6.4 Stratospheric ozoneIB Environmental Systems and Societies SL: Subtopic test

1. 1
   The Sun emits electromagnetic radiation over a range of wavelengths. Approximate typical wavelengths are: radio waves about 1 m, infrared about 10 000 nm, visible light about 500 nm and ultraviolet (UV) about 300 nm. Frequency increases as wavelength decreases. A student notes that sunlight reaching the ground contains plenty of visible and infrared light but much less UV than the radiation arriving at the top of the atmosphere.

   (a)

   Which of these types of radiation has the highest frequency and therefore carries the most energy per photon?

   [1 mark]

   • AUltraviolet
   • BVisible light
   • CInfrared
   • DRadio waves

   (b)

   What is the main role of ozone in the stratosphere?

   [1 mark]

   • AIt traps infrared radiation, causing the greenhouse effect
   • BIt provides the oxygen needed for respiration
   • CIt absorbs ultraviolet radiation from the Sun, reducing the amount reaching the Earth's surface
   • DIt reflects visible light back into space

   (c)

   Using the wavelengths given, explain why UV radiation is more dangerous to living organisms than visible light.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ozone (O3) in the stratosphere is continually being formed, when UV radiation splits oxygen molecules (O2) into atoms that join other oxygen molecules, and continually being broken down, when ozone absorbs UV radiation and splits again. Measurements made before the 1970s showed that the total concentration of stratospheric ozone stayed almost constant from year to year.

   (a)

   What is meant by a steady state equilibrium in the stratospheric ozone concentration?

   [1 mark]

   • AOzone formation has stopped
   • BThe rate of ozone formation equals the rate of ozone destruction, so the concentration stays constant
   • COzone is destroyed faster than it is formed
   • DOzone is formed only at night

   (b)

   How do ozone-depleting substances (ODSs) affect the natural balance of ozone?

   [1 mark]

   • AThey increase the rate of ozone formation
   • BThey have no effect because ozone is constantly replaced
   • CThey absorb UV radiation instead of ozone
   • DThey destroy ozone molecules, adding to the natural breakdown, so the concentration falls

   (c)

   Explain why the ozone concentration stayed almost constant before the 1970s even though ozone was being formed and destroyed all the time.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Punta Arenas in southern Chile lies close to Antarctica and is sometimes beneath the edge of the Antarctic ozone hole in spring. A student is given simplified, rounded October mean values of total ozone over Antarctica in Dobson units (DU): 300 DU in 1979, 150 DU in 1990 and 100 DU in 2000.

   (a)

   Calculate the percentage decrease in October mean ozone over Antarctica between 1979 and 2000, and state the effect this has on the radiation reaching the surface.

   [3 marks]

   (b)

   Explain how increased UVB radiation could affect organisms living near Punta Arenas, including humans.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The Montreal Protocol, agreed in 1987, regulates the production, trade and use of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs). CFCs were widely used in refrigerators, aerosol sprays and foam, and are very stable, so they persist in the atmosphere for decades. Every UN member state has ratified the treaty, developing countries were given extra time and financial help through a Multilateral Fund, and about 99% of controlled ODSs have been phased out. The ozone layer over Antarctica is projected to recover to 1980 levels by about 2066.

   (a)

   Explain how the use of CFCs led to a reduction in ozone and why this is harmful.

   [6 marks]

   (b)

   Evaluate the success of the Montreal Protocol.

   [6 marks]

   Total for question 4: 12 marks

End of questions