What punched a hole in the ozone layer, and why does it matter?
Topic 9.1 Stratospheric Ozone Depletion: explain how CFCs deplete stratospheric ozone and describe the consequences of a thinner ozone layer.
A focused answer to APES Topic 9.1, covering the protective role of stratospheric ozone, how CFCs release chlorine that catalytically destroys ozone, the Antarctic ozone hole, the consequences of increased UV (skin cancer, cataracts, harm to ecosystems), and the difference from ground-level ozone, with a worked catalytic-destruction reasoning example.
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What this topic is asking
The College Board (Topic 9.1) wants you to explain how CFCs deplete stratospheric ozone and describe the consequences of a thinner ozone layer.
The protective ozone layer and how CFCs destroy it
Why one CFC destroys so much ozone
Consequences of a thinner ozone layer
Why this matters
Ozone depletion is the AP exam's classic example of a global problem solved by international action (Topic 9.2). It builds on the stratosphere of Unit 4 and is endlessly confused with two other problems: the greenhouse effect (carbon dioxide, warming) and ground-level ozone (a harmful pollutant in smog). Keeping these three straight is essential.
Try this
Q1. Identify the chemicals primarily responsible for stratospheric ozone depletion. [1 point]
- Cue. Chlorofluorocarbons (CFCs), which release ozone-destroying chlorine.
Q2. Explain why a single CFC molecule can destroy many ozone molecules. [2 points]
- Cue. UV releases chlorine from the CFC, and chlorine acts as a catalyst: it destroys an ozone molecule and is regenerated, so the same chlorine atom goes on to destroy many more ozone molecules.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Explain the role of the stratospheric ozone layer. (b) Explain how CFCs deplete ozone. (c) Identify two consequences for living things of a thinner ozone layer. (d) Explain why one CFC molecule can destroy many ozone molecules.Show worked answer →
A 4-point FRQ on ozone depletion.
(a) Explain (1 point): the stratospheric ozone layer absorbs most of the Sun's harmful ultraviolet (UV) radiation, protecting life at the surface.
(b) Explain (1 point): CFCs rise to the stratosphere where UV breaks them down, releasing chlorine atoms that catalyze the breakdown of ozone into oxygen.
(c) Identify (1 point): any two of skin cancer, cataracts, immune suppression, harm to phytoplankton, or crop damage.
(d) Explain (1 point): chlorine acts as a catalyst, so it is regenerated after destroying an ozone molecule and goes on to destroy many more.
Markers reward the UV-absorbing protective role, the CFC-to-chlorine catalytic mechanism, two valid consequences of more UV, and the catalyst regeneration point.
AP 2018 (style)1 marksSection I (multiple choice). Stratospheric ozone depletion is caused primarily by: (A) carbon dioxide from fossil fuels (B) chlorine released from CFCs (C) sulfur dioxide from power plants (D) methane from agriculture. Justify your choice.Show worked answer →
A 1-point MCQ on ozone depletion. The answer is (B).
Ozone depletion is driven by chlorine (and bromine) released when UV breaks down CFCs in the stratosphere; the chlorine then catalytically destroys ozone. Carbon dioxide (A) and methane (D) are greenhouse gases driving warming, and sulfur dioxide (C) causes acid rain, not ozone depletion. The trap is confusing ozone depletion (CFCs and chlorine) with the greenhouse effect (carbon dioxide and methane); they are different problems.
Related dot points
- Topic 9.2 Reducing Ozone Depletion: describe the strategies and international agreements used to reduce ozone depletion and how the ozone layer is recovering.
A focused answer to APES Topic 9.2, covering the Montreal Protocol, the phase-out of CFCs, substitutes (HCFCs and HFCs) and their trade-offs, why ozone recovery is slow, the success of international cooperation, and the lesson for other global problems, with a worked recovery-timescale reasoning example.
- Topic 9.3 The Greenhouse Effect: explain the greenhouse effect, identify the major greenhouse gases, and distinguish the natural effect from the enhanced effect.
A focused answer to APES Topic 9.3, covering how the greenhouse effect works, the major greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor, CFCs), the difference between the natural and enhanced greenhouse effect, global warming potential and residence time, with a worked global warming potential calculation.
- Topic 4.4 Earth's Atmosphere: describe the composition of the atmosphere and the four main layers, and explain how temperature changes with altitude.
A focused answer to APES Topic 4.4, covering atmospheric composition, the four layers (troposphere, stratosphere, mesosphere, thermosphere), the temperature profile, the ozone layer, and the role of the atmosphere in weather and protection, with a worked composition calculation.
- Topic 9.10 Human Impacts on Biodiversity: identify the major human causes of biodiversity loss (HIPPCO) and explain why declining biodiversity matters.
A focused answer to APES Topic 9.10, covering the HIPPCO causes of biodiversity loss, why habitat loss is the largest, the importance of biodiversity for ecosystem services and resilience, the sixth mass extinction, and conservation responses, with a worked species-loss reasoning example.
- Topic 7.1 Introduction to Air Pollution: identify the major air pollutants and their sources and distinguish primary from secondary pollutants.
A focused answer to APES Topic 7.1, covering the major air pollutants, their natural and human sources, the criteria pollutants, and the distinction between primary and secondary pollutants, with a worked emissions calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)