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Ozone

Ozone


Ozone is a gas found in the atmosphere in very trace amounts. Depending on where it is located, ozone can be beneficial ("good ozone") or detrimental ("bad ozone"). On average, every ten million air molecules contains only about three molecules of ozone. Indeed, if all the ozone in the atmosphere were collected in a layer at Earth's surface, that layer would only have the thickness of three dimes. But despite its scarcity, ozone plays very significant roles in the atmosphere. In fact, ozone frequently "makes headlines" in the newspapers because its roles are of importance to humans and other life on Earth.


What Is Ozone?

Chemically, the ozone molecule consists of three atoms of oxygen arranged in the shape of a wide V. Its formula is O3 (the more familiar form of oxygen that one breathes has only two atoms of oxygen and a chemical formula of O2). Gaseous ozone is bluish in color and has a pungent, distinctive smell. In fact, the name ozone is derived from the Greek word ozein, meaning "to smell or reek." The smell of ozone can often be noticed near electrical transformers or nearby lightning strikes. It is formed in these instances when an electrical discharge breaks an oxygen molecule (O2) into free oxygen atoms (O), which then combine with O2 in the air to make O3. In addition to its roles in the atmosphere, ozone is a chemically reactive oxidizing agent that is used as an air purifier, a water sterilizer, and a bleaching agent.

Where Is Ozone Found in the Atmosphere?

Ozone is mainly found in the two regions of the atmosphere that are closest to the earth's surface. About 10 percent of the atmosphere's ozone is in the lowest-lying atmospheric region, the troposphere. This ozone is formed in a series of chemical reactions that involve the interaction of nitrogen oxides, volatile organic compounds, and sunlight. Most ozone (about 90%) resides in the next atmospheric layer, the stratosphere. The stratosphere begins between 8 and 18 kilometers (5 and 11 miles) above the earth's surface and extends up to about 50 kilometers (30 miles). The ozone in this region is commonly known as the ozone layer. Stratospheric ozone is formed when the sun's ultraviolet (UV) radiation breaks apart molecular oxygen (O2) to form O atoms, which then combine with O2 to make ozone. Note that this formation mechanism differs from the one mentioned above for ozone in the lower atmosphere.


What Roles Does Ozone Play in the Atmosphere and How Are Humans Affected?

The ozone molecules in the stratosphere and the troposphere are chemically identical. However, they have very different roles in the atmosphere and very different effects on humans and other living beings, depending on their location.

A useful statement summarizing ozone's different effects is that it is "good up high, bad nearby." In the upper atmosphere, stratospheric ozone plays a beneficial role by absorbing most of the sun's biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually defines the stratosphere (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the earth's atmosphere. Without the filtering action of the ozone layer, more of the sun's UV-B radiation would penetrate the atmosphere and reach the earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown that excessive exposure to UV-B radiation has harmful effects. Serious long-term effects can include skin cancers and eye damage. The UV-absorbing role of stratospheric ozone is what lies behind the expression that ozone is "good up high."

In the troposphere, ozone comes into direct contact with life-forms. Although some amount of ozone is naturally present in the lower atmosphere, excessive amounts of this lower-atmospheric ozone are undesirable (or bad ozone). This is because ozone reacts strongly with other molecules, including molecules that make up the tissues of plants and animals. Several studies have documented the harmful effects of excessive ozone on crop production, forest growth, and human health. For example, people with asthma are particularly vulnerable to the adverse effects of ozone. Thus, ozone is "bad nearby."


What Are the Environmental Issues Associated with Ozone?

The dual role of ozone links it to two separate environmental issues often seen in the newspaper headlines. One issue relates to increases in ozone in the troposphere (the bad ozone mentioned above). Human activities that add nitrogen oxides and volatile organic compounds to that atmosphere, such as the fossil fuel burning associated with power-generating plants and vehicular exhaust, are contributing to the formation of larger amounts of ozone near the earth's surface. This ozone is a key component of photochemical smog, a familiar problem in the atmosphere of many cities around the world. Higher amounts of surface-level ozone are increasingly being observed in rural areas as well. Thus, the environmental issue is that human activities can lead to more of the bad ozone.

