AP Environmental Science: Ozone Depletion (CFCs, Montreal Protocol)

AP Environmental Science – Ozone Depletion (CFCs, Montreal Protocol)

AP Environmental Science: Ozone Depletion (CFCs & Montreal Protocol) – Exam-Ready Study Guide

What This Is

Ozone depletion refers to the thinning of the stratospheric ozone layer (10–50 km above Earth), which absorbs 97–99% of the sun’s harmful ultraviolet (UV-B) radiation. Without this layer, life on Earth would face increased skin cancer, cataracts, weakened immune systems, and ecosystem damage. The primary cause is chlorofluorocarbons (CFCs), human-made chemicals once used in refrigerants, aerosols, and solvents. The Montreal Protocol (1987) is a landmark international treaty that phased out CFCs, proving global cooperation can successfully address environmental crises. Example: The "ozone hole" over Antarctica (discovered in 1985) is a real-world consequence of CFCs breaking down ozone molecules, especially during polar spring.

Key Terms & Concepts

• Stratospheric Ozone (O?): A molecule made of three oxygen atoms that forms a protective layer in the stratosphere, absorbing UV radiation.
• Tropospheric Ozone (Ground-Level Ozone): A harmful air pollutant (component of smog) formed by reactions between NO? and VOCs in sunlight. Not the same as the protective ozone layer!
• Chlorofluorocarbons (CFCs): Human-made chemicals (e.g., CCl?F, CCl?F?) used in refrigeration, aerosols, and foam-blowing. They are stable in the troposphere but break down in the stratosphere, releasing chlorine atoms that destroy ozone.
• Ozone Depletion Reaction (Simplified):
• CFC + UV light-Cl (chlorine radical)
• Cl + O?-ClO + O? (destroys ozone)
• ClO + O-Cl + O? (regenerates Cl, allowing it to destroy 100,000+ ozone molecules before leaving the stratosphere).
• Montreal Protocol (1987): An international treaty that phased out CFCs and other ozone-depleting substances (ODS). First UN treaty ratified by every country (197 parties).
• Ozone Hole: A region of severely depleted ozone over Antarctica, most pronounced in September–November (Southern Hemisphere spring) due to polar stratospheric clouds (PSCs) and cold temperatures accelerating ozone destruction.
• Polar Stratospheric Clouds (PSCs): Ice clouds in the Antarctic stratosphere that provide surfaces for heterogeneous chemical reactions, speeding up ozone depletion.
• Hydrochlorofluorocarbons (HCFCs): Temporary CFC replacements (e.g., CHClF?) that are less ozone-depleting but still harmful. Phased out under the Montreal Protocol’s later amendments.
• Hydrofluorocarbons (HFCs): CFC replacements (e.g., CH?FCF?) that do not deplete ozone but are potent greenhouse gases (regulated under the Kigali Amendment, 2016).
• UV-B Radiation: Medium-wavelength UV light (280–315 nm) that causes skin cancer, cataracts, and reduced plant photosynthesis. The ozone layer blocks most UV-B.
• Rowland-Molina Hypothesis (1974): The theory (later proven) that CFCs could deplete the ozone layer, proposed by Mario Molina and Sherwood Rowland (Nobel Prize in Chemistry, 1995).

Step-by-Step / Process Flow

How Ozone Depletion Works (Chemical Process)

1. CFCs are released at Earth’s surface (e.g., from old refrigerators or aerosol cans).
2. CFCs rise into the stratosphere (takes 5–10 years) because they are chemically stable and don’t break down in the troposphere.
3. UV radiation breaks CFCs apart, releasing chlorine (Cl) atoms.
4. Example: CCl?F + UV-Cl + CCl?F
5. Chlorine destroys ozone (O?) in a catalytic cycle (one Cl atom can destroy thousands of O? molecules).
6. Cl + O?-ClO + O?
7. ClO + O-Cl + O? (Cl is regenerated)
8. Ozone depletion is worst over Antarctica because:
9. Polar vortex traps cold air, forming PSCs (ice clouds).
10. PSCs provide surfaces for heterogeneous reactions, speeding up ozone destruction.
11. Sunlight returns in spring, triggering rapid ozone loss.

How to Answer an FRQ on Ozone Depletion

1. Identify the cause: Always mention CFCs (or other ODS like halons, methyl bromide) as the primary culprit.
2. Explain the chemical process: Use the Cl catalytic cycle (Cl + O?-ClO + O?, then ClO + O-Cl + O?).
3. Link to environmental effects: Increased UV-B radiation-skin cancer, cataracts, ecosystem damage (e.g., phytoplankton decline).
4. Discuss the solution: Montreal Protocol (phased out CFCs, replaced with HCFCs-HFCs).
5. Compare to climate change: Unlike the Kyoto Protocol (CO?), the Montreal Protocol worked because:
6. Clear scientific consensus (Rowland-Molina hypothesis).
7. Affordable alternatives (HFCs, HCFCs).
8. Global cooperation (all countries ratified).

