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Preserving Earth’s Stratosphere PUBLIC ACCESS

Ozone Levels in the Arctic and Antarctic Should Begin to Recover Due to Reductions in Chlorofluorocarbon Production, but Greenhouse Warming May Exacerbate Losses of the Gas in the Polar Regions.

Mechanical Engineering 120(10), 88-91 (Oct 01, 1998) (4 pages) doi:10.1115/1.1998-OCT-5

This article discusses that ozone levels in the Arctic and Antarctic should begin to recover due to reductions in chlorofluorocarbon production, but greenhouse warming may exacerbate losses of the gas in the polar region. It is important to understand the life cycle of ozone molecules because it plays such a vital role in screening harmful ultraviolet radiation. The concentrations of certain gases, such as these highly reactive chlorine compounds, have a critical effect on ozone levels. The chlorine found in the stratosphere comes principally from chlorofluorocarbons (CFC). A CFC release becomes well mixed throughout the troposphere in about one year. The CFCs, which enter the stratosphere from the tropical upper-troposphere region, have been measured by the Cryogenic Limb Array Etalon Spectrometer on the Upper Atmosphere Research Satellite (UARS). Recent research has suggested that greenhouse warming may lead to significant cooling of the polar region. If so, this cooling may exacerbate ozone losses despite decreasing chlorine and bromine levels.

Scientists’ understanding of ozone and chlorofluorocarbons (CFCs) in the upper atmosphere has continued to evolve over the last 30 years as their knowledge of the stratosphere has increased. About 90 percent of the ozone in Earth’s atmosphere is contained in the stratosphere—the band stretching from about 10 to 50 kilometers above the Earth’s surface—while the remaining 10 percent is contained in the troposphere, the lowest layer of the atmosphere, where weather patterns are observed. While ozone is lethal when breathed at high dosage levels, it remains a critical component of Earth’s atmosphere because it absorbs harmful solar ultraviolet radiation—that is, radiation at wavelengths less than about 320 nanometers (nm). Ozone concentrations are greatest between about 15 and 30 km above the surface of the Earth.

CFCs, on the other hand, are nontoxic and have a multitude of beneficial uses in refrigerants, spray propellants, and other products. Ultimately, however, CFCs lead to ozone loss and a consequent increase in ultraviolet radiation. Since ozone is the principal factor in the screening of such radiation, it is critical to understand both the distribution of ozone in the atmosphere and the processes that control the levels of ozone.

Because of the strong absorption of solar ultraviolet by ozone in the stratosphere, it is virtually impossible for ultraviolet rays between 200 and 300 nm to penetrate to the earth’s surface. At 290 nm, the radiation is 350 million times weaker than at the top of the atmosphere. If our eyes detected light at 290 nm instead of in the visual range, the ozone would make the world very dark indeed.

Ozone’s absorption of ultraviolet rays is critical for the well-being of humankind, since this radiation is energetic enough to break the bonds of DNA molecules.

While plants and animals are generally able to repair damaged DNA, on occasion damaged DNA molecules can continue to replicate, leading to dangerous forms of skin cancer in humans. The probability that DNA can be damaged by UV varies with wavelength, shorter wavelengths being the most dangerous. Fortunately, at the wavelengths that easily damage DNA, ozone strongly absorbs UV and, at the longer wavelengths where ozone absorbs weakly, DNA damage is unlikely. But given a 10-percent decrease in ozone in the atmosphere, the amount of DNA-damaging UV would be expected to increase by about 22 percent.

UV radiation is typically broken down into three parts: UV-a (320 to 400 nm), UV-b (280 to 320 nm), and UV-c (200 to 280 nm). UV-c is quickly absorbed by small amounts of ozone, so that none gets to the Earth’s surface. UV-b is partially absorbed and about half of the UV-a is absorbed by ozone or scattered. Ozone is so effective at absorbing the extremely harmful UV-c that sunscreen manufacturers don’t need to worry about these wavelengths. UV-b radiation levels are critical, since ozone doesn’t fully absorb at these wavelengths.

