Stratospheric Chemistry and the Ozone Layer

Stratospheric Chemistry and the Ozone Layer
Chapter 15 Stratospheric Chemistry and the Ozone Layer A Thin Veil of Protection Copyright W. H. Freeman and Company · New York

Copyright W. H. Freeman and Company · New York
Chemistry Applied Familiar Metals: And their alloys, including mercury, gold and nickel in body studs Homogeneous Substances: Elements, compounds, atoms, molecules. Heterogeneous Mixtures: Colloids, especially emulsions in foods and lotions States of Matter: Including liquid crystals on TV and Copyright W. H. Freeman and Company · New York

Chapter 15 The ozone layer lies approximately 15—30 km above the earth’s surface and filters out harmful UV radiation from the sun. Copyright W. H. Freeman and Company · New York

The Ozone Layer An ozone hole opens over Antarctica every spring. Ozone concentration in the atmosphere is measured in Dobson units and varies with the season at any one location: Copyright W. H. Freeman and Company · New York

The Ozone Layer Atmospheric scientists have discovered that since the late 1970’s the springtime (Sept. – Nov.) ozone concentration over Antarctica has been falling. Normal ozone levels are subsequently recovered by each midsummer. Copyright W. H. Freeman and Company · New York

The Ozone Layer The total springtime loss amounted to 50% by the mid 1980’s. The geographical area affected by the loss also grew during this same period. Copyright W. H. Freeman and Company · New York

The Ozone Layer In the 1970’s scientists predicted that chlorine from synthetic chlorofluorocarbon (CFC) gases would destroy ozone, but only to a small extent. Subsequent research revealed that the ozone hole was indeed due to chlorine pollution. Use of CFC gases (refrigerants, propellants) has now been curtailed by most developed countries. Because of existing CFC gases already in the atmosphere the Antarctic ozone hole will probably continue till the middle of this century and a similar hole may develop over the Arctic region. Copyright W. H. Freeman and Company · New York

The Ozone Layer Molecules selectively absorb light. Sunlight is made of many different colors, each color representing a wave of light having a different wavelength. The wavelength of light is the distance between two successive crests along the light wave. Sunlight is not limited to the wavelength that we can see, however. Copyright W. H. Freeman and Company · New York

The Ozone Layer Substances vary greatly in their ability to absorb light of a given wavelength. Oxygen, O2, does not absorb visible light, but does absorb in the UV region. Copyright W. H. Freeman and Company · New York

The Ozone Layer O2 and O3 filter much of the ultraviolet component from sunlight. Oxygen molecules in and above the stratosphere filter all of the UV light from 120 to 220 nm from the sunlight reaching the surface of the Earth. UV light having a wavelength shorter than 120 nm is also totally filtered out by O2 and other atmospheric components, such as N2. O2 also filters out part of the UV light in the wavelength range: 220 – 240 nm. UV light in the wavelength range 220 – 320 nm is filtered out mainly by ozone molecules, O3. This ozone is located in the ozone layer of the atmosphere. Copyright W. H. Freeman and Company · New York

The Ozone Layer O2 and O3 filter much of the ultraviolet component from sunlight. Oxygen molecules in and above the stratosphere filter all of the UV light from 120 to 220 nm from the sunlight reaching the surface of the Earth. UV light having a wavelength shorter than 120 nm is also totally filtered out by O2 and other atmospheric components, such as N2. O2 also filters out part of the UV light in the wavelength range: 220 – 240 nm. Copyright W. H. Freeman and Company · New York

The Biological Consequences of Ozone Depletion
Skin cancer and cataracts in humans are related to UV exposure. UV light in the wavelength range 220 – 320 nm is filtered out mainly by ozone molecules, O3. This ozone is located in the ozone layer of the atmosphere. Note that ozone does not filter out all of the UV light in the 290 – 320 nm range. Copyright W. H. Freeman and Company · New York

The UV region of the sun’s spectrum is divided into three regions: UV-A ( nm), UV-B (280 – 320 nm), and UV-C ( nm). Oxygen and other molecules completely filter out UV-C radiation from the sunlight reaching the earth’s surface. Ozone and oxygen filter out all of the UV light in the nm range which includes part of the UV-B radiation, however ozone does not absorb light well in the nm range and much of this UV light reaches the earth’s surface. No component of the unpolluted atmosphere absorbs UV-A ( nm). This is the least damaging of the types of UV light, however. Copyright W. H. Freeman and Company · New York

The absorption of UV light by DNA molecules in the skin cells can cause two thymine bases to become covalently linked. This linkage, if not removed prior to cell division, results in a permanent mutation in the DNA sequence of the cells involved which can lead to several forms of skin cancer. The large majority of skin cancers are slow growing basal cell carcinomas which can be easily removed. Copyright W. H. Freeman and Company · New York

