AP Environmental Science

## Learning Outcomes

- Explain how stratospheric ozone is naturally formed and destroyed
- Identify the main human-caused ozone-depleting substances, especially CFCs and halons
- Describe how ultraviolet radiation breaks apart ozone-depleting substances in the stratosphere
- Explain the catalytic role of chlorine and bromine radicals in ozone destruction
- Understand why Antarctica is especially vulnerable to severe seasonal ozone loss
- Distinguish ozone depletion from climate change and tropospheric ozone pollution
- Apply this cause-and-effect chain in AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand causes of ozone depletion, with a focus on the following syllabus points:

- Know that most atmospheric ozone is found in the **stratosphere**, where it protects life by absorbing harmful ultraviolet radiation
- Understand the natural ozone balance, including ozone formation from oxygen and ultraviolet radiation and natural ozone breakdown
- Identify the main human-caused **ozone-depleting substances (ODS)**, especially **chlorofluorocarbons (CFCs)** and **halons**
- Explain why CFCs and related compounds are especially damaging: they are stable in the troposphere, persist long enough to reach the stratosphere, and then break apart under UV radiation
- Describe the chlorine catalytic cycle that destroys ozone and recognise that one chlorine atom can destroy many ozone molecules
- Understand that bromine-containing compounds also deplete ozone and are more effective per atom than chlorine
- Explain why severe ozone depletion is concentrated over Antarctica, including the roles of polar stratospheric clouds, extreme cold, the polar vortex, and returning spring sunlight
- Distinguish stratospheric ozone depletion from **global warming** and from **tropospheric ozone** as an air pollutant

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why do CFCs survive long enough to damage the ozone layer?
2. How does ultraviolet radiation turn a CFC into an ozone-destroying chemical?
3. Why is one chlorine atom able to destroy many ozone molecules rather than just one?
4. Why is Antarctica the place where ozone depletion is most severe?
5. How is stratospheric ozone depletion different from global climate change?

## Introduction

Stratospheric ozone depletion is one of the best examples in AP Environmental Science of a clear human cause producing a global atmospheric problem. The key idea is not just that some chemicals are “bad,” but that their **properties**, especially long-term stability, let them reach the stratosphere and disrupt the normal ozone cycle.

This topic often appears in questions that test mechanism. You need to move beyond “CFCs damage ozone” and explain the full chain: release of ozone-depleting substances, transport to the stratosphere, UV breakdown, release of chlorine or bromine radicals, and catalytic destruction of ozone. If you can explain that chain clearly, you are prepared for both MCQs and FRQs.

> **Key Term: stratospheric ozone depletion**
> The thinning of ozone in the stratosphere caused mainly by chemicals that release chlorine or bromine, reducing protection from ultraviolet radiation.

### The natural ozone balance and why it can be disrupted

Before you can explain ozone depletion, you need to understand that ozone is naturally unstable. The ozone layer is not a static sheet. Ozone is constantly being made and broken down in the stratosphere.

The main natural process is often called the **Chapman cycle**. It works like this:

- Ultraviolet radiation breaks oxygen molecules apart:
  - O₂ + UV → O + O
- A free oxygen atom then joins an oxygen molecule:
  - O + O₂ → O₃
- Ozone can also absorb UV and break apart:
  - O₃ + UV → O₂ + O
- A free oxygen atom can react with ozone:
  - O + O₃ → 2O₂

These reactions matter because they show that ozone is part of a balance. Ozone forms naturally and is destroyed naturally. Under normal conditions, enough ozone remains in the stratosphere to absorb most harmful UVB and nearly all UVC radiation.

> **Key Term: Chapman cycle**
> The natural set of reactions in the stratosphere that forms and breaks down ozone, creating the normal ozone balance.

What, then, is depletion? Depletion happens when another set of reactions speeds up ozone destruction faster than natural formation can replace it. Human-produced chemicals do exactly that.

A common student mistake is to think ozone is “used up” once and gone. That is too simple. The real problem is that certain atoms, especially chlorine and bromine, act as agentss. They are regenerated and can repeat the destruction cycle many times.

> **Test Tip:** If an exam question asks why ozone depletion became severe, do not stop at “because ozone broke down.” The stronger answer is “because catalytic reactions involving chlorine and bromine sped up ozone destruction beyond the natural balance.”

### Why CFCs and halons are the main human cause

The main human cause of ozone depletion is the release of **ozone-depleting substances**, especially **CFCs** and **halons**. These substances were widely used because they seemed ideal for industry: nonflammable, stable, and useful in refrigeration, aerosol sprays, solvents, and fire suppression.

That stability is exactly why they became a global problem.