The screening of UV by ozone involves other factors, such as time of day and season. The map on page 90 shows the UV radiation responsible for sunburn (erythemal exposure) during July 1988. This image is derived from the National Aeronautics and Space Administration’s satellite observations of solar UV reflected off the Earth’s atmosphere, using the Total Ozone Mapping Spectrometer (TOMS) . Intense sunburn-causing UV is shown by the red-orange colors, while weaker values are shown by the blue-purple colors. Tropical regions have very intense exposure, while high-latitude regions have rather weak exposures. Higher altitudes in the Rocky Mountains and the Himalayas have high exposures as well. In addition, clouds affect the amount of UV that is incident on the Earth’s surface.

Because ozone plays such a critical role in screening harmful ultraviolet radiation, it is important to understand the life cycle of ozone molecules. An ozone molecule’s career begins when intense ultraviolet radiation (at a wavelength less than 240 nm) breaks apart an oxygen molecule (O2) into two oxygen atoms, and these atoms re-act with other oxygen molecules to form two ozone molecules. As a chemical equation, this is represented as O2 + UV radiation → O+O < 240nm. The rate at which ozone in the lower stratosphere is formed is slow, since most of the UV is absorbed in the upper stratosphere at wavelengths less than 240 nm. If all the ozone at around 20 km was destroyed, it would take about one year to replace this destroyed ozone by solar radiation alone.

The ozone molecule spends most of its life absorbing UV. This absorption process occurs when the UV breaks the ozone (O3) into an oxygen molecule (O2) and an oxygen atom (O), followed by the recombination of the oxygen atom with another oxygen molecule to reform ozone. This is represented chemically as

O3 + UV radiation → O2, + O

O + O2 → O3

Net: UV is converted to heat.

The molecule’s life ends when it reacts with one of a variety of chemicals in the stratosphere, such as chlorine, nitrogen, bromine, or hydrogen compounds. These loss reactions typically occur in a catalytic process whereby the ozone molecule is lost, while the catalyst (the chlorine, nitrogen, bromine, or hydrogen compound) is reformed to destroy another ozone molecule. A typical reaction occurs when CIO reacts with an oxygen atom to form Cl and O2. The Cl atom then reacts with an ozone molecule to reform CIO and another O2.

CIO + O → O2+ Cl

Cl + O3 → O2+ CIO

Net: O+ O3 → 2 O2

The net effect is the formation of two oxygen molecules from an oxygen atom and an ozone molecule, while the CIO molecule is unaffected. At 40 km, this CI-ClO catalytic chain can destroy nearly 1,000 ozone molecules before the Cl or CIO is converted to an ozone benign chlorine form such as HCl or CIONO2.

The total amount of ozone is a balance between production by solar energy and loss by this CI-ClO and other catalytic reactions. If it were possible to increase the sun’s UV output at wavelengths below 240 nm, ozone levels would rise. Likewise, if the amounts of chlorine, nitrogen, bromine, or hydrogen in the stratosphere increase, the level of ozone will decrease.

The net effect is the formation of two oxygen molecules from an oxygen atom and an ozone molecule, while the CIO molecule is unaffected. At 40 km, this CI-ClO catalytic chain can destroy nearly 1,000 ozone molecules before the Cl or CIO is converted to an ozone benign chlorine form such as HCl or CIONO,.

The total amount of ozone is a balance between production by solar energy and loss by this CI-ClO and other catalytic reactions. If it were possible to increase the sun’s UV output at wavelengths below 240 nm, ozone levels would rise. Likewise, if the amounts of chlorine, nitrogen, bromine, or hydrogen in the stratosphere increase, the level of ozone will decrease.

ASME Division Focuses Energy Systems on

Since the signing of the Montreal Protocol more than 10 years ago, concerns about ozone depletion have joined cost reduction as the most significant factors driving technical innovation in refrigeration and air-conditioning applications. The three articles starting on page 88 highlight the potential dangers that ozone-producing substances pose to Earth’s stratosphere. They also report on the progress mechanical engineers are making in developing alternative refrigerants that can maintain our quality of life while minimizing damage to the conditions that make life possible.

