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Difficulty: MediumClimate Change, Global Warming and Ozone Layer Depletion

With reference to the chemical mechanism of stratospheric ozone depletion over Antarctica, consider the following statements:

1. Polar Stratospheric Clouds (PSCs) provide catalytic surfaces for heterogeneous reactions that convert inactive chlorine reservoirs (HClHCl and ClONO2ClONO_2) into reactive chlorine species.
2. The catalytic destruction of ozone molecules by active chlorine free radicals occurs rapidly during the dark polar winter.
3. The polar vortex plays a crucial role by confining cold air over Antarctica, preventing mixing with mid-latitude air masses and maintaining low temperatures required for PSC formation.

Which of the statements given above is/are correct?

  1. 1 and 3 onlyAnswer
  2. B
    1 and 2 only
  3. C
    2 and 3 only
  4. D
    1, 2 and 3

Answer

1 and 3 only
The correct answer states that 1 and 3 only are true. PSC surfaces enable the chemical transformation of reservoir compounds into chlorine gas (Cl2Cl_2). The Antarctic polar vortex isolates cold polar air to maintain temperatures under −78∘C-78^\circ\text{C}. However, active ozone destruction requires sunlight to photolyze Cl2Cl_2 into free chlorine radicals, which happens during Antarctic spring rather than dark polar winter.

Step-by-Step Solution

1
Evaluate Statement 1 regarding Polar Stratospheric Clouds (PSCs)
Statement 1 is TRUE. PSCs formed in extremely cold temperatures (below −78∘C-78^\circ\text{C}) provide ice particle surfaces for heterogeneous reactions converting stable reservoir species (HClHCl and ClONO2ClONO_2) into photolytically active forms like Cl2Cl_2 and HOClHOCl.
Heterogeneous chemistry on PSC surfaces is a fundamental precursor step to ozone hole formation.
2
Evaluate Statement 2 regarding timing of ozone destruction
Statement 2 is FALSE. During dark winter, molecular chlorine (Cl2Cl_2) accumulates. Photolysis (splitting of Cl2Cl_2 into reactive Cl∙Cl^\bullet radicals by solar UV radiation) requires sunlight, which occurs when spring arrives (September-October). Rapid catalytic ozone destruction occurs during early spring, not during polar winter.
Chlorine catalytic cycles require photon absorption (hνh\nu) from solar ultraviolet radiation.
3
Evaluate Statement 3 regarding the polar vortex
Statement 3 is TRUE. A strong ring of circulating wind (polar vortex) seals polar stratospheric air, preventing warmer, ozone-rich air from mid-latitudes from entering and sustaining ultra-cold conditions necessary for PSC stability.
Isolation by the polar vortex maintains environmental conditions necessary for PSC persistence.

Key Concept

Heterogeneous chemistry on Polar Stratospheric Clouds and photochemical destruction of stratospheric ozone by chlorine radicals during Antarctic spring.
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Topics

Climate Change, Global Warming and Ozone Layer Depletion