Less frequent are malignant melanoma cancers which can easily spread to other parts of the body if not treated. This type of cancer has been linked to short periods of very high UV exposure, particularly early in life. The lag period between first exposure and melanoma is years. Copyright W. H. Freeman and Company · New York

Sunscreen products are available that prevent UV radiation from reaching the skin. These products are not completely effective, however, and many actually lead to increased skin cancer rates since they allow very long exposures to the sun without a subsequent sunburn. A 1-2% increase in malignant skin cancer rates is predicted for each 1% decrease in the overhead ozone. Because of the lag time between exposure and symptoms, the medical effects of ozone depletion would not be apparent yet. Copyright W. H. Freeman and Company · New York

Other effects of UV radiation on human health include cataracts and suppression of the immune system. Copyright W. H. Freeman and Company · New York

Increased UV exposure affects both plants and animals. Increases in UV-B exposure interferes with the photosynthetic process in plants, leading to less leaf, seed, and fruit. Organisms living within the first 5 meters below the surface in bodies of clear water are also at risk. Phytoplankton living near the surface of seawater are at the base of the ocean’s food chain, and if significantly by increased UV exposure, would have serious consequences. The drop in amphibian populations (frogs) in increase in deformities has been linked to increased UV levels. Copyright W. H. Freeman and Company · New York

The Interplay of Light with Chemistry in the Stratosphere
Light’s energy varies with wavelength. Light behaves like a wave in many instances but when it is absorbed by matter, its energy can only be absorbed in finite packets, called photons. The energy of a photon of light is directly proportional to the frequency of the light and inversely proportional to the wavelength of the light. High energy low energy UV-C > UV-B > UV-A > visible > infrared Short wavelength long wavelength Copyright W. H. Freeman and Company · New York

Ozone is created primarily in the stratosphere. Above the stratosphere: O2 + UV-C light  O + O At any one time there are many more O2 molecules present than O atoms so an O atom is much more likely to bump into an O2 molecule than another O atom as it moves around. In this process, ozone is created: O + O2  O3 + heat The release of heat by this process results in a temperature inversion in the stratosphere which results in a very stable layer with little mixing. Copyright W. H. Freeman and Company · New York

Stratospheric ozone is continually destroyed and re-formed. Ozone absorbs UV light with wavelengths shorter than 320 nm by the process: O3 + (UV-B or UV-C light)  O2 + O The oxygen atom formed will then either encounter another O2 molecule or an O3 molecule: O2 + O  O3 + heat or O3 + O  2 O2 The first reaction continues the process for a second cycle, the second reaction removes an ozone molecule. Copyright W. H. Freeman and Company · New York

The second reaction does not usually occur, however because the collisions between ozone molecules and oxygen atoms do not usually have enough energy to allow the reaction to happen. Chapman Cycle Copyright W. H. Freeman and Company · New York

The Ozone Hole and Other Sites of Ozone Depletion
A catalytic ozone-destruction cycle operates over the South Pole in the spring. 2 Cl + 2 O3  2 ClO + 2 O3 2 ClO  ClOOCl ClOOCl  2 Cl + O2 (UV light) Note that chlorine atoms are regenerated to act again on other ozone molecules. The overall reaction is: 2 O3  3 O2 Each chlorine atom destroys about 50 O3 molecules each day during the spring. Copyright W. H. Freeman and Company · New York

About ¾ of the ozone destruction in the Antarctic hole occurs by this mechanism. A minor catalytic route involves both Cl and Br atoms. Copyright W. H. Freeman and Company · New York

Catalytically inactive chlorine can be temporarily activated. Most of the chlorine in the stratosphere exists as HCl and ClONO2. These are formed by the reactions: ClO + NO2  ClONO2 Cl + CH4  HCl + CH3 Special weather conditions in the lower stratosphere above the South Pole convert these inactive chlorine compounds into chlorine gas during the Antarctic winter. Copyright W. H. Freeman and Company · New York

During the Antarctic winter, total darkness results in very low temperatures in the stratosphere: the heat from the O + O2  O3 reaction is not released since UV light is required to produce O atoms. The very low temperatures allow ice crystals to form from the little water vapor present at that altitude. In addition, the low temperatures causes a drop in air pressure, and a vortex around the South Pole develops, similar to the rotation about a low pressure area in normal weather patterns. This vortex effectively isolates the stratosphere above Antarctica from the surrounding atmosphere, trapping the ice crystals and surrounding air. Copyright W. H. Freeman and Company · New York

