Smog is essentially caused by the atmospheric presence of

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2019 Q101 — Environment & Ecology PYQ with Answer

State PCS 2019 Q101 — Environment & Ecology PYQ with Answer