey pall that killed thousands during the Great Smog of London in December 1952 and forced Britain's Clean Air Act of 1956. The photochemical or Los Angeles type, also called oxidising smog, forms when oxides of nitrogen (NO and NO2, produced whenever combustion is hot enough to burn atmospheric nitrogen — chiefly in vehicle engines and power plants) react with volatile organic compounds under strong sunlight, generating ground-level ozone, peroxyacetyl nitrate (PAN) and aldehydes, and giving the characteristic brown haze. Take the two families together and the causal agents are exactly the oxides of nitrogen and of sulphur. Delhi and the National Capital Region in winter suffer both routes at once — vehicular NOx, coal-fired SO2, crop-residue burning and a cold-season inversion that traps the lot.

• (a)Oxygen and ozone — Oxygen makes up about 21 per cent of clean dry air and is not a pollutant at all. Ozone does belong in the smog story, but on the wrong side of the arrow: in photochemical smog ground-level ozone is a SECONDARY pollutant, formed after oxides of nitrogen and hydrocarbons react in sunlight. Naming it as a cause reverses the chemistry.
• (b)Ozone and nitrogen — Molecular nitrogen (N2) is about 78 per cent of clean air and is chemically inert at ordinary temperatures — it becomes a problem only after the heat of an engine or a furnace converts it into oxides of nitrogen. That single word of difference, 'nitrogen' versus 'oxides of nitrogen', is precisely what this question is testing; and ozone, again, is a product of smog rather than its cause.
• (c)Oxygen and nitrogen — These two gases together are roughly 99 per cent of the dry atmosphere — they ARE ordinary clean air. If their presence caused smog, every breath anyone has ever taken would have been smog. The option is a pure definitional decoy.

Air pollutants are classified as primary — emitted directly, such as sulphur dioxide, oxides of nitrogen, carbon monoxide and particulate matter — or secondary, formed in the atmosphere by reactions among primary pollutants, such as ground-level ozone, peroxyacetyl nitrate and sulphuric acid aerosol. Smog is the visible, health-damaging mixture that results. Sulphurous (London) smog is a cold, damp, sunless phenomenon dominated by sulphur dioxide and soot; photochemical (Los Angeles) smog is a warm, sunlit phenomenon dominated by oxides of nitrogen and hydrocarbons. A temperature inversion — warm air sitting over cold surface air — is the meteorological lid that lets either kind build up instead of dispersing.

The fastest route to the answer is elimination by composition. Nitrogen and oxygen are the two main constituents of ordinary air, so any option built from them describes clean air, not pollution — that removes (c) immediately, and weakens (a) and (b), each of which is half clean air. What remains is the precursor-versus-product distinction: ozone appears in options (a) and (b), but ozone is manufactured inside photochemical smog, not responsible for it. Only option (d) names substances that are unambiguously emitted pollutants and unambiguously the drivers of both smog families. The trap works on candidates who remember 'ozone is in smog' without remembering which way the reaction runs.

• 'Smog' = smoke + fog; the term was first applied to the coal-smoke winter fogs of London in the early twentieth century.
• Classical / London / sulphurous / reducing smog: sulphur dioxide plus smoke particles trapped in cold, humid, stagnant air. The Great Smog of London (5-9 December 1952) led directly to Britain's Clean Air Act, 1956.
• Photochemical / Los Angeles / oxidising smog: oxides of nitrogen plus volatile organic compounds, cooked by strong sunlight, yielding ground-level ozone, peroxyacetyl nitrate (PAN, a severe eye irritant) and aldehydes.
• Ground-level ozone is a secondary pollutant — a product of smog chemistry, never a direct emission. Stratospheric ozone, by contrast, is protective; the same molecule is 'good up high, bad nearby'.
• The same two oxides also produce acid rain: SO2 to sulphuric acid and NOx to nitric acid. India's National Clean Air Programme was launched on 10 January 2019, eleven months before this exam.

• Treating ozone as a cause of smog — in the photochemical route it is a product, and any option naming it as a cause is chemically backwards.
• Confusing nitrogen gas (N2, inert, 78 per cent of air) with the oxides of nitrogen (NO, NO2) — the wording of the options turns on exactly this.
• Assuming all smog is photochemical. The London type is sulphurous and forms in cold, damp, sunless conditions, which is why the correct option has to name sulphur too.

UPSC has asked photochemical smog from both ends — what it is a reaction among (2013) and what it produces (2003) — while UPPSC prefers the blunter 'what causes smog'. Memorise the two smog families with their precursor list and their product list, and every phrasing of this question is covered.

• practice — not a real PYQ

  Which one of the following is a secondary air pollutant?

  • (a)Sulphur dioxide
  • (b)Ground-level ozone
  • (c)Carbon monoxide
  • (d)Lead

  Answer(b) Ground-level ozone — it is not emitted directly by any source but is formed in the atmosphere when oxides of nitrogen and volatile organic compounds react in sunlight; sulphur dioxide, carbon monoxide and lead are all primary pollutants released straight from combustion or industry.

• practice — not a real PYQ

  Peroxyacetyl nitrate (PAN), a powerful eye irritant, is characteristically associated with which one of the following?

  • (a)Classical or London-type smog
  • (b)Photochemical smog
  • (c)Depletion of the stratospheric ozone layer
  • (d)Eutrophication of lakes

  Answer(b) Photochemical smog — PAN is formed along with ozone and aldehydes when oxides of nitrogen react with volatile organic compounds in strong sunlight, and it is the compound responsible for the stinging eyes typical of Los Angeles-type smog.