FAQ on Stratospheric Ozone
From TWENTY QUESTIONS AND ANSWERS ABOUT THE OZONE LAYER: 2006 UPDATE
lead author: D.W. Fahey
I Ozone in our atmosphere
Q1: What is ozone and where is it in the atmosphere?
Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule contains three atoms of oxygen and is denoted chemically as O3. Ozone is found primarily in two regions of the atmosphere. About 10% of atmospheric
ozone is in the troposphere, the region closest to Earth (from the surface to about 10-16 kilometers (6-10 miles)). The
remaining ozone (90%) resides in the stratosphere, primarily between the top of the troposphere and about 50 kilometers
(31 miles) altitude. The large amount of ozone in the stratosphere is often
referred to as the “ozone layer.”
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Q2: How is ozone formed in the atmosphere?
Ozone is formed throughout the atmosphere in multi-step chemical processes that require sunlight. In the stratosphere,
the process begins with an oxygen molecule (O2) being broken apart by ultraviolet radiation from
the Sun. In the lower atmosphere (troposphere), ozone is formed in a different set of chemical reactions
involving hydrocarbons and nitrogen-containing gases.
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Q3: Why do we care about atmospheric ozone?
Ozone in the stratosphere absorbs some of the sun’s biologically harmful ultraviolet radiation. Because of this
beneficial role, stratospheric ozone is considered “good” ozone. In contrast, excess ozone at Earth’s surface that
is formed from pollutants is considered “bad” ozone because it can be harmful to humans, plants, and animals.
The ozone that occurs naturally near the surface and in the lower atmosphere is also beneficial because ozone
helps remove pollutants from the atmosphere.
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Q4: Is total ozone uniform over the globe?
No, the total amount of ozone above the surface of Earth varies with location on time scales that range from
daily to seasonal and longer. The variations are caused by stratospheric winds and the chemical production
and destruction of ozone. Total ozone is generally lowest at the equator and highest near the poles because
of the seasonal wind patterns in the stratosphere.
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Q5: How is ozone measured in the atmosphere?
The amount of ozone in the atmosphere is measured by instruments on the ground and carried aloft on
balloons, aircraft, and satellites. Some measurements involve drawing air into an instrument that contains
a system for detecting ozone. Other measurements are based on ozone’s unique absorption of light
in the atmosphere. In that case, sunlight or laser light is carefully measured after passing through a portion
of the atmosphere containing ozone.
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II The ozone depletion process
Q6: What are the principal steps in stratospheric ozone depletion caused by human activities?
The initial step in the depletion of stratospheric ozone by human activities is the emission, at Earth’s surface, of ozone-depleting gases containing chlorine and bromine. Most of these gases accumulate in the lower atmosphere because they are unreactive and do not dissolve readily in rain or snow. Eventually, these emitted source gases are transported to the stratosphere, where they are converted to more reactive gases containing chlorine and bromine. These more reactive gases then participate in reactions that destroy ozone. Finally, when air returns to the lower atmosphere, these reactive chlorine and bromine gases are removed from Earth’s atmosphere by rain and snow.
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Q7: What emissions from human activities lead to ozone depletion?
Certain industrial processes and consumer products result in the emission of “halogen source gases” to the atmosphere. These gases bring chlorine and bromine to the stratosphere, which cause depletion of the ozone layer. For example, chlorofluorocarbons (CFCs), once used in almost all refrigeration and air conditioning systems, eventually reach the stratosphere, where they are broken apart to release ozone-depleting chlorine atoms. Other examples of human-produced ozone-depleting gases are the “halons,” which are used in fire extinguishers and contain ozone-depleting bromine atoms. The production and consumption of all principal halogen source gases by human activities are regulated worldwide under the Montreal Protocol.
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Q8: What are the reactive halogen gases that destroy stratospheric ozone?
Emissions from human activities and natural processes include large sources of chlorine- and bromine-containing gases that eventually reach the stratosphere. When exposed to ultraviolet radiation from the Sun, these halogen source gases are converted to more reactive gases also containing chlorine and bromine. Important examples of the reactive gases that destroy stratospheric ozone are chlorine monoxide (ClO) and bromine monoxide (BrO). These reactive gases participate in “catalytic” reaction cycles that efficiently destroy ozone. Volcanoes can emit some chlorine-containing gases, but these gases are ones that readily dissolve in rainwater and ice and are usually “washed out” of the atmosphere before they can reach the stratosphere.
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Q9: What are the chlorine and bromine reactions that destroy stratospheric ozone?
Reactive gases containing chlorine and bromine destroy stratospheric ozone in “catalytic” cycles made up of two or more separate reactions. As a result, a single chlorine or bromine atom can destroy many hundreds of ozone molecules before it reacts with another gas, breaking the cycle. In this way, a small amount of reactive chlorine or bromine has a large impact on the ozone layer. Certain ozone destruction reactions become most effective in polar regions because the reactive gas chlorine monoxide reaches very high levels there in the late winter/early spring season.
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Q10: Why has an “ozone hole” appeared over Antarctica when ozone-depleting gases are
present throughout the stratosphere?