The surfaces of the ice crystals are covered by a very thin layer of liquid water, in which the HCl and ClONO2 can react to form chlorine gas: HCl(g) + ClONO2  Cl2(g) + HNO3(aq) The chlorine gas accumulates during the winter months and which the sun rises in the spring, are decomposed by UV light into chlorine atoms which begin destroying ozone molecules. By early October (spring) almost all the ozone between the altitudes of 15 and 20 km has been wiped out. Copyright W. H. Freeman and Company · New York

The surfaces of the ice crystals are covered by a very thin layer of liquid water, in which the HCl and ClONO2 can react to form chlorine gas: HCl(g) + ClONO2  Cl2(g) + HNO3(aq) The chlorine gas accumulates during the winter months and which the sun rises in the spring, are decomposed by UV light into chlorine atoms which begin destroying ozone molecules. By early October (spring) almost all the ozone between the altitudes of 15 and 20 km has been wiped out. A few weeks after the vortex disappears, the destructive cycle disappears and the ozone levels rise again, but never quite to their natural levels. Copyright W. H. Freeman and Company · New York

One the vortex disappears, the ozone poor air above the South Pole mixes with the surrounding air, temporarily lowering the ozone concentration over the adjoining geographical areas. Copyright W. H. Freeman and Company · New York

Different ways of measuring the size of the Antarctic ozone hole give different results. Surface area covered by low overhead ozone. This has grown until it now covers the entire vortex. The rate of stratospheric ozone decrease has leveled off, however the length of time depletion occurs each year has increased. There has also been an increase in the vertical region over which ozone depletion occurs. Copyright W. H. Freeman and Company · New York

Stratospheric ozone depletion is increasing over the Arctic region. An ozone hole did not start to form above the Arctic region until recently. The Arctic winters are not as cold as in the Antarctic so ice crystals do not form in the stratosphere as often or for as long. The vortex also breaks up before the Arctic sunrise which allows any Cl2 formed to be returned back into its inactive forms before destroying much ozone. Springtime temperatures above the Arctic are now decreasing, however, and ozone depletion there is accelerating in the lower stratosphere. Copyright W. H. Freeman and Company · New York

The Arctic vortex was particularly cold in the winter and spring of , resulting in significant ozone losses as late as mid-April. Stratospheric chlorine concentrations have begun to fall, however springtime ozone holes over the Arctic may occur for the next decade or two. Copyright W. H. Freeman and Company · New York

Stratospheric ozone has decreased in non-polar areas. Since the 1980’s the ozone levels over non-polar areas have decreased by several percent worldwide. This depletion has been the greatest over northern mid-altitude regions and during the March-April period. It is suspected that the chlorine activation reactions occur in the non-polar regions on cold liquid droplets consisting mainly of sulfuric acid. Copyright W. H. Freeman and Company · New York

The dominant , though temporary, source of sulfuric acid in the stratosphere is volcanic eruptions. Temporary declines in the stratospheric ozone concentration can be directly correlated to the eruptions of Mt. Pinatubo in 1991 and El Chichon in 1982. Volcanic eruptions do not account for all of the ozone loss over the non-polar regions, however. Copyright W. H. Freeman and Company · New York

The Chemicals That Cause Ozone Destruction
Both natural and synthetic sources of chlorine and bromine compounds reach the stratosphere. Chlorine reaches the stratosphere naturally through upward migration of CH3Cl gas produced mainly in the sea by the reaction of NaCl with decaying vegetation. Recently, this source has been overshadowed by synthetic chlorine and bromine compounds released during production or use. Only halogen compounds without a sink in the troposphere are a serious problem. The peak chlorine concentration in the stratosphere was in 1999 and was 4 times the naturally occurring levels. Copyright W. H. Freeman and Company · New York

CFCs and carbon tetrachloride are responsible for much of the increase in atmospheric chlorine. Chlorofluorocarbons have no tropospheric sink, are not soluble in water (rain), do not react with any other molecules, and are not photo-dissociated by UV-A. They have a very long lifetime in the atmosphere and once released, slowly diffuse up into the stratosphere. Copyright W. H. Freeman and Company · New York

Copyright W. H. Freeman and Company · New York

Eventually, CFCs reach the mid-stratosphere where there is enough UV-C light to photo dissociate them: CFC-12: CF2Cl2  CF2Cl + Cl (UV-C light). The second chlorine atom is also released. Other significant CFCs are CFC-11 and carbon tetrachloride. Copyright W. H. Freeman and Company · New York

CFC replacements contain hydrogen and therefore decompose in the troposphere. The direct replacements for CFCs all contain hydrogen atoms bonded to carbon. These molecules can be decomposed in the troposphere when they are attacked by a hydroxyl radical: OH + H-C-  H2O + other products. These replacements are called HCFCs, hydrofluorochlorocarbons. A commonly used HCFC is HCFC-22 whose ozone reducing potential is only 5% that of CFC-11. Copyright W. H. Freeman and Company · New York

