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In this lesson, you will learn the importance of the ozone layer and how it is depleted. You will also study the research and policy that has been instrumental in its recovery.

Ozone Is Beneficial

You probably don’t realize that every time you step outside, ozone, a gaseous molecule of three oxygens, makes it possible for you to do so without reaping the harmful effects of the sun’s radiation.

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This helpful ozone is found in the stratosphere, which is the atmospheric layer just above where life exists and weather occurs.When that same gaseous ozone is found in our lower layer (called the troposphere), it is considered an air pollutant and is very harmful to human health. However, we need it in the stratosphere because even at the low concentration of 12 parts per million, ozone is so effective at absorbing the sun’s UV radiation that this small amount is plenty enough to protect us on Earth.

Just how harmful is UV radiation? In humans, UV radiation causes skin cancer and cataracts. UV radiation also affects the fertility of other animals, as well as the viability of their offspring. Plants are affected by UV radiation because it affects their ability to grow and develop correctly. As you’ll see later, UV radiation also influences how chemicals break down and react, and this can lead to catastrophic changes in environments and ecosystems.

CFCs Break Down Ozone

You’ve likely heard about the ozone hole or thinning of the ozone layer, but you may not know what causes these problems. These discoveries came about through many years of scientific research, so let’s take a trip back in time to gain a better understanding of how the ozone layer has been affected by human activity.It all began in the 1960s when scientists noticed that their measurements of ozone in the atmosphere were lower than what their models predicted they should be. The scientists believed that either naturally or artificially created chemicals were somehow depleting the ozone.It turns out that these chemicals were something called chlorofluorocarbons, or CFCs. These are compounds used for refrigerants, fire extinguishers, and aerosol propellants. CFCs were invented in the 1920s and were originally thought to be environmentally friendly, but in fact, they break down ozone in the presence of UV radiation.

What happens is this: a chlorine molecule from the CFC is released from the compound by the UV radiation. It then reacts with an ozone molecule (O3), leaving a molecule of oxygen gas (O2) and a molecule of chlorine monoxide (ClO). The chlorine on this molecule is free to react with a single oxygen atom and then breaks away, leaving another O2 molecule and the free chlorine atom. This chlorine atom is now floating around the stratosphere, ready to break apart another ozone molecule into atmospheric oxygen, continuing the cycle.

Early Discoveries

Ok, so now that we know how CFCs break apart ozone, let’s go to the early 1970s where CFCs are being mass produced at an annual rate of one million metric tons and growing by 20% each year!In 1974, two scientists named Sherwood Rowland and Mario Molina were able to demonstrate how CFCs were splitting ozone molecules into atmospheric oxygen and chlorine monoxide molecules, and this work earned them the 1995 Nobel Prize in chemistry. Their research helped put new policy into motion, and by 1979, the U.S. and other countries had banned CFCs in aerosol sprays, but not for other uses.Fast forward to 1985 when a new and shocking discovery sparked action across the world.

British scientists, who had been measuring ozone concentrations at a research station in Antarctica since the 1950s, discovered that the ozone above this region had declined by as much as 60% in the previous ten years. This extremely thin layer of ozone above the region became known as the ozone hole, and over the next several years, it was confirmed that CFCs were the cause of so much thinning.

Policy to Protect Ozone

Let’s jump one more time, now to the Montreal Protocol of 1987.

When it was originally drafted, 24 different nations signed this treaty agreeing to reduce CFC production by at least 50%. Since then, five more agreements have followed up this original one with more severe cuts to CFC production and other ozone-depleting chemicals. At present, it includes almost 200 different signatory nations from all over the world.Finally, we come to the present, where the use of ozone-depleting chemicals is down 95% from what it was in the 1980s.

The ozone is slowly beginning to recover and rebuild, but a major concern is that many of the ozone-depleting chemicals that were emitted years ago have yet to react with UV and split ozone molecules in the stratosphere. As mentioned before, CFCs take a very long time to break down, so there are many chlorine molecules still floating around continuing their destructive cycle.

Lesson Summary

Ozone is quite the double-edged sword. When found at ground level atmosphere, this gas serves as a dangerous, harmful pollutant. As a molecule in Earth’s stratosphere, it protects us by absorbing harmful UV radiation from the sun that causes a variety of health problems in humans, plants, and other animals, as well as damages ecosystems.The ozone layer in the stratosphere is thin but powerful.

However, because it occurs in such a low concentration, it’s vulnerable to ozone-depleting chemicals, such as CFCs. CFCs are compounds that react with ozone in the presence of UV light by splitting the three oxygen atoms into one O2 molecule and one ClO molecule. The ClO molecule may react with a single oxygen atom, creating more atmospheric oxygen and a single chlorine atom, which is then free to start the cycle anew.It took many decades and several dedicated scientists, such as Rowland and Molina, to describe the harmful effects of CFCs on the ozone layer. The Montreal Protocol was originally signed by 24 different countries in 1987, and now almost 200 nations have vowed to reduce their CFC production and use in order to curb this global problem.

Because of this and other legislation that followed, the production and use of ozone-depleting chemicals has been reduced by 95%. However, because CFCs may take a thousand years or more to break down, the recovery of the ozone layer may be a long, slow process.

Learning Outcome

Following this lesson, you’ll be able to:

  • Explain when ozone is helpful and when it is harmful
  • Describe how CFCs react with ozone and the effects that this reaction causes
  • Summarize the research findings on the ozone layer, beginning in the 1970s
  • Identify the importance of the Montreal Protocol as it was originally signed and as it stands today

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