Contributed by ASME’s Advanced Energy Systems Division, the articles reflect the division’s active role in providing the expertise needed by engineers and the public to assess the scope of this problem and to devise solutions. AESD comprises several technical committees, including Direct Thermal Power Conversion and Thermal Management, Emerging Technologies, Energy Storage and Transport Technologies, Energy System Miniaturization, Fuel Cell Power Systems, Geothermal Energy Systems, Hydrogen Technology, and Systems Analysis. As the AESD articles in this issue demonstrate, the competitiveness of companies and governments in a global economy-not to mention the comfort and well-being of all Earth’s citizens-rests on the contributions of the engineers in these fields

Keith E. Herold, Past Chair, ASME’s AES Division

In this vertical profile of ozone in the mid-latitudes of the Northern Hemisphere, the width of each bar indicates the amount of energy as a function of altitude.

Grahic Jump LocationIn this vertical profile of ozone in the mid-latitudes of the Northern Hemisphere, the width of each bar indicates the amount of energy as a function of altitude.

In this plot of ultraviolet flux versus wavelength for various altitudes, a large action spectrum value implies the existence of significant biological damage.

Grahic Jump LocationIn this plot of ultraviolet flux versus wavelength for various altitudes, a large action spectrum value implies the existence of significant biological damage.

This map plots surface erythemal uv exposure as determined by NASA’S Total Ozone Mapping Spectrometer.

Grahic Jump LocationThis map plots surface erythemal uv exposure as determined by NASA’S Total Ozone Mapping Spectrometer.

The concentrations of certain gases, such as these highly reactive chlorine compounds, have a critical effect on ozone levels. The chlorine found in the stratosphere comes principally from chlorofluorocarbons. Although vast amounts of chlorine are found on the earth in the form of salt (NaCl), it , fortunately, is water soluble, so this chlorine molecule is completely rained out, with none getting to the stratosphere. However, CFCs are not water soluble and are very nonreactive (this is the wonderful property that makes them nontoxic). Their stability means that CFCs can only be destroyed through photolysis by the extremely energetic UV that exists above most of the ozone layer in the upper stratosphere.

A CFC release (for example, refrigerator venting) becomes well mixed throughout the troposphere in about one year. The CFCs, which enter the stratosphere from the tropical upper-troposphere region, have been measured by the Cryogenic Limb Array Etalon Spectrometer on the Upper Atmosphere Research Satellite (UARS). Concentrations are measured in parts per trillion by volume. After entering the lower stratosphere, the CFCs may either be mixed to higher latitudes or slowly carried into the upper stratosphere, where they can be broken down by the sun’s UV radiation; CFC values accordingly decrease with higher altitude, as the molecules are broken down. It would take about a year for a CFC molecule to get to the upper stratosphere from the tropical upper troposphere, but most of the air entering the stratosphere gets recycled back into the troposphere before it reaches the upper stratosphere. It takes a few decades or more to cycle all the air in the troposphere through the upper stratosphere. This slow circulation of CFCs through the upper stratosphere means that it will take decades before all of the CFCs finally arrive at the upper stratosphere.

Once chlorine is liberated from the CFC by UV photolysis, it can catalytically destroy ozone. Eventually, the chlorine atoms react with methane to form HCl. This byproduct of CFC photolysis is measured by the Halogen Occultation Experiment (HALOE) aboard the UARS satellite. Near the bottom of the stratosphere at 16 km, relative HCl levels are quite low, while near the top of the stratosphere at 60 km the levels are quite high . The HALOE data also show that HCl values have increased from the early 1990s. This HCl increase is consistent with the CFC concentrations observed in the troposphere resulting from release of man-made CFCs into the atmosphere. These increased chlorine concentrations lead to greater ozone loss in the stratosphere.

