GC: Lesson - Stratospheric Ozone Depletion (Topic 9.1) đź“–

⏳ Estimated Reading/Watching Time: 11 - 13 minutes

Learning Objective

Explain the importance of stratospheric ozone to life on Earth.

 

Atmospheric Ozone

Sun shining on Earth. UV-A penetrates Ozone layer, most of UV-B absorbed by the Ozone layer, all of the UV-C is absorbed
The ozone layer protects Earth from the most harmful rays of the sun's radiation.

You know that the atmosphere is divided into five layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. In this lesson, we are going to focus on the ozone layer, which is located in the stratosphere. The temperature of the stratosphere is stable to about 20 kilometers and tends to get warmer as you increase in altitude because of the absorption of UV by the ozone layer, which is located in the stratosphere. 

The ozone layer is a protective shield that surrounds the Earth and regulates its temperature and radiation exposure. It blocks out the most harmful ultraviolet (UV) rays from the sun, which can cause skin damage and cancers. There are three types of UV rays: UV-A, UV-B, and UV-C. UV-A rays can reach the Earth's surface and contribute to skin aging and some skin cancers. UV-B rays are partly blocked by the ozone layer and can cause sunburn and skin cancers. UV-C rays have the highest energy and the most potential for harm, but they are completely stopped by the ozone layer.

Watch the short video below to learn more about the ozone layer:

Ninety percent of the ozone in the atmosphere is located in the ozone layer. The other 10% is mostly "smog" ozone, found in the troposphere. Ozone (O3) is harmful in the troposphere but beneficial in the stratosphere which leads to the saying,

“Ozone: good up high but bad nearby.”

In the stratosphere, ozone is bombarded by ultraviolet (UV) light and uses that energy input to become oxygen (O2) molecules. 

The image is a diagram illustrating the ozone formation and destruction cycle. It is divided into two sections: the top half labeled “Ozone Formation” and the bottom half labeled “Ozone Destruction.” Each section contains two chemical reactions encircled by arrows indicating a continuous cycle. The reactions are: Ozone Formation: O2​+UVB→O+O O+O2​→O3​ Ozone Destruction: O+O3​→O2​+O2​ O3​+UVB→O+O2​ The chemical species are represented by their chemical symbols, such as “O,” “O2,” and “UVB.” The formation reactions are outlined in pink, and the destruction reactions are outlined in blue. Arrows between the ovals show the directionality from one step to another. This visual representation simplifies the complex processes involved in the ozone cycle.
The amount of ozone in the atmosphere is constantly changing because the formation reactions and destruction reactions are always occurring. 

 

Essential Knowledge

The stratospheric ozone layer is important to the evolution of life on Earth and the continued health and survival of life on Earth.

 

The Ozone Hole

Whether you notice it or not, stratospheric ozone varies seasonally.  In the spring, stratospheric ozone normally decreases over the two poles, especially the South pole. Some chemicals can act as a catalyst, or speed up, the destruction reactions.  Chlorofluorocarbons (CFCs), which are used as refrigerants, are especially adept at this. When the ozone breaks down, more ultraviolet light can penetrate to Earth’s surface. Starting in 1985, ozone thinning got so extreme it created what we think of today when we hear about the ozone hole. Watch the brief video from NASA below to learn more about the ozone hole:

In 1985, the hole was the size of the United States and ozone layers were 50% lower than normal.  A second hole has now developed over the North Pole.  

 

Causes of Ozone Depletion

The image displays a circular map representing the Antarctic ozone hole, with colors indicating varying ozone concentrations. The center, over Antarctica, shows deep blues and purples for lower ozone levels, surrounded by greens, yellows, and reds for higher concentrations. Latitude and longitude lines are overlaid for reference. The period “21–30 September 2017” is indicated, and a scale from 100 to 600 Dobson units correlates the colors from blue to red, denoting the ozone measurements.
The dark blue and purple regions show the severe ozone depletion or “ozone hole” found every spring. Minimum values of total ozone inside the hole are close to 150 DU compared to values of about 350 DU in the early 1970s.

Now that we know how the ozone layer works, and a little bit about the ozone hole, let's dive into the causes of ozone depletion. Stratospheric ozone depletion is caused by anthropogenic factors, such as chlorofluorocarbons (CFCs), and natural factors, such as the melting of ice crystals in the atmosphere at the beginning of the Antarctic spring. We can’t do a lot about the melting of ice crystals, so let's concentrate on the anthropogenic factors that cause ozone depletion.

Why are certain chemicals more likely to deplete the ozone layer than others? 