Substances containing only hydrogen fluorine and carbon, HFCs are the main long-term replacements for CFCs. HFC-134a, or CH2FCF3 is already being used in many applications. Copyright W. H. Freeman and Company · New York

Is it really the CFCs that are to blame? The argument that natural sources of chlorine (seawater, volcanoes) spew much more chlorine into the atmosphere that do CFCs is invalid. Natural sources emit their chlorine into the troposphere. NaCl (seawater) and HCl (volcanoes) are both water soluble and are removed by precipitation before they can diffuse up into the stratosphere. Copyright W. H. Freeman and Company · New York

Halons and methyl bromide are ozone-depleting compounds that contain bromine. Halons are bromine containing, hydrogen free carbon compounds (CF3Br and CF2BrCl). The have no tropospheric sinks and eventually rise to the stratosphere where the bromine and chlorine atoms are photochemically released. Halons are a significant fraction of the ozone destroying halogen catalysts in the stratosphere. Copyright W. H. Freeman and Company · New York

Methyl bromide, CH3Br, is released naturally from the environment and is also used as a soil fumigant . The atmospheric lifetime of CH3Br is about one year which is time for some of it to reach the stratosphere where it can be photochemically decomposed into bromine atoms. Copyright W. H. Freeman and Company · New York

International agreements on the production of CFCs and other ozone-depleting substances have been implemented. The use of CFCs in most aerosol products was banned in the late 1970’s in North America and some Scandinavian countries. International agreement on remedies for the ozone depletion culminated in the Montreal Protocol in 1987. All legal CFC production in developed countries ended in 1995 and developing countries must comply by 2040 Copyright W. H. Freeman and Company · New York

Methyl bromide has also been added to the list of ozone-depleting banned substances. This has resulted in strong opposition from US farmers because alternatives are both less effective and more costly. Copyright W. H. Freeman and Company · New York

Summarizing the Main Ideas
The ozone layer is located in the middle and lower parts of the stratosphere which lies above the troposphere (15-20 km above ground level) For the last 25 years, an ozone hole has developed over Antarctica each spring. Ozone depletion also occurs in non-polar regions, but to a lesser extent. UV light has a shorter wavelength than visible light and contains enough energy to break chemical bonds. O2 and O3 filter UV-C and some UV-B from sunlight by absorbing them. Exposure to UV-B can produce skin cancer and perhaps affect the immune system. Copyright W. H. Freeman and Company · New York

Stratospheric ozone is created when O2 molecules absorb UV-C from sunlight and dissociate into oxygen atoms. These atoms then combine with other oxygen molecules to form ozone. The release of heat as ozone is formed results in a temperature inversion in the stratosphere. Ozone molecules are destroyed when they absorb UV light or encounter an oxygen atom. They are also catalytically destroyed by atoms such as chlorine. 2 Cl + 2 O3  2 ClO + 2 O2 2 ClO  ClOOCl ClOOCl  O2 + 2 Cl (UV light) Copyright W. H. Freeman and Company · New York

Most stratospheric chlorine is in the inactive forms: HCl and ClONO2. In the Antarctic ozone hole, these molecules are converted to active chlorine atoms on the liquid surfaces of frozen particles. These particles form because of the very low temperatures at the South Pole, and because their isolation from surrounding regions by a vortex above the South Pole. In recent years much of the ozone above the south pole has been destroyed in the spring by the catalytic chlorine mechanism. The ozone hole heals later in the year. Copyright W. H. Freeman and Company · New York

A full ozone hole over the Arctic region has not occurred because the temperatures there are not as cold and the vortex breaks up before much ozone has been destroyed. Partial ozone depletion above the Arctic has been observed in recent years. In non-polar regions, ozone losses may be due to the presence of cold liquid H2SO4 droplets which are thought to catalyze the release of chlorine atoms from atmospheric HCl and ClONO2. Chlorine and bromine atoms occur naturally in the atmosphere due to methyl bromide and chloride release from the oceans. Copyright W. H. Freeman and Company · New York

Ozone depleting substances are anthropogenic chemicals that supply chlorine or bromine to the stratosphere. These include CFCs, carbon tetrachloride, methyl chloroform, HCFCs, halons and methyl bromide. The Montreal Protocol agreements have resulted in a phaseout in the production and use of most of these compounds. The chlorine concentration in the atmosphere has peaked but it will take 50 years for the concentration to fall to the point where the Antarctic ozone hole will be healed. Copyright W. H. Freeman and Company · New York

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