Reducing
Ozone Depletion

Explore the science behind the ozone layer, the chemicals that destroy it, and the international policies that have helped it recover.

AP Environmental Science

What Is the Ozone Layer?

The ozone layer is a region of the stratosphere (roughly 15 to 35 km altitude) where \(\text{O}_3\) molecules are concentrated. These molecules absorb most of the Sun's harmful ultraviolet-B (UV-B) radiation, protecting life on Earth.

The Ozone-Oxygen Cycle

Why Ozone Depletion Matters

Skin Cancer

Increased UV-B radiation raises rates of melanoma and non-melanoma skin cancers in humans.

Crop Damage

Excess UV-B inhibits photosynthesis, reducing agricultural yields and harming food security.

Marine Harm

UV-B penetrates shallow water, killing phytoplankton that form the base of ocean food webs.

CFCs and the Chain Reaction

Chlorofluorocarbons (CFCs) are synthetic chemicals once widely used in refrigerants, aerosol sprays, and foam insulation. In the stratosphere, UV radiation breaks them apart, releasing free chlorine atoms that catalytically destroy ozone.

The Antarctic Ozone Hole

In 1985, scientists discovered a dramatic thinning of the ozone layer over Antarctica each spring. Polar stratospheric clouds (PSCs) form during the extremely cold Antarctic winter, providing surfaces on which chlorine reservoir species convert into reactive chlorine. When sunlight returns in spring, this chlorine rapidly destroys ozone.

1

Winter darkness: Polar vortex isolates Antarctic air, temperatures drop below \(-78^\circ\text{C}\).

2

PSC formation: Ice clouds provide surfaces that convert \(\text{ClONO}_2\) and \(\text{HCl}\) into reactive \(\text{Cl}_2\).

3

Spring sunlight: UV splits \(\text{Cl}_2\) into free Cl atoms, triggering massive ozone loss.

ODS vs. Greenhouse Gases

	ODS (e.g., CFCs)	GHGs (e.g., \(\text{CO}_2\))
Primary effect	Destroy stratospheric \(\text{O}_3\)	Trap heat in troposphere
Layer affected	Stratosphere	Troposphere
UV connection	Increases UV-B at surface	No direct UV effect
Key treaty	Montreal Protocol	Paris Agreement
Recovery status	Recovering	Still rising

Compare ozone-depletion potential with global-warming potential as separate metrics.

Use this illustration to explore the concept described in its caption.

Conceptual Earth horizon under separate ozone and climate pathways. UV releases reactive chlorine from a chlorine-containing ozone-depleting substance; a refrigerant gas absorbs infrared. A high-GWP HFC and a selected lower-GWP candidate have zero ozone-depletion potential. — A refrigerant substitute can have zero ozone-depletion potential while still contributing strongly to warming. Equal cooling service is held fixed here; other performance and lifecycle effects remain conditional.

The Montreal Protocol

Signed in 1987, the Montreal Protocol is widely considered the most successful international environmental treaty. It required nations to phase out the production and consumption of ozone-depleting substances (ODS), including CFCs, halons, carbon tetrachloride, and methyl chloroform.

Universal

Ratified by all 198 UN member states: the first treaty to achieve universal ratification.

Phased

Developed nations phased out CFCs by 1996; developing nations by 2010 with financial aid.

Adaptive

Amended multiple times (London, Copenhagen, Kigali) to address new threats like HFCs.

The Kigali Amendment (2016)

HFCs were introduced as CFC replacements because they do not deplete ozone. However, HFCs are extremely potent greenhouse gases (up to 14,800 times the warming potential of \(\text{CO}_2\)). The Kigali Amendment added HFCs to the Montreal Protocol phasedown schedule.

HFC-134a

Common in car AC; GWP of 1,430 over 100 years.

Alternatives

Natural refrigerants like propane, ammonia, and \(\text{CO}_2\) have near-zero GWP.

Measuring Ozone: Dobson Units

Ozone concentration is measured in Dobson Units (DU). One DU equals a layer of pure ozone 0.01 mm thick at standard temperature and pressure. Normal stratospheric ozone is about 300 DU. The "ozone hole" is defined as the area where total column ozone falls below 220 DU.

Recovery Timeline

1974

Rowland-Molina Hypothesis

Scientists warn CFCs can destroy stratospheric ozone.

1985

Ozone Hole Discovered

British Antarctic Survey detects severe ozone thinning over Antarctica.

1987

Montreal Protocol Signed

International agreement to phase out ODS production.

1996

Developed Nations Phase Out CFCs

Major CFC production halted in the U.S., EU, and Japan.

2016

Kigali Amendment

HFCs added to Montreal Protocol phasedown schedule.