The second environmental issue relates to the loss of ozone in the stratosphere. Ground-based and satellite instruments have measured decreases in the amount of stratospheric ozone in our atmosphere, which is called ozone-layer depletion. The most extreme case occurs over some parts of Antarctica, where up to 60 percent of the total overhead amount of ozone (known as the column ozone) disappears during some periods of the Antarctic spring (September through November). This phenomenon, which has been occurring only since the early 1980s, is known as the Antarctic ozone hole. In the arctic polar regions, similar processes occur that have also led to significant chemical depletion of the column ozone during late winter and spring in many recent years. Arctic ozone loss from January through late March has been typically 20 to 25 percent, and shorter-period losses have been higher, depending on the meteorological conditions encountered in the Arctic stratosphere. Smaller, but nevertheless significant, stratospheric ozone decreases have been seen at other, more populated latitudes of the earth, away from the polar regions. Instruments on satellites and on the ground have detected higher amounts of UV-B radiation at the earth's surface below areas of depleted ozone.


What Human Activities Affect the Stratospheric Ozone Layer?

Initially, theories about the cause of ozone-layer depletion abounded. Many factors were suggested, from the sun to air motions to human activity. In the 1970s and 1980s, the scientific evidence showed conclusively that human-produced chemicals are responsible for the observed depletions of the ozone layer. The ozone-depleting compounds contain various combinations of carbon with the chemical elements chlorine, fluorine, bromine, and hydrogen (the halogen family in the periodic table of the elements). These are often described by the general term halocarbons. The compounds include chlorofluorocarbons (CFCs which are used as refrigerants, foam-blowing agents, electronics cleaners, and industrial solvents) as well as halons (which are used in fire extinguishers). The compounds are useful and benign in the troposphere, but when they eventually reach the stratosphere, they are broken apart by the sun's ultraviolet radiation. The chlorine and bromine atoms released from these compounds are responsible for the breakdown of stratospheric ozone. The ozone destruction cycles are catalytic, meaning that the chlorine or bromine atom enters the cycle, destroys ozone, and exits the cycle unscathed and therefore able to destroy another ozone molecule. In fact, an individual chlorine atom can destroy as many as 10,000 different ozone molecules before the chlorine atom is removed from the stratosphere by other reactions.


What Actions Have Been Taken to Protect the Ozone Layer?

Research on ozone depletion advanced very rapidly in the 1970s and 1980s, leading to the identification of CFCs and other halocarbons as the cause. Governments and industry acted quickly on the scientific information. Through a 1987 international agreement known as the Montréal Protocol on Substances That Deplete the Ozone Layer, governments decided to eventually discontinue production of CFCs (known in the United States by the industry trade name "Freons"), halons, and other halocarbons (except for a few special uses). Concurrently, industry developed more ozone-friendly substitutes for the CFCs and other ozone-depleting halocarbons. If nations adhere to international agreements, the ozone layer is expected to recover by the year 2050. The interaction of science in identifying the problem, technology in developing alternatives, and governments in devising new policies is thus an environmental "success story in the making." Indeed, the Montréal Protocol serves as a model for other environmental issues now facing the global community.


What Actions Have Been Taken to Reduce the Amount of Ozone at Ground Level?

Ozone pollution at the earth's surface is formed within the atmosphere by the interaction of sunlight with chemical precursor compounds (or starting ingredients): the nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the United States, the efforts of the Environmental Protection Agency (EPA) to reduce ozone pollution are therefore focused on reducing the emissions of the precursor compounds. VOCs, a primary focus of many regulations, arise from the combustion of fossil fuel and from natural sources (emissions from forests). Increasingly, attention is turning to reducing the emissions of NOx compounds, which also arise from the combustion of fossil fuels. The use of cleaner fuels and more efficient vehicles has caused a reduction in the emission of ozone precursors in urban areas. This has led to a steady decline in the number and severity of episodes and violations of the one-hour ozone standard established by the U.S. Environmental Protection Agency (EPA) (which is 120 parts per billion or ppb, meaning that out of a billion air molecules, 120 are ozone). In 1999 there were thirty-two areas of the country that were in violation of the ozone standard, down from 101 just nine years earlier. Despite these improvements, ground-level ozone continues to be one of the most difficult pollutants to manage. An additional, more stringent ozone standard proposed by the EPA to protect public health, eighty ppb averaged over eight hours, was cleared in early 2001 for implementation in the United States. For comparison, Canada's standard is sixty-five ppb averaged over eight hours.

