The Chemicals That Cause Ozone Destruction

The increase in levels of stratospheric chlorine and bromine that occurred in the last half of the twentieth century was due primarily to the release into the atmosphere of organic compounds containing chlorine and bromine that are anthropogenic, that is to say, they are man-made. These anthropogenic contributions to stratospheric halogen levels completely overshadowed the natural input. In this section, we investigate

• why the levels of chlorine and bromine increased due to the release into the air of compounds having certain characteristics,

• how international agreements were put in place to control such substances,

• the strategy underlying the formulations of compounds to replace the original halogen compounds, and the practical difficulties and controversy about phasing out methyl bromide, and

• how two practical replacements developed by green chemistry for the now-banned chemicals can be employed.

The chlorine- and bromine-containing compounds that give rise to increased levels of the halogens in the stratosphere are those that do not have a sink—i.e., a natural removal process such as dissolution in rain or oxidation by atmospheric gases—in the troposphere. After a few years of traveling in the troposphere, they begin to diffuse into the stratosphere, where eventually they undergo photochemical decomposition by UV-C from sunlight and thereupon release their halogen atoms.

The variation in the total concentration of stratospheric chlorine and bromine atoms, expressed as the equivalent of chlorine in terms of ozone destruction power, measured over the course of the last quarter-century and projected to the middle of the twenty-first century, is illustrated by the topmost curve in Figure 2-10. The peak chlorine-equivalent concentration of about 3.8 ppb that occurred in the late 1990s was almost four times as great as the "natural" level due to methyl chloride and methyl bromide releases from the sea. The Antarctic ozone hole appeared first when the chlorine concentration reached about 2 ppb (dotted horizontal line).

CFC Decomposition Increases Stratospheric Chlorine

As is clear from inspection of Figure 2-10, the recent increase in stratospheric chlorine is due primarily to the use and release of chlorofluorocarbons— compounds containing only chlorine, fluorine, and carbon, which are commonly called CFCs. In the 1980s, about 1 million tonnes (i.e., metric tons, 1000 kg each) of CFCs were released annually to the atmosphere. These compounds are nontoxic, nonflammable, nonreactive, and have useful

FIGURE 2-10 Actual and projected concentration of stratospheric chlorine versus time, showing the contributions of various gases. Note that ozone-depleting effects of bromine atoms in halons and methyl bromide have been converted to their chlorine equivalents. [Source: DuPont.)

CT"

Total chlorine

/Methyl bromide

' Halons

HCFCs

Total chlorine

HCFCs

Methyl Bromide Dupont

tetrachloride

CFCs

Chlorine level at which Antarctic ozone hole appeared

"Natural" methyl bromide

"Natural" methyl chloride

tetrachloride

CFCs

Chlorine level at which Antarctic ozone hole appeared

"Natural" methyl bromide

"Natural" methyl chloride

1979 '84 '89 '94 '99 2004 '09 '14 '19 '24 929 '34 '39 *44 '49 '54

Year-

condensation properties (making them suitable for use as coolants, for example); because of these favorable characteristics, they found a multitude of uses. Large volumes of several CFCs were manufactured commercially and employed worldwide throughout the mid-to-late 1900s. Most of the amounts produced eventually leaked from the devices in which they were originally placed and entered the atmosphere as gases.

CFCs have no tropospheric sink, so all their molecules eventually rise to the stratosphere. In contrast to intuitive expectation, this vertical transport in the atmosphere is not affected by the fact that the mass of such molecules is greater than the average molecular mass of nitrogen and oxygen in air, because the differential force of gravity is much less than that due to the constant collisions of other molecules, which randomize the directions of even heavy molecules.

The CFC molecules eventually migrate to the middle and upper parts of the stratosphere where there is sufficient unfiltered UV-C from sunlight to photochemically decompose them, thereby releasing chlorine atoms. CFCs do not absorb sunlight with wavelengths greater than 290 nm, and they generally require light of 220 nm or less for photolysis. The CFCs must rise to the mid-stratosphere before decomposing, since UV-C does not penetrate to lower altitudes. Because vertical motion in the stratosphere is slow, their atmospheric lifetimes are long. It is because of their long stratospheric lifetimes that the chlorine concentration in Figure 2-10 falls so slowly with time.

PROBLEM 2-4

Reactions of the type

OH + CF2C12-» HOF + CFC1, ate conceivable tropospheric sinks for CFCs. Can you deduce why they don't occur, given that C—F bonds are much stronger than O—F bonds?