2066

Projected Full Recovery

Antarctic ozone layer expected to return to 1980 levels.

How to Evaluate ODS Policies

1

Identify the ODS: Which chemical is being regulated? Is it a CFC, HCFC, halon, or HFC?

2

Check the mechanism: Does the substance release reactive halogens (Cl, Br) in the stratosphere?

3

Assess the ODP: Compare its Ozone Depletion Potential relative to CFC-11 (ODP = 1.0).

4

Evaluate replacement: Does the substitute avoid ozone harm? Check if it has a high GWP instead.

5

Consider equity: Are developing nations given adequate financial and technical support for transition?

Example: CFC-12 Phase-Out

Scenario: CFC-12 (dichlorodifluoromethane) was the primary refrigerant in automobile air conditioning. It has an ODP of 1.0 and a GWP of 10,900. Evaluate the environmental benefit of replacing it.

1

ODS identification: CFC-12 contains two chlorine atoms, both of which are released in the stratosphere.

2

Replacement: HFC-134a has ODP = 0 (no chlorine or bromine), but GWP = 1,430.

3

Net effect: Ozone destruction eliminated. GWP reduced by ~87% (10,900 to 1,430).

An old cooling unit is a chemical reservoir

Retired cooling unit exposes an intact coil circuit beside professional recovery equipment and a sealed cylinder. No venting cloud or procedural hose instructions depicted; gauges are uncalibrated context. Separate exact panel: An installed refrigerant stock transfers to recovered storage, then to reclamation or destruction; a separate conditional leakage branch reaches air.

Recovery contains refrigerant, reclamation prepares eligible material for reuse, and destruction changes the chemical. Installed legacy refrigerant banks can persist after new manufacture declines.

Refrigerant recovery transfers material; destruction is a separate process.

Recovery contains refrigerant, reclamation prepares eligible material for reuse, and destruction changes the chemical. Installed legacy refrigerant banks can persist after new manufacture declines.

Example: Comparing ODP Values

Problem: A factory uses 500 kg/yr of Halon-1301 (ODP = 10.0) for fire suppression. If it switches to HFC-227ea (ODP = 0), what is the equivalent CFC-11 reduction?

Example: Multi-Factor Analysis

Scenario: A country must choose between two CFC replacements for its refrigeration sector. Evaluate both options.

	HCFC-22	R-290 (Propane)
ODP	0.055	0
GWP (100-yr)	1,810	3
Atmospheric lifetime	12 years	12 days
Safety concern	Low toxicity	Flammable
Montreal status	Phase-out by 2030	Not restricted

Common Mistakes

Confusing ozone depletion with global warming

Ozone loss increases UV-B at the surface; it does not directly cause Earth's surface to warm. They are separate problems with different causes.

Thinking the ozone "hole" is a literal hole

It is a region of severely thinned ozone (below 220 DU), not a physical gap in the atmosphere.

Assuming all replacements are safe

HFCs solved the ozone problem but created a climate problem. Always evaluate both ODP and GWP of any replacement chemical.

Question 1 of 5

Which atmospheric layer contains the highest concentration of ozone that shields Earth from UV-B radiation?

The stratosphere (15-35 km altitude) contains the ozone layer. The troposphere is the lowest layer where weather occurs; the mesosphere and thermosphere are above the stratosphere and contain very little ozone.

Question 2 of 5

A single chlorine atom released from a CFC molecule can destroy approximately how many ozone molecules before being deactivated?

One chlorine atom can catalytically destroy approximately 100,000 ozone molecules because it is regenerated after each reaction cycle: \(\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2\), then \(\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2\).

Question 3 of 5

The Kigali Amendment to the Montreal Protocol primarily targets which group of chemicals?

The Kigali Amendment (2016) targets HFCs. While HFCs have zero ODP and don't harm the ozone layer, they are potent greenhouse gases. CFCs and halons were already phased out under the original Montreal Protocol.

Question 4 of 5

A factory uses 200 kg/yr of a substance with an ODP of 0.5. What is the CFC-11 equivalent emission?

ODP-weighted mass = \(\text{mass} \times \text{ODP} = 200 \times 0.5 = \) 100 kg CFC-11 equivalent. This means the substance destroys ozone at half the rate of CFC-11 per kilogram.

Question 5 of 5

Why does the ozone hole form specifically over Antarctica rather than uniformly around the globe?

The extreme cold of the Antarctic winter (below \(-78^\circ\text{C}\)) allows polar stratospheric clouds (PSCs) to form. These clouds provide surfaces that convert stable chlorine compounds into reactive \(\text{Cl}_2\). When spring sunlight arrives, this chlorine rapidly destroys ozone, creating the seasonal "hole."