see also Air Pollution; Asthma; CFCs (Chlorofluorocarbons); Electric Power; Halon; MontrÉal Protocol; NOx (Nitrogen Oxides); Smog; Vehicular Pollution; Ultraviolet Radiation; VOCs (Volatile Organic Compounds).

Bibliography

World Meteorological Organization. (2003). Scientific Assessment of Ozone Depletion: 2002. Global Ozone Research and Monitoring Project, Report No. 47. Geneva: World Meteorological Organization.


internet resources

University Corporation for Atmospheric Research. "Cycles of the Earth and AtmosphereModule Review." Available from http://www.ucar.edu/learn/1.htm.

U.S. Environmental Protection Agency. "Automobiles and Ozone." Available from http://www.epa.gov/otaq/04-ozone.htm.

U.S. Environmental Protection Agency. "Ozone Depletion." Available from http://www.epa.gov/docs/ozone.

Christine A. Ennis

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Ozone

Ozone

Ozone is an allotrope (a physically or chemically different form of the same substance) of oxygen with the chemical formula O3. This formula shows that each molecule of ozone consists of three atoms. By comparison, normal atmospheric oxygenalso known as dioxygenconsists of two atoms per molecule and has the chemical formula O2.

Ozone is a bluish gas with a sharp odor that decomposes readily to produce dioxygen. Its normal boiling point is 112°C (170°F), and its freezing point is 192°C (314°F). It is more soluble in water than dioxygen and also much more reactive. Ozone occurs in the lower atmosphere in very low concentrations, but it is present in significantly higher concentrations in the upper atmosphere. The reason for this difference is that energy from the Sun causes the decomposition of oxygen molecules in the upper atmosphere:

O2 (solar energy) 2O

The nascent (single-atom) oxygen formed is very reactive. It may combine with other molecules of dioxygen to form ozone:

O + O2 O3

Words to Know

Allotropes: Forms of a chemical element with different physical and chemical properties.

Chlorofluorocarbons (CFCs): A family of chemical compounds consisting of carbon, fluorine, and chlorine.

Dioxygen: The name sometimes used for ordinary atmospheric oxygen with the chemical formula O2.

Electromagnetic radiation: A form of energy carried by waves.

Nascent oxygen: Oxygen that consists of molecules made of a single oxygen atom, O.

Ozone hole: A term invented to describe a region of very low ozone concentration above the Antarctic that appears and disappears with each austral (Southern Hemisphere) summer.

Ozone layer: A region of the stratosphere in which the concentration of ozone is relatively high.

Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.

Standard (pollution): The highest level of a harmful substance that can be present without a serious possibility of damaging plant or animal life.

Stratosphere: The region of Earth's atmosphere ranging between about 15 and 50 kilometers (9 and 30 miles) above Earth's surface.

Troposphere: The lowest layer of Earth's atmosphere, ranging to an altitude of about 15 kilometers (9 miles) above Earth's surface.

Ultraviolet radiation: A form of electromagnetic radiation with wavelengths just less than those of visible light (4 to 400 nanometers, or billionths of a meter).

Ozone layer depletion

Most of the ozone in our atmosphere is concentrated in a region of the stratosphere between 15 and 30 kilometers (9 and 18 miles) above Earth's surface. The total amount of ozone in this band is actually relatively small. If it were all transported to Earth's surface, it would form a layer no more than 3 millimeters (about 0.1 inch) thick. Yet stratospheric ozone serves an invaluable function to life on Earth.

