CDS (I) 2021 General Knowledge · Question 55 of 120

Correct — D, Stratosphere. The ozone layer sits in the lower and middle stratosphere, roughly fifteen to thirty-five kilometres up, where it absorbs essentially all incoming UV-C and most UV-B. That absorption is not incidental to the layer's structure — it is what creates it. Because ozone converts ultraviolet energy into heat, temperature rises with height through the stratosphere instead of falling, and that inversion makes the layer stable, free of vertical convection and almost cloudless. The same stability is why jet aircraft cruise in the lower stratosphere and fly smoothly there. Ozone is a triatomic molecule of oxygen, and column amounts are measured in Dobson Units.

• (a)Ionosphere — The ionised region above the mesosphere, where solar radiation strips electrons from atoms. It reflects radio waves back to the earth, which is what makes long-distance short-wave broadcasting possible, but it holds no ozone layer.
• (b)Troposphere — The lowest layer, where all weather occurs and temperature falls with height. Ozone does occur here, but as a pollutant formed in photochemical smog at ground level — harmful to breathe, and not the protective layer the question describes.
• (c)Mesosphere — The layer above the stratosphere, where temperature falls again with height to the coldest point in the atmosphere. It is where meteors burn up, and it lies above the ozone concentration, not within it.

The atmosphere is layered by how temperature behaves with height. In the troposphere temperature falls; in the stratosphere it rises, because ozone absorbs ultraviolet; in the mesosphere it falls again to the atmosphere's coldest point; in the thermosphere it rises steeply. Ozone's own chemistry is a cycle — ultraviolet splits oxygen molecules, the freed atoms combine with other oxygen molecules to make ozone, and ultraviolet then breaks ozone apart again, absorbing energy at each step. Chlorine and bromine released from man-made compounds accelerate the destruction side of that cycle without being consumed themselves.

The one distinction the syllabus keeps returning to is good ozone above and bad ozone below. In the stratosphere ozone is a shield; in the troposphere it is a pollutant produced by sunlight acting on vehicle exhaust, and it aggravates asthma. A question that mentions absorption of ultraviolet is asking about the shield. The other habitual confusion is between the ozone regime and the climate regime, and they are genuinely separate: the Vienna Convention of March 1985 and the Montreal Protocol of 16 September 1987 govern ozone-depleting substances, while the UNFCCC, Kyoto and Paris govern greenhouse gases. The link between them was established by Molina and Rowland in 1974, who showed that chlorofluorocarbons destroy stratospheric ozone; they shared the 1995 Nobel Prize in Chemistry with Paul Crutzen. As of the 2021 exam the position was already one of recovery rather than crisis, and it has held: the Montreal Protocol achieved universal ratification in 2009, its Kigali Amendment of 2016 extended the regime to hydrofluorocarbons — which do not deplete ozone but are potent greenhouse gases — and the layer is assessed as on track to recover.

• The ozone layer lies in the stratosphere at roughly fifteen to thirty-five kilometres, and absorbs essentially all UV-C and most UV-B.
• Ozone is a triatomic molecule of oxygen, and column ozone is measured in Dobson Units.
• Absorption of ultraviolet by ozone is why temperature rises with height in the stratosphere, making the layer stable and cloud-free.
• The link between chlorofluorocarbons and ozone depletion was established by Molina and Rowland in 1974; they shared the 1995 Nobel Prize in Chemistry with Paul Crutzen.
• The Antarctic ozone hole was reported in 1985 by the British Antarctic Survey, and the Montreal Protocol was agreed on 16 September 1987.

• Confusing the protective stratospheric layer with ground-level ozone, which is a pollutant.
• Filing the Montreal Protocol with the climate treaties; it belongs to the ozone regime, and Kyoto and Paris to the climate regime.
• Assuming the ionosphere holds the ozone layer because both involve solar radiation; the ionosphere reflects radio waves and lies far above the ozone concentration.

Asked as a layer-identification item, and in harder versions as a two-statement item pairing the layer with the treaty or with the chemistry of depletion.

• practice — not a real PYQ

  Temperature increases with height in the stratosphere mainly because

  • (a)the air is thinner there
  • (b)ozone absorbs ultraviolet radiation and converts it to heat
  • (c)it is closer to the sun
  • (d)water vapour traps outgoing infrared radiation

  Answer(b) ozone absorbs ultraviolet radiation and converts it to heat — the inversion this creates is what makes the stratosphere stable and cloud-free.

• practice — not a real PYQ

  The Montreal Protocol, agreed in 1987, deals with

  • (a)the reduction of greenhouse gas emissions
  • (b)substances that deplete the ozone layer
  • (c)transboundary movement of hazardous wastes
  • (d)the conservation of biological diversity

  Answer(b) substances that deplete the ozone layer — its Kigali Amendment of 2016 later added hydrofluorocarbons, which harm the climate but not the ozone layer.