Some human-made compounds release bromine or chlorine when they are exposed to UV radiation. When bromine or chlorine atoms come into contact with ozone molecules, the molecules of ozone are broken apart and, thus, lose their ability to protect us from damaging UV radiation. These ozone-damaging compounds are called Ozone Depleting Substances (ODS). Chemicals that release chlorine atoms include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, and methyl chloroform. ODS that release bromine atoms include halons and methyl bromide. Even though these substances are emitted at earth's surface, they can still make their way up to the ozone layer over several years through atmospheric processes.

These compounds are typically nonreactive, nonflammable, and nontoxic.  However, when these compounds react with ozone and ultraviolet light, a free chlorine atom is released:

CFCl3 (a CFC) + UV → Cl + CFCl2 (free chlorine atoms are released)

These chlorine atoms cause chaos when released into the atmosphere. ONE chlorine atom has the potential to destroy over 100,000 ozone molecules before it leaves the stratosphere. They react with ozone and free oxygen atoms in the atmosphere, producing more free chlorine:

Cl + O3 → ClO + O2 (chlorine atoms attack ozone reducing it to oxygen)

ClO + O → Cl + O2

These two reactions are known as the chlorine catalytic cycle because they are continuously regenerating chlorine, allowing chlorine to continually break down ozone. The image below shows the cycle of ozone destruction when chlorine (or another halogen) is introduced to the atmosphere:

The image is a colorful diagram that illustrates the ozone destruction cycle. It features several labeled molecules and atoms, including oxygen (O2), ozone (O3), and chlorine (Cl), with arrows indicating the reactions between them. The diagram shows the process where chlorine monoxide (ClO) reacts with an oxygen atom (O), resulting in the creation of a chlorine atom and two oxygen molecules, which contributes to ozone depletion. Each molecule and atom is represented by circles with their respective chemical symbols, and the colors blue, yellow, and orange are used to differentiate between different parts of the cycle. Accompanying text descriptions explain the reactions, such as “Cl + O3 → ClO + O2” and “ClO + O → Cl + O2,” with a note at the bottom stating “Note: O + O3 → 2O2” for an additional reaction. The diagram is labeled “Ozone Destruction Cycle” at the top.
The amount of ozone in the atmosphere is constantly changing because the formation reactions and destruction reactions are always occurring. 

There are two other ozone destructive cycles that occur over polar regions involving either chlorine or bromine atoms. For this course, knowing the chemical reactions of Chlorine Catalytic Cycle (above) will suffice. Being aware that any halogen gas in the atmosphere can reduce ozone to oxygen atoms is very important as oxygen allows more ultraviolet light to penetrate to Earth’s surface. 

 

Essential Knowledge

Stratospheric ozone depletion is caused by anthropogenic factors, such as chlorofluorocarbons (CFCs), and natural factors, such as the melting of ice crystals in the atmosphere at the beginning of the Antarctic spring.

 

Effects of Ozone Depletion

Ozone depletion can have wide-ranging effects because we lose our UV-B filter when the ozone layer is depleted, doubling the amount of UV-B reaching Earth's surface.

 

Environmental Effects of Ozone Depletion

A person is snorkeling in clear, blue water over a vibrant coral reef. They are wearing a dark wetsuit with a pink and black patterned front, along with a snorkel and mask. The water’s surface is reflecting sunlight, creating patterns of light and shadow.
Water doesn't completely protect aquatic organisms from UV radiation.

Marine Ecosystems

If you’ve ever been to the beach and stayed in the water all day, you know that you can still get a sunburn or sun exposure even if you are in the water. This is equivalent to what animals and plants that live in the water experience. If they are exposed to more UV than they are adapted to handle, they can die, reducing biodiversity and disrupting the food chain.

Studies have shown that phytoplankton populations decline as UV-B levels increase. Loss of phytoplankton can impact the entire aquatic food chain.

UV-B radiation has also been found to cause damage to the early developmental stages of fish, shrimp, crab, amphibians, and other marine animals.

 

Terrestrial Ecosystems

Scientists have found that UV-B can potentially impact biogeochemical cycles on land and in water, altering both the sources and sinks of greenhouse gases and other chemically important trace gases such as carbon dioxide, carbon monoxide, carbonyl sulfide, ozone, and other gases.

These potential changes could contribute to changes in biosphere-atmosphere feedback loops that determine the concentrations of greenhouse gases.

 

Because the ozone layer is so important to life on Earth, scientists have prioritized its protection since the discovery of the ozone hole in the late 1900s. Progress has been made in repairing the ozone layer, but much remains to be done. In the next lesson, we will learn what steps we've taken to protect the ozone layer.

 

Essential Knowledge

A decrease in stratospheric ozone increases the UV rays that reach the Earth's surface. Exposure to UV rays can lead to skin cancer and cataracts in humans.

 

AP Exam Tip

You NEED to know the equations in the Chlorine Catalytic Cycle. One year, students had to provide the three equations on their FRQ!

 

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