In the lower atmosphere, or troposphere, many chemicals react quickly, dissolve in rain, or are removed by biological processes. CFCs do not. They are highly persistent. Because they do not react much in the troposphere, they can remain in the atmosphere for years and slowly mix upward into the stratosphere.

> **Key Term: ozone-depleting substances (ODS)**
> Human-made chemicals that release chlorine or bromine in the stratosphere and thereby destroy ozone.
>
> **Key Term: chlorofluorocarbons (CFCs)**
> Very stable human-made compounds containing carbon, chlorine, and fluorine that can reach the stratosphere and release chlorine under ultraviolet radiation.

The textbook emphasis for APES is clear: CFCs are the standard answer. Historically, they were produced on a huge scale. By the 1970s, production was extremely high, and leakage from refrigeration and air-conditioning systems was a major source.

Their long residence time means two things students should remember:

- ozone depletion can continue long after emissions are reduced
- policy action does not produce immediate full recovery

This delay is why the ozone problem did not vanish as soon as international restrictions began.

Halons and other halocarbons also matter. Halons were often used in fire extinguishers. Methyl bromide was used in fumigation and pest control. These compounds are important because they release **bromine**, which is even more effective per atom at ozone destruction than chlorine.

> **Key Term: halons**
> Bromine- or chlorine-containing compounds, often used in fire suppression, that deplete stratospheric ozone.

Some natural halogen compounds do exist. For example, natural processes such as biomass burning, volcanic activity, and marine organisms can release some halogen-containing gases. But APES questions on the main cause are centered on human-produced ODS, especially CFCs and halons. That is the safest exam answer unless a question specifically brings in another cause.

A useful distinction is this:

- **Cause at the APES level:** human release of CFCs, halons, and related ODS
- **Reason they are dangerous:** they are stable enough to survive to the stratosphere

Without that persistence, the chlorine and bromine would not reach the place where they can do the most damage.

### How UV radiation creates chlorine and bromine radicals

The next part of the chain is what happens after ODS reach the stratosphere. Here, ultraviolet radiation is more intense than in the troposphere. It can break apart the stable molecules.

This process is called **photodissociation** or photolysis.

A CFC molecule reaches the stratosphere and is hit by UV radiation. The molecule breaks apart and releases chlorine atoms. Those chlorine atoms are highly reactive.

> **Key Term: photodissociation**
> The breaking apart of a molecule by ultraviolet radiation, releasing reactive atoms or radicals.

Once chlorine is free, the core ozone-destruction cycle begins:

- Cl + O₃ → ClO + O₂
- ClO + O → Cl + O₂

Net effect:

- O₃ + O → 2O₂

This is the key reaction set for AP Environmental Science. Notice what happens: the chlorine atom appears at the start and then reappears at the end. It is regenerated.

That means chlorine is acting as a **facilitator**.

> **Key Term: driver**
> A substance that speeds up a chemical reaction and is regenerated, so it can act repeatedly.

Because chlorine is regenerated, one chlorine atom can destroy a very large number of ozone molecules. That is why a small amount of chlorine can produce a large effect.

Students often miss the difference between the intermediate and the net reaction. Chlorine does not “become part of ozone” permanently. Instead, it cycles through the reaction and keeps attacking more ozone.

Bromine does the same kind of damage through catalytic cycles. Even though bromine is present in much lower concentrations, it is more efficient per atom than chlorine. That is why halons and methyl bromide are especially damaging.

A side-by-side distinction helps:

- **Chlorine:** more abundant historically, major driver of global ozone loss
- **Bromine:** less abundant, but more effective per atom

That is why exam questions sometimes mention both.

> **Test Tip:** For FRQs, write the chlorine cycle explicitly if asked for mechanism. A response that includes “Cl is regenerated, so it acts catalytically” is stronger than one that only says “chlorine destroys ozone.”

A further detail that can earn marks in explanation questions is that not all chlorine in the stratosphere is always in its reactive form. Some exists in temporary storage compounds.

### Why Antarctica experiences the most severe ozone depletion

The Antarctic ozone hole is not caused by a different chemistry from the rest of the world. It is caused by the same chlorine- and bromine-based process, but under conditions that make that chemistry much more intense.

Three linked conditions make Antarctica especially vulnerable:

- extremely cold stratospheric temperatures
- isolation of air in the polar vortex
- return of sunlight in spring

First, the Antarctic stratosphere in winter becomes cold enough for **polar stratospheric clouds (PSCs)** to form. These clouds provide surfaces on which key reactions occur.

> **Key Term: polar stratospheric clouds (PSCs)**
> Very cold clouds that form in the polar stratosphere and convert less reactive chlorine compounds into forms that can rapidly destroy ozone.

Much of the chlorine in the stratosphere is often tied up in **reservoir species** such as HCl and ClONO₂. These forms are less reactive. They do not destroy much ozone directly.