Radiation from the Sun that reaches Earth's outer atmosphere consists of a whole range of electromagnetic radiation: cosmic rays, gamma rays, ultraviolet radiation, infrared radiation, and visible light. Various forms of radiation can have both beneficial and harmful effects. Ultraviolet radiation, for example, is known to affect the growth of certain kinds of plants, to cause eye damage in animals, to disrupt the function of DNA (the genetic material in an organism), and to cause skin cancer in humans.

Fortunately for living things on Earth, ozone molecules absorb radiation in the ultraviolet region. Thus, the ozone layer in the stratosphere protects plants and animals on Earth's surface from most of these dangerous effects.

Human effects on the ozone layer. In 1984, scientists reported that the ozone layer above the Antarctic appeared to be thinning. In fact, the amount of ozone dropped to such a low level that the term "hole" was used to describe the condition. The hole was a circular area above the Antarctic in which ozone had virtually disappeared. In succeeding years, that hole reappeared with the onset of each summer season in the Antarctic (September through December).

The potential threat to humans (and other organisms) was obvious. Increased exposure to ultraviolet radiation because of a thinner ozone layer would almost certainly mean higher rates of skin cancer. Other medical problems were also possible.

At first, scientists disagreed as to the cause of the thinning ozone layer. Eventually, however, the evidence seemed to suggest that chemicals produced and made by humans might be causing the destruction of the ozone. In particular, a group of compounds known as the chlorofluorocarbons (CFCs) were suspected. These compounds had become widely popular in the 1970s and 1980s for a number of applications, including as chemicals used in refrigeration, as propellants in aerosol sprays, as blowing agents in the manufacture of plastic foams and insulation, as drycleaning fluids, and as cleaning agents for electronic components.

One reason for the popularity of the CFCs was their stability. They normally do not break down when used on Earth's surface. In the upper atmosphere, however, the situation changes. Evidence suggests that CFCs break down to release chlorine atoms which, in turn, attack and destroy ozone molecules:

CFC solar energy Cl atoms

Cl + O3 ClO + O2

This process is especially troublesome because one of the products of the reaction, chlorine monoxide (ClO) reacts with other molecules of the same kind to generate more chlorine atoms:

ClO + ClO Cl + Cl + O2

Once CFCs get into the stratosphere and break down, therefore, a continuous supply of chlorine atoms is assured. And those chlorine atoms destroy ozone molecules.

Scientists and nonscientists alike soon became concerned about the role of CFCs in the depletion of stratospheric ozone. A movement then developed to reduce and/or ban the use of these chemicals. In 1987, a conference sponsored by the United Nations Environment Programme resulted in the so-called Montreal Protocol. The Protocol set specific time limits for the phasing out of both the production and use of CFCs. Only three years later, concern had become so great that the Protocol deadlines were actually moved up. One hundred and sixty-five nations signed this agreement. Because of the Protocol, the United States, Australia, and other developed countries have completely phased out the production of CFCs. According to the Protocol, developing nations have until the year 2010 to complete their phase out.

Ozone in the troposphere

Ozone is a classic example of a chemical that is both helpful and harmful. In the stratosphere, of course, it is essential in protecting plants and animals on Earth's surface from damage by ultraviolet radiation. But in the lower regions of the atmosphere, near Earth's surface, the story is very different.

The primary source of ozone on Earth is the internal-combustion engine. Gases released from the tailpipe of a car or truck can be oxidized in the presence of sunlight to produce ozone. Ozone itself has harmful effects on both plants and animals. In humans and other animals, the gas irritates and damages membranes of the respiratory system and eyes. It can also induce asthma. Sensitive people are affected at concentrations that commonly occur on an average city street during rush-hour traffic.

Ozone exposure also brings on substantial damage to both agricultural and wild plants. Its primary effect is to produce a distinctive injury that reduces the area of foliage on which photosynthesis can occur. (Photosynthesis is a complicated process in which plants utilize light energy to form carbohydrates and release oxygen as a by-product.) Most plants are seriously injured by a two- to four-hour exposure to high levels of ozone. But long-term exposures to even low levels of the gas can cause decreases in growth. Large differences among plants exist, with tobacco, spinach, and conifer trees being especially sensitive.