Other Chlorine-Containing, Ozone-Depleting Substances

Another widely used carbon-chlorine compound that lacks a tropospheric sink—although some of it ends up dissolving in ocean waters—is carbon tetrachloride, CCI4, which also is photochemically decomposed in the stratosphere. Like CFCs, then, it is classified as an ozone-depleting substance (ODS). Commercially, carbon tetrachloride was used as a solvent and was an intermediate in the manufacture of several CFCs, during the production of which some was lost to the atmosphere. Its use as a dry-cleaning solvent was discontinued in most developed countries some décades ago, but until recently it has continued to be used in many other countries. Because of its relatively long atmospheric lifetime (26 years), it will continue to make a significant contribution to stratospheric chlorine for several more decades (Figure 2-10).

Methyl chloroform, CH3—CCI3, or 1,1,1'trichloroethane, was produced in large quantities and used in metal cleaning in such a way that much of it was released into the atmosphere. Although about half of it is removed from the troposphere by reaction with the hydroxyl radical, the remainder survives long enough to migrate to the stratosphere. Because its average lifetime is only five years and its production has been largely phased out, its concentration in the atmosphere has declined rapidly since the 1990s. According to Figure 2-10, the contribution of methyl chloroform to stratospheric chlorine was substantial in the 1990s but by 2010 will become negligible.

Green Chemistry: The Replacement of CFC and Hydrocarbon Blowing Agents with Carbon Dioxide in Producing Foam Polystyrene

Polystyrene is a common polymer that is used to make many everyday items. This polymer varies in appearance from a rigid solid plastic to foam polystyrene. Rigid plastic polystyrene is used in disposable silverware; audiocassette, CD, and DVD cases; and appliance casings. Foam polystyrene is utilized as insulation in coolers and houses, foam cups, meat and poultry trays, egg cartons; in some countries it is still used in fast-food containers. Globally, about 10 million tonnes of polystyrene are produced on an annual basis, with approximately half used to produce the foam form.

In order to produce foam polystyrene, the melted polymer is combined with a gas under pressure. This mixture is then extruded into an environment of lower pressure where the gas expands, leaving a foam which is about 95% gas and 5% polymer.

In the past, CFCs were employed as blowing agents for rigid plastic foams, and foam polystyrene is no exception. When these foams are crushed or they degrade, the CFCs are released into the atmosphere where they can migrate to the stratosphere and act to destroy ozone. Low-molecular-weight hydrocarbons, such as pentane, have also been used as blowing agents; although these compounds do not deplete the ozone layer, they do contribute to ground-level smog when they are emitted into the atmosphere, as we will see in Chapter 3. Low-molecular-weight hydrocarbons are also very flammable and reduce worker safety.

The search for a replacement for CFC and hydrocarbon blowing agents led the Dow Chemical Company of Midland, Michigan, to develop a process employing 100% carbon dioxide as a blowing agent for polystyrene foam sheets. For this discovery, Dow was the recipient of a Presidential Green Chemistry Challenge Award in 1996. Carbon dioxide, C02, is not flammable nor does it deplete the ozone layer. Nonetheless, we will see in Chapter 6 that it is a greenhouse gas and thus contributes to the environmental problem of global warming, so one might wonder whether we are trading one environmental problem for another. However, waste carbon dioxide from other processes (natural gas production and the preparation of ammonia) that would otherwise be emitted into the atmosphere can be captured and used as a blowing agent. In addition, we will see in Chapter 6 that CFCs not only dramatically affect the ozone layer but also are greenhouse gases significantly more potent than carbon dioxide.

Dow Chemical found an added advantage with the polystyrene foam sheets made with carbon dioxide in that they remained flexible for a much longer period of time than those made with CFCs. This results in less breakage during use and a longer shelf life. In addition, foam sheets made with CFCs had to be degassed of the CFCs prior to recycling them, while carbon dioxide rapidly escapes from the polystyrene, leaving a sheet composed of 95% air and 5% polystyrene within a few days.

CFC Replacements

Compounds such as CFCs and CC14 have no tropospheric sinks because they do not undergo any of the normal removal processes: They are not soluble in water and thus are not rained out from air; they are not attacked by the hydroxyl radical or any other atmospheric gases and so do not decompose; and they are not photochemically decomposed by either visible or UV-A light.