Ozone-depleting gases are present throughout the stratospheric ozone layer because they are transported great distances by atmospheric air motions. The severe depletion of the Antarctic ozone layer known as the “ozone hole” occurs because of the special weather conditions that exist there and nowhere else on the globe. The very low temperatures of the Antarctic stratosphere create ice clouds called polar stratospheric clouds (PSCs). Special reactions that occur on PSCs and the relative isolation of polar stratospheric air allow chlorine and bromine reactions to produce the ozone hole in Antarctic springtime.
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III Stratospheric ozone depletion
Q11: How severe is the depletion of the Antarctic ozone layer?
Severe depletion of the Antarctic ozone layer was first observed in the early 1980s. Antarctic ozone depletion is seasonal, occurring primarily in late winter and early spring (August-November). Peak depletion occurs in early October when ozone is often completely destroyed over a range of altitudes, reducing overhead total ozone by as much as two-thirds at some locations. This severe depletion creates the “ozone hole” in images of Antarctic total ozone made from space. In most years the maximum area of the ozone hole far exceeds the size of the Antarctic continent.
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Q12: Is there depletion of the Arctic ozone layer?
Yes, significant depletion of the Arctic ozone layer now occurs in some years in the late winter/early spring
period (January-April). However, the maximum depletion is less severe than that observed in the Antarctic
and is more variable from year to year. A large and recurrent “ozone hole,” as found in the Antarctic stratosphere,
does not occur in the Arctic.
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Q13: How large is the depletion of the global ozone layer?
The ozone layer has been depleted gradually since 1980 and now is about an average of 4% lower over the globe. The average depletion exceeds the natural variability of the ozone layer. The ozone loss is very small near the equator and increases with latitude toward the poles. The larger polar depletion is primarily a result of the late winter/early spring ozone destruction that occurs there each year.
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Q14: Do changes in the Sun and volcanic eruptions affect the ozone layer?
Yes, factors such as changes in solar radiation, as well as the formation of stratospheric particles after volcanic eruptions, do influence the ozone layer. However, neither factor can explain the average decreases observed in global total ozone over the last two decades. If large volcanic eruptions occur in the coming decades, ozone depletion will increase for several years after the eruption.
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IV Controlling ozone-depleting gases
Q15: Are there regulations on the production of ozone-depleting gases?
Yes, the production of ozone-depleting gases is regulated under a 1987 international agreement known as the “Montreal Protocol on Substances that Deplete the Ozone Layer” and its subsequent Amendments and Adjustments. The Protocol, now ratified by over 190 nations, establishes legally binding controls on the national production and consumption of ozone-depleting gases. Production and consumption of all principal halogen-containing gases by developed and developing nations will be significantly phased out before the middle of the 21st century.
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Q16: Has the Montreal Protocol been successful in reducing ozone-depleting gases in the atmosphere?
Yes, as a result of the Montreal Protocol, the total abundance of ozone-depleting gases in the atmosphere has begun to decrease in recent years. If the nations of the world continue to follow the provisions of the Montreal Protocol, the decrease will continue throughout the 21st century. Some individual gases, such as halons and hydrochlorofluorocarbons (HCFCs), are still increasing in the atmosphere but will begin to decrease in the next decades if compliance with the Protocol continues. Around mid-century, the effective abundance of ozone-depleting gases should fall to values that were present before the Antarctic “ozone hole” began to form in the early 1980s.
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V Implications of ozone depletion
Q17: Does depletion of the ozone layer increase ground-level ultraviolet radiation?
Yes, ultraviolet radiation at Earth’s surface increases as the amount of overhead total ozone decreases, because ozone absorbs ultraviolet radiation from the Sun. Measurements by ground-based instruments and estimates made using satellite data have confirmed that surface ultraviolet radiation has increased in regions where ozone depletion is observed.
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Q18: Is depletion of the ozone layer the principal cause of climate change?
No, ozone depletion itself is not the principal cause of climate change. However, because ozone absorbs solar radiation and is a greenhouse gas, ozone changes and climate change are linked in important ways. Stratospheric ozone depletion and increases in global tropospheric ozone that have occurred in recent decades both contribute to climate change. These contributions to climate change are significant but small compared with the total contribution from all other greenhouse gases. Ozone and climate change are indirectly linked because both ozone-depleting gases and substitute gases contribute to climate change.
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VI Stratospheric ozone in the future
Q19: How will recovery of the ozone layer be identified?
Scientists expect to identify the recovery of the ozone layer with detailed ozone measurements in the atmosphere and with global models of ozone amounts. Increases in global ozone and reductions in the extent and severity of the Antarctic “ozone hole” will be important factors in gauging ozone recovery. Natural variations in ozone amounts will limit how soon recovery can be detected with future ozone measurements.
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Q20: When is the ozone layer expected to recover?
Substantial recovery of the ozone layer is expected near the middle of the 21st century, assuming global compliance with the Montreal Protocol. Recovery will occur as chlorine- and bromine-containing gases that cause ozone depletion decrease in the coming decades under the provisions of the Protocol. However, the influence of changes in climate and other atmospheric parameters could accelerate or delay ozone recovery, and volcanic eruptions in the next decades could temporarily reduce ozone amounts for several years.
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