Common Mistakes

• Mistake: Confusing stratospheric ozone (good) with tropospheric ozone (bad).

• Correction: Stratospheric ozone protects us from UV; tropospheric ozone is a pollutant that harms lungs and plants.

• Mistake: Thinking CO? causes ozone depletion.

• Correction: CO? is a greenhouse gas, not an ozone-depleting substance. CFCs (and other halocarbons) are the main culprits.

• Mistake: Assuming the ozone hole is permanent.

• Correction: The ozone layer is slowly recovering (expected to return to 1980 levels by 2060–2070) due to the Montreal Protocol.

• Mistake: Forgetting that HFCs (CFC replacements) are greenhouse gases.

• Correction: HFCs do not deplete ozone but contribute to climate change (regulated under the Kigali Amendment).

• Mistake: Overlooking the role of polar stratospheric clouds (PSCs) in ozone depletion.

• Correction: PSCs accelerate ozone destruction by providing surfaces for chlorine reactions, which is why the ozone hole is worst over Antarctica.

AP Exam Insights

• Frequently Tested:
• Chemical reactions (Cl catalytic cycle).
• Montreal Protocol vs. Kyoto Protocol (why one succeeded, the other struggled).
• Effects of UV-B radiation (human health, ecosystems).

• CFC replacements (HCFCs-HFCs-alternatives like hydrocarbons).

• Tricky Distinctions:

• Ozone depletion vs. climate change: Different causes (CFCs vs. CO?/CH?), but some chemicals (like HFCs) contribute to both.

• Stratospheric vs. tropospheric ozone: One is protective, the other is harmful.

• Common FRQ Prompts:

• Explain the chemical process of ozone depletion.
• Compare the Montreal Protocol to another environmental agreement (e.g., Paris Agreement).
• Describe human health and ecological effects of ozone depletion.

• Evaluate the effectiveness of CFC replacements (HCFCs, HFCs).

• Multiple-Choice Traps:

• “Which gas is responsible for ozone depletion?”-CFCs (not CO? or methane).
• “Where is the ozone hole located?”-Antarctica (not the Arctic).
• “What replaced CFCs?”-HCFCs first, then HFCs (but HFCs are greenhouse gases).

Quick Check Questions

1. Multiple Choice

Which of the following is the primary chemical reaction responsible for ozone depletion in the stratosphere? (A) O? + UV-O? + O (B) Cl + O?-ClO + O? (C) CO? + UV-C + O? (D) NO? + O?-NO? + O?

Correct Answer: (B) Explanation: Chlorine (Cl) from CFCs catalyzes the destruction of ozone (O?) by forming chlorine monoxide (ClO) and oxygen (O?).

2. Short FRQ (3 pts)

The Montreal Protocol has been called the most successful international environmental agreement in history. (a) Identify ONE reason why the Montreal Protocol was successful. (1 pt) (b) Explain how the Montreal Protocol addressed the problem of ozone depletion. (1 pt) (c) Describe ONE unintended consequence of the chemicals used to replace CFCs. (1 pt)

Sample Answers: (a) The Montreal Protocol had clear scientific evidence (Rowland-Molina hypothesis) linking CFCs to ozone depletion, making it easier to gain global support. (b) It phased out CFCs and other ozone-depleting substances (ODS), replacing them with less harmful alternatives (e.g., HCFCs, then HFCs). (c) HFCs (hydrofluorocarbons), while not ozone-depleting, are potent greenhouse gases that contribute to climate change.

Last-Minute Cram Sheet

1. CFCs (chlorofluorocarbons) = main cause of ozone depletion (e.g., CCl?F, CCl?F?).
2. Ozone depletion reaction: Cl + O?-ClO + O? (Cl is regenerated, destroying 100,000+ O? molecules).
3. Montreal Protocol (1987) = phased out CFCs; first UN treaty ratified by all countries.
4. Ozone hole = worst over Antarctica in September–November (Southern Hemisphere spring).
5. Polar stratospheric clouds (PSCs) = speed up ozone destruction by providing surfaces for reactions.
6. UV-B radiation = causes skin cancer, cataracts, reduced plant growth.
7. HCFCs = temporary CFC replacements (less ozone-depleting but still harmful).
8. HFCs = CFC replacements that do not deplete ozone but are greenhouse gases (regulated by Kigali Amendment, 2016).
9. Rowland-Molina Hypothesis (1974) = predicted CFCs would deplete ozone (Nobel Prize, 1995).
10. Ozone depletion-climate change (different causes, but some chemicals like HFCs affect both).