Many nations, states, and cities have now set standards for maximum permissible concentrations of ozone in their air. At the present time in the United States, the standard is 120 ppb (parts per billion). That number had been raised from 80 ppb in 1979 because many urban areas could not meet the lower standard. Areas in which ozone pollution is most severesuch as Los Angeles, Californiacannot meet even the higher standard. Measurements of 500 ppb for periods of one hour in Los Angeles are not uncommon.

Long-term problem

Despite a relatively rapid and effective international response to CFC emissions, the recovery of the ozone layer may take up to 50 years or more. This is because these chemicals are very persistent in the environment: CFCs already present will also be around for many decades. Moreover, there will continue to be substantial emissions of CFCs for years after their manufacture, and uses are banned because older CFC-containing equipment and products already in use continue to release these chemicals.

In studies released in late 2000, scientists said they were stunned by findings that up to 70 percent of the ozone layer over the North Pole has been lost and that the ozone hole over the South Pole grew to an expanse larger than North America. According to the National Oceanic and Atmospheric Administration (NOAA), the hole in the ozone layer over the South Pole expanded to a record 17.1 million square miles (44.3 million square kilometers).

Scientists blamed the record ozone holes on two main reasons: extreme cold and the continued use of bromine. Recent very cold winters in the two poles have slowed the recovery of the ozone layer. Cold air slows the dissipation and decay of CFCs, which allows them to destroy ozone faster. Bromine is a chemical cousin to chlorine and is used for some of the same purposesfire fighting, infection control, and sanitation. NOAA believes bromine is 45 times more damaging to ozone in the atmosphere than chlorine. But bromine has not been regulated as strictly as chlorine because countries could not stand the loss of income if it were regulated more. Some scientists, however, believe governments will be under growing pressure in the coming years to limit the chemical.

[See also Greenhouse effect ]

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Ozone

Ozone


Earth's ozone layer plays a critical role in protecting Earth's surface from the Sun's harmful ultraviolet (UV) radiation. Every ozone molecule, which consists of three oxygen atoms, has the ability to absorb a certain amount of UV radiation. Under normal circumstances, the ozone layer, which is located in the stratosphere between 15 and 50 kilometers (9 and 31 miles) above Earth, remains in a continuous balance between natural processes that both produce and destroy ozone.

Ozone is produced in the upper atmosphere through a two-step chemical process that involves oxygen and UV radiation.

O + UV radiation O + O

O + O2 O3

The process begins with UV radiation breaking apart molecular oxygen (O2), thus producing two oxygen (O) atoms. In the second step, an oxygen atom (O) recombines with an oxygen molecule (O2) to form an ozone (O3) molecule.

Ozone can also be naturally destroyed through reactions with chlorine, nitrogen, and hydrogen. For example, chlorine can be a very effective destroyer of ozone via the following set of reactions.

Cl + O3 ClO + O2

ClO + O Cl + O2

In this process, a chlorine atom (Cl) reacts with ozone (O3) to produce chlorine monoxide (ClO) and an oxygen molecule (O2). ClO can then combine with an oxygen atom (O) to reform Cl and O2. In this reaction set, because chlorine is reformed after destroying ozone, the cycle can repeat itself very quickly.

In recent years global chlorine levels have increased due to the use of chlorofluorocarbons (CFCs) , a large class of chemicals useful in a variety of industries. Under certain circumstances, even a single chlorine atom released from a CFC's molecule can destroy many thousands of ozone molecules through a chemical chain reaction. Current declines in global ozone levels and the development of the Antarctic ozone hole have both been linked to CFC use.

Although ozone concentrations in the upper atmosphere play an important role in protecting Earth's surface from harmful UV radiation, ozone at its surface is a pollutant harmful to human health. Enhanced levels of surface ozone are often the result of automobile exhaust and pose a serious health risk. Fortunately, current levels of surface ozone (also known as smog) over most major cities have declined to healthier levels due in part to domestic and international governmental regulations.

see also Atmospheric Chemistry.