The compounds being implemented as the direct replacements for CFCs all contain hydrogen atoms bonded to carbon. Consequently, a majority (though not necessarily 100%) of the molecules will be removed from the troposphere by a sequence of reactions which begins with hydrogen abstraction by OH:

OH + H—C— -* H20 + C-centered free radical->

I C02 and other products eventually

Reactions of this type are discussed in more detail in Chapters 3 and 5. Because methyl chloride, methyl bromide, and methyl chloroform each contain hydrogen atoms, a fraction of such molecules are removed in the troposphere before they have a chance to rise to the stratosphere.

The temporary replacements for CFCs employed in the 1990s and the early years of the twenty-first century contain hydrogen, chlorine, fluorine, and carbon; they are called HCFCs, hydrochloro/luorocarbons. One important example is CHF2C1, the gas called HCFC-22 (or just CFC-22). It is employed in modern domestic air conditioners and in some refrigerators and freezers, and it has found some use in blowing foams such as those used in food containers. Since it contains a hydrogen atom and thus is mainly removed from air before it can rise to the stratosphere,, its long-term ozone-reducing potential is small—only 5% of that of the CFC that it replaced. This advantage is offset, however, by the fact that HCFC-22 decomposes to release chlorine more quickly than does the CFC, so its short-term potential for ozone destruction is greater than that implied by this percentage. But because most HCFC-22 is destroyed within a few decades after its release, it is responsible for almost no long-term ozone destruction. However, most concerns about stratospheric ozone destruction are centered on the next few decades, before substantial reduction of stratospheric chlorine occurs from the phase-out of CFCs. Notice the contributions of HCFCs to the curve in Figure 2-10. They should be significant only until about 2030.

Reliance exclusively on HCFCs as CFC replacements would have eventually led to a renewed buildup of stratospheric chlorine, because the volume ofHCFC consumption would presumably rise with increasing world population and affluence. Products that are entirely free of chlorine, and that therefore pose no hazard to stratospheric ozone, will be the ultimate replacements for CFCs and HCFCs.

Hydrofluorocarbons, HFCs, substances that contain hydrogen, fluorine, and carbon, are the main long-term replacements for CFCs and HCFCs. The compound CH2F—CF3, called HFC-134a, has an atmospheric lifetime of several decades before finally succumbing to OH attack. HFC-134a is now used as the working fluid in new refrigerators and air conditioners for the North American market, including those in automobiles. All HFCs eventually react to form hydrogen fluoride, HF. Unfortunately, one atmospheric degradation pathway for HFC-134a, and for several HOFOb as well, produces tri/luoroacetic acid, TFA (CF3—COOH), as an intermediate, which is then removed from the air by rainfall. Some scientists worry that TFA represents an environmental hazard to wetlands since it will accumulate in aquatic plants and could inhibit their growth. However, some of the TFA in the environment arises from the degradation under heating of polymers such as Teflon, not from CFC replacements. Polyfluorocarboxylic acids, of which the acid form of TFA is an example, have been used in certain commercial products but are now being phased out, as discussed in Chapter 12.

Another environmental concern with HFCs involves their accumulation in air after their inadvertent release during use. While present in the troposphere, before they are destroyed, HFCs contribute to global warming by enhancing the greenhouse effect, a topic discussed in detail in Chapter 6. Outside North America, industry usually uses cyclopentane or isobutane, rather than an HFC, as a refrigerant. Such hydrocarbons have a much shorter lifetime in air than HFCs. Some environmentalists hope that developing countries will follow the hydrocarbon rather than the HFC route when they start to manufacture goods requiring coolants. Fully fluorinated compounds are unsuitable replacements for CFCs because they have no tropospheric or stratospheric sinks, and if released into the air, they would contribute to global warming for very long periods of time.

Halons

Halon chemicals are bromine-containing, hydrogen-free substances such as CFjBr and CF2BrCl. Because they have no tropospheric sinks, they eventually rise to the stratosphere. There they are photochemically decomposed, with the release of atomic bromine (and chlorine, if present), which, as we have already discussed, is an efficient X catalyst for ozone destruction. Thus halons also are ozone-depleting substances. Bromine from halons will continue to account for a significant fraction of the ozone-destroying potential of stratospheric halogen catalysts for decades to come (Figure 2-10).

Halons are used in fire extinguishers. They operate to quell fires by releasing atomic bromine, which combines with the free radicals in the combustion to form inert products and less reactive free radicals. The halons release their bromine atoms even at moderately high temperatures, since their C—Br bonds are relatively weak. Since they are nontoxic and leave no residues upon evaporation, halons are very useful for fighting fires, particularly in inhabited, enclosed spaces, such as military aircraft, and those housing electronic equipment, such as computer centers. The substitution of other chemicals for halons in the testing of the extinguishers drastically reduces halon emissions to the atmosphere, since only a minority of the releases are from the fighting of actual fires. Fine sprays of water can be substituted for halons in fighting many fires.