In the life cycle of an ozone molecule, oxygen and ozone molecules are photolyzed by UV; a catalytic reaction then results in the conversion of O3, and O atoms into oxygen molecules.

Grahic Jump LocationIn the life cycle of an ozone molecule, oxygen and ozone molecules are photolyzed by UV; a catalytic reaction then results in the conversion of O3, and O atoms into oxygen molecules.

The growth of the chlorine and bromine levels in the stratosphere has produced very large losses of ozone over the polar regions, producing the famous Antarctic ozone hole. The illustration above shows the total amount of ozone in a column above the surface over Antarctica during October of various years as measured by a series of satellite instruments such as the TOMS. The top panels show ozone levels for years in the 1970s, when chlorine and bromine levels were low, while the bottom panels show recent ozone levels. Recent years have ozone levels that are less than half of what was observed previously. Losses of a similar magnitude have been observed over the Arctic during the last few years. Because of the different climates of the Arctic and Antarctic, the Arctic ozone levels are naturally higher, yet both polar regions have shown large declines over the last decade.

Polar ozone losses are caused directly by chlorine and bromine catalytic reactions. The chlorine and bromine reactive forms are exacerbated by chemical reactions that take place on the surfaces of cloud particles. Laboratory chemists realized long ago that some gases were affected by contact with the walls of reaction chambers. These “surface” reactions modified the results obtained for a pure gas reaction with another gas. In an identical fashion, it was realized that the surfaces of individual cloud particles could enable reactions to occur that otherwise would not take place in the stratosphere.

As the stratosphere cools to very cold temperatures over the Antarctic during the southern winter, these polar stratospheric clouds (PSCs) from. Forms of chlorine that do not affect ozone (for example, Hcl) can react on the surfaces of these PS Cs and produce chlorine products that can catalytically destroy ozone. These chlorine and bromine reactions are so fast that all of the ozone over Antarctica between 12 and 20 km is destroyed within a few weeks during the August- September period, culminating in extremely low values in October.

The two key ingredients for polar ozone losses are high chlorine and bromine levels and cold temperatures during the late winter. Increasing levels of CFCs and bromine compounds over the last few decades are thus the cause of the Antarctic ozone hole, since Antarctic temperatures are always cold in late winter. Arctic losses app ear to be related to these higher chlorine and bromine levels combined with colder temperatures, which have begun to appear over the Arctic during the 1990s.

Total column ozone is shown for various years in the 1970s and 1990s, as measured in Dobson units. (One Dobson unit equals .01 mm.)

Grahic Jump LocationTotal column ozone is shown for various years in the 1970s and 1990s, as measured in Dobson units. (One Dobson unit equals .01 mm.)

Concern for the health of the stratospheric ozone layer led to an international agreement in 1987 (the landmark Montreal Protocol) that restricted CFC production. This international agreement and its amendments have led to a curtailment of CFC production around the world. Ground-based measurements have shown that CFC levels have stopped increasing, and have slightly decreased in the troposphere. Stratospheric levels lag behind these tropospheric levels by a few years, but should begin to decrease within the next few years. As these chlorine and bromine levels begin to decrease, Antarctic and Arctic ozone levels should begin to recover.

While the CFC impact on the ozone layer is being dealt with, however, other effects, such as greenhouse warming, need to be studied. Recent research has suggested that greenhouse warming may lead to significant cooling of the polar regions. If so, this cooling may exacerbate ozone losses in spite of decreasing. chlorine and bromine levels.

Our ability to monitor the stratosphere, investigate its phenomena, and assess its future has dramatically improved because of investments by government, industry, and the academic community. Forecasting the future is always a tricky process, as any weather forecaster knows. But we are now largely able to determine the long-term stratospheric effects of new chemicals and technologies, and thereby heal and preserve the ozone layer for future generations.

Total column ozone is shown for the Arctic (top) and the Antarctic (bottom).

Grahic Jump LocationTotal column ozone is shown for the Arctic (top) and the Antarctic (bottom).

Copyright © 1998 by ASME
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