On PSC surfaces, these reservoir compounds are converted into more photoreactive substances such as Cl₂ and HOCl. Then, when sunlight returns in Antarctic spring, these compounds are broken apart by light and release active chlorine atoms.

That produces a burst of ozone destruction.

> **Key Term: reservoir species**
> Less reactive chlorine-containing compounds, such as HCl and ClONO₂, that temporarily store chlorine until it is converted back to active forms.

Second, the **polar vortex** matters because it traps very cold polar air and reduces mixing with warmer, ozone-richer air from lower latitudes. That isolation allows reactive chlorine to build up.

Third, sunlight returns after the dark polar winter. This is the trigger. During winter, chlorine is prepared and stored in reactive forms on PSCs. In spring, light activates it. The result is rapid seasonal ozone loss.

This is why the ozone hole is most severe in Antarctic spring, not simply all year at the same rate.

The Arctic also experiences ozone depletion, but it is usually less severe than Antarctica. The reason is not that CFCs “prefer” Antarctica. It is that the Arctic has more variable temperatures and greater mixing with midlatitude air, so the conditions that intensify depletion are less extreme and less persistent.

This cause-and-effect chain is high value for APES:

1. ODS release chlorine and bromine into the stratosphere
2. Antarctic winter forms PSCs
3. PSCs convert inactive chlorine reservoirs into reactive forms
4. Polar vortex traps this air
5. Spring sunlight releases active chlorine
6. Rapid catalytic ozone destruction follows

If you can explain this clearly, you understand not just what happens, but why it happens there and then.

### Other contributing chemistry and common APES distinctions

The central APES answer remains CFCs and halons, but there is one useful refinement. **Nitrous oxide (N₂O)** can also contribute to stratospheric ozone depletion because it can form nitrogen oxides in the stratosphere, and these can also participate in catalytic ozone destruction.

However, students need to be careful here. In AP Environmental Science, the headline cause of ozone depletion is still ODS-derived chlorine and bromine. N₂O is a valid secondary contributor, but it is not usually the best first answer unless a question specifically asks about nitrogen chemistry.

That matters because students often confuse several atmospheric problems:

- **Stratospheric ozone depletion**: mainly caused by CFCs, halons, and related ODS
- **Global warming**: mainly driven by greenhouse gases such as CO₂ and CH₄
- **Tropospheric ozone pollution**: formed near Earth’s surface from NOₓ, VOCs, and sunlight

These are not interchangeable.

For example, CO₂ is a greenhouse gas, but it is not the primary cause of the ozone hole. Tropospheric ozone is a harmful pollutant, but stratospheric ozone is beneficial. Direct NOₓ from vehicles mostly matters for smog in the troposphere, not as the main APES explanation for ozone depletion.

> **Key Term: tropospheric ozone**
> Ozone in the lower atmosphere, where it acts as a pollutant and respiratory irritant rather than a protective UV shield.

A strong exam answer often includes one sharp distinction. For example:

- “Stratospheric ozone helps block UVB; tropospheric ozone harms lungs.”
- “Ozone depletion is not the same issue as climate change, even though some gases affect both.”
- “CFCs do not destroy ozone in the lower atmosphere; they must first reach the stratosphere and be broken apart by UV.”

These distinctions are often what separate a partial answer from a precise one.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Stratospheric ozone is naturally formed and destroyed in the Chapman cycle
- Ozone depletion occurs when catalytic reactions speed up ozone destruction beyond the natural balance
- The main human-caused ozone-depleting substances are CFCs and halons
- CFCs are especially damaging because they are stable in the troposphere and persist long enough to reach the stratosphere
- Ultraviolet radiation photodissociates CFCs and releases reactive chlorine atoms
- Chlorine destroys ozone catalytically through the cycle Cl + O₃ → ClO + O₂ and ClO + O → Cl + O₂
- Bromine from halons also depletes ozone and is more effective per atom than chlorine
- Some chlorine is stored in reservoir species such as HCl and ClONO₂ until converted into more reactive forms
- Polar stratospheric clouds help convert inactive chlorine into photoreactive compounds in the Antarctic stratosphere
- The Antarctic ozone hole is especially severe because of extreme cold, the polar vortex, PSC formation, and returning spring sunlight
- Stratospheric ozone depletion is different from global warming and different from tropospheric ozone pollution
- The Montreal Protocol targeted ozone-depleting substances, but recovery is slow because many ODS are long-lived

## Key Terms and Concepts

- stratospheric ozone depletion
- Chapman cycle
- ozone-depleting substances (ODS)
- chlorofluorocarbons (CFCs)
- halons
- photodissociation
- driver
- polar stratospheric clouds (PSCs)
- reservoir species
- tropospheric ozone


Stratospheric Ozone Depletion - Causes of ozone depletion