CDS (I) 2022 General Knowledge · Question 86 of 120

Correct — B, Formation of an ‘inversion lid’. Smog is a concentration problem before it is an emission problem. Normally air cools with height, so warm air near the ground is buoyant, rises, and carries pollutants up and away by convection. A temperature inversion turns that upside down: a layer of warmer air comes to sit over the cooler air at the surface, convection is shut off, vertical mixing stops, and everything emitted below the inversion stays below it. The trapped layer thickens hour by hour until the inversion breaks. That is why the same city can have clear air one week and a smog episode the next on identical emissions, and it is why the Great Smog of London in December 1952, blamed for something of the order of ten to twelve thousand deaths, is described as an inversion event rather than as a sudden increase in coal burning. Of the four options, the inversion is the one that decides whether pollutants accumulate or disperse, and so it is the most important single factor.

• (a)Long winter nights — Long winter nights help, but only because they favour the ground cooling that produces a radiation inversion in the first place. They are a contributing condition, not the mechanism; and smog occurs in places and seasons without long nights, as Los Angeles shows.
• (c)Presence of many air pollutant resources — Sources of pollution are necessary but not sufficient. Cities with heavy emissions and good ventilation do not build up smog, while cities with moderate emissions under a persistent inversion do. This option supplies the material; the inversion supplies the trap.
• (d)Rapid fall in temperature with increasing height above the sea level — A rapid fall of temperature with height is the ordinary lapse rate, which makes the air unstable and helps pollutants rise and disperse. It is the opposite of the condition that produces smog, and stating it as a cause inverts the physics.

Two different things are called smog. Classical or London smog forms in cold, damp conditions when sulphur dioxide and particulates from coal combine with fog droplets; it is a reducing mixture and a winter phenomenon. Photochemical or Los Angeles smog forms when nitrogen oxides and volatile hydrocarbons from vehicles react in strong sunlight to make ground-level ozone, peroxyacetyl nitrate and aldehydes; it is an oxidising mixture and a warm-weather phenomenon. Both need the same thing to become dangerous — a stable layer of air that will not mix.

The question asks for the most important factor, which means the options have to be ranked rather than merely judged true or false. Sources of pollution and long nights are both real contributors; the inversion is the one without which the others do not produce smog at all, and it is the one that explains the timing of episodes. Read option (d) carefully as well, because it is written to look technical: a rapid fall of temperature with height is the normal state of the troposphere and promotes mixing, so it works against smog rather than for it. Two printed slips in this item are worth noticing and neither changes the answer. The stem reads Which one of following, without the word the; and option (c) reads air pollutant resources where the sense plainly wants sources. Both are the paper's own, and the bank reproduces them as printed.

• A temperature inversion is a layer of warmer air lying over cooler air, which stops the convection that normally disperses pollutants upward.
• The Great Smog of London, December 1952, is the standard example of an inversion episode and was blamed for an estimated 10,000 to 12,000 deaths.
• Classical or London smog is a winter, coal-and-fog, reducing mixture; photochemical or Los Angeles smog is a sunlight-driven, vehicle-emission, oxidising mixture.
• Ground-level ozone is the marker of photochemical smog, formed from nitrogen oxides and hydrocarbons in the presence of sunlight, along with peroxyacetyl nitrate.
• The normal environmental lapse rate is a fall of about 6.5 degrees Celsius per kilometre, and it favours mixing rather than trapping.

• Treating the number of pollution sources as the decisive factor; ventilation decides whether the emissions accumulate.
• Reading a rapid fall of temperature with height as a smog-forming condition; that is the normal lapse rate and it aids dispersal.
• Assuming all smog is winter smog; photochemical smog needs strong sunlight and is a warm-weather phenomenon.

As a most-important-factor item like this one, as a question on what is produced during photochemical smog, or as a comparison between the two kinds of smog.

• practice — not a real PYQ

  A temperature inversion contributes to the build-up of smog chiefly because it

  • (a)increases the rate of emission of pollutants
  • (b)prevents the vertical mixing of air near the ground
  • (c)raises the ground-level temperature sharply
  • (d)increases wind speed at the surface

  Answer(b) prevents the vertical mixing of air near the ground — with convection shut off, pollutants stay in the shallow layer below the inversion.

• practice — not a real PYQ

  Photochemical smog is characterised by the presence of

  • (a)sulphur dioxide and soot
  • (b)ground-level ozone and peroxyacetyl nitrate
  • (c)carbon dioxide and methane
  • (d)chlorofluorocarbons

  Answer(b) ground-level ozone and peroxyacetyl nitrate — secondary pollutants formed from nitrogen oxides and hydrocarbons in sunlight.