Questions: Stratospheric Ozone Depletion and the Ozone Hole
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
In the cycle Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂, why is chlorine called a catalyst?
AIt is a product of the first step and a reactant in neither step
BIt is consumed in the first step and regenerated in the second, so the net reaction O₃ + O → 2 O₂ leaves chlorine unchanged
CIt absorbs ultraviolet light and converts it into heat
DIt combines permanently with ozone to form a stable compound
Adding the two steps cancels Cl and ClO, leaving O₃ + O → 2 O₂. Because the chlorine atom comes out of the cycle unchanged, it can repeat the cycle many times. It stops only when it reacts into a reservoir compound such as HCl or ClONO₂.
Question 2 Short Answer
Why is the Antarctic ozone hole deepest in the Southern Hemisphere spring (September–October) rather than in winter?
Think about your answer, then reveal below.
Model answer: During the dark polar winter, very cold air inside the polar vortex forms polar stratospheric clouds. Reactions on cloud surfaces convert stable reservoir chlorine (HCl, ClONO₂) into Cl₂. No destruction happens until sunlight returns in spring and splits Cl₂ into reactive chlorine atoms, which then destroy ozone rapidly.
Ozone loss needs both the cold-cloud chemistry of winter and the sunlight of spring. This timing is why the hole appears each spring and fills in as the vortex breaks up and warmer, ozone-rich air mixes in.
Question 3 True / False
CFC molecules are heavier than the average air molecule, so they stay near the ground and cannot reach the stratosphere.
TTrue
FFalse
Answer: False
Below roughly 100 km, turbulent mixing dominates over settling by molecular weight. CFCs are chemically inert in the troposphere and last for decades, so winds carry them upward. In the stratosphere, short-wave UV breaks them apart and releases chlorine.
Question 4 Multiple Choice
Which statement best describes the effect of the Montreal Protocol?
AIt banned all ozone in the lower atmosphere
BIt phased out production of most ozone-depleting substances, and stratospheric chlorine has been declining since the late 1990s
CIt immediately closed the Antarctic ozone hole by 1990
DIt controlled carbon dioxide emissions to stop stratospheric cooling
Signed in 1987 and strengthened by later amendments, the protocol cut production of CFCs, halons, and related gases. Because these gases last for decades, recovery is slow. Scientific assessments project that Antarctic ozone will return to 1980 levels around the 2060s.