Eugene C. Cordero

Bibliography

Graedel, T. E., and Crutzen, Paul J. (1993). Atmospheric Change: An Earth System Perspective. New York: W. H. Freeman.

Internet Resources

Stratospheric Ozone: An Electronic Textbook. Available from <http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm>.

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ozone

ozone (ō´zōn), an allotropic form of the chemical element oxygen (see allotropy). Pure ozone is an unstable, faintly bluish gas with a characteristic fresh, penetrating odor. The gas has a density of 2.144 grams per liter at STP. Below its boiling point (-112°C) ozone is a dark blue liquid; below its melting point (-193°C) it is a blue-black crystalline solid. Ozone is triatomic oxygen, O3, and has a molecular weight of 47.9982 atomic mass units (amu). It is the most chemically active form of oxygen. It is formed in the ozone layer of the stratosphere by the action of solar ultraviolet light on oxygen. Although it is present in this layer only to an extent of about 10 parts per million, ozone is important because its formation prevents most ultraviolet and other high-energy radiation, which is harmful to life, from penetrating to the earth's surface. Ultraviolet light is absorbed when its strikes an ozone molecule; the molecule is split into atomic and diatomic oxygen: 03+ ultraviolet light →0+02. Later, in the presence of a catalyst, the atomic and diatomic oxygen reunite to form ozone. Some environmental scientists fear that certain man-made pollutants (e.g., nitric oxide, NO) may interfere with this delicate balance of reactions that maintains the ozone's concentration, possibly leading to a drastic depletion of stratospheric ozone. Ozone is also formed when an electric discharge passes through air; for example, it is formed by lightning and by some electric motors and generators. Ozone is produced commercially by passing dry air between two concentric-tube or plate electrodes connected to an alternating high voltage; this is called the silent electric discharge method. Ozone is used commercially as a disinfectant and decontaminant for air and water, and as a bleaching agent for waxes, oils, and other organic compounds. The major commercial use is in the production by ozonolysis of azelaic acid (used in making plastics); it is also used in the synthesis of cortisone and certain synthetic sex hormones. Ozonization, the reaction of ozone with the double or triple bonds of unsaturated organic molecules, is useful in determining the structure of organic compounds.

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ozone

o·zone / ˈōˌzōn/ • n. a colorless unstable toxic gas with a pungent odor and powerful oxidizing properties, formed from oxygen by electrical discharges or ultraviolet light. It differs from normal oxygen (O2) in having three atoms in its molecule (O3). ∎ short for ozone layer. ∎ inf. fresh invigorating air, esp. that blowing onto the shore from the sea. DERIVATIVES: o·zon·ic / ōˈzänik/ adj.

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ozone

ozone (oh-zohn) n. a poisonous gas containing three oxygen atoms per molecule. Ozone is a very powerful oxidizing agent. It is found in the atmosphere at very high altitudes (the ozone layer) and is responsible for absorbing a large proportion of the sun's ultraviolet radiation.

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ozone

ozone Chemically composed of three atoms of oxygen, O3. A powerful germicide, used to sterilize water and in antiseptic ice for preserving fish.

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ozone

ozone XIX. — G. ozon m Gr. ózon, n. prp. of ózein smell, rel. to odmé ODOUR; so named from its peculiar smell.

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ozone

ozoneflagstone, ragstone •Blackstone, jackstone •sandstone • capstone • hearthstone •headstone • gemstone • whetstone •hailstone • gravestone •freestone, keystone •greenstone • Wheatstone •Tinseltown • ringtone • pitchstone •millstone • whinstone • siltstone •holystone • semitone •stepping stone • coping stone •baritone • acetone • dulcitone •tritone • drystone • milestone •limestone •grindstone, rhinestone •cobblestone • gallstone • brownstone •lodestone • soapstone • duotone •microtone • bluestone • tombstone •moonstone • touchstone •bloodstone, mudstone •sunstone • ironstone • undertone •monotone • cornerstone •Silverstone • overtone •kerbstone (US curbstone) •turnstone •birthstone • flavone • endzone •cortisone • ozone

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