Fluorine atoms are liberated in the stratosphere as a result of the decomposition of halons, as well as CFCs, HCFCs, and HFCs. In principle, the fluorine atoms could catalytically destroy ozone (see Problem 2-6). However, the reaction of atomic fluorine with methane and other hydrogen-containing molecules in the stratosphere is rapid and produces HF, a very stable molecule.

Because the H—F bond is much stronger than the O—H bond, the reactivation of fluorine by the attack of the hydroxyl radical on hydrogen fluoride molecules is very endothermic. Consequently, its activation energy is high and the reaction is extremely slow at atmospheric temperatures (see Box 1-2). Thus fluorine is quickly and permanently deactivated before it can destroy any significant amount of ozone.

PROBLEM 2-5

The free radical CF3O is produced during the decomposition of HCF-134a. Show the sequence of reactions by which it could destroy ozone acting as an X catalyst in a manner reminiscent of OH. (Note that it is too short-lived to actually destroy much ozone.)

PRO BUM 2-6

(a) Write the set of reactions by which atomic fluorine could operate as an X catalyst by Mechanisms I and II in the destruction of ozone, (b) An alternative to the second step of Mechanism I in the case of X = F is the reaction between FO and ozone to give atomic fluorine and two molecules of oxygen. Write out this mechanism, and deduce its overall reaction.

International Agreements That Restrict ODSs

In contrast to almost all other environmental problems, such as global warming (Chapter 7), international agreement on remedies to stratospheric ozone depletion was obtained and successfully implemented in a fairly short period of time. The use of CFCs in most aerosol products was banned in the late 1970s in North America and some Scandinavian countries. This decision was made on the basis of predictions, made by Sherwood Rowland and Mario Molina, chemists at the University of California, Irvine, concerning the effect of chlorine on the thickness of the ozone layer. There was no experimental indication of any depletion at the time of their prediction. Rowland and Molina, together with the German chemist Paul Crutzen, were jointly awarded the Nobel Prize in Chemistry in 1995 to honor their work in researching the science underlying ozone depletion.

The growing awareness of the seriousness of chlorine buildup in the atmosphere led to international agreements to phase out CFC production in the world. The breakthrough came at a conference in Montreal, Canada, in 1987 that gave rise to the Montreal Protocol; this agreement has been strengthened at several follow-up conferences. As a result of this international agreement, all ozone-depleting chemicals are now destined for phase-out in all nations. All legal CFC production in developed countries ended in 1995. Developing countries have been allowed until 2010 to reach the same goal. Figure 2-11 shows how the tropospheric concentrations of the two most

widely used CFCs have changed in recent decades. The level of CFC-11 (cfci3), the average atmospheric lifetime of which is about 50 years, peaked about 1994, seven years after its production and emission started a precipitous decline, and has dropped slowly since then; the level of CFC'12 (CF2CI7), which has a lifetime of more than 100 years, did not peak until about 2002.

The production of carbon tetrachloride and methyl chloroform has been phased out. Developed countries have agreed to end production of HCFCs by 2030, and developing countries by 2040, with no increases allowed after 2015.

Halon production was halted in developed countries in 1994 by the terms of the Montreal Protocol. However, use of existing stocks continues, as do releases from fire-fighting equipment. In addition, in the 1990s, China and Korea—which, as developing countries, have until 2010 to terminate production—increased their production of these chemicals. For these reasons, the atmospheric concentration of this chemical continued to rise.

The other bromine-containing ODS is the pesticide gas methyl bromide, CHjBr. Scientifically we do not yet have a good handle on atmospheric methyl bromide. In particular:

• Significant new sources of natural emission of the gas to the atmosphere continue to be discovered. Consequently, even the approximate ratio of synthetic to natural emissions is uncertain, as is the lifetime of about one year.

* The tropospheric concentration of the gas has changed much more since 1999 than had been anticipated by production levels and controls. Its concentration is still declining, though now at a slower rate.

Methyl bromide was added to the Montreal Protocol during the 1992 revision of the international treaty. It was agreed that developed countries would phase out methyl bromide production and importation completely in 2005. Its consumption in all developing countries combined, which amounted to less than half the U.S. usage, was to have been frozen at 1995-1998 levels in 2002, was to have been reduced by 20% in 2005, and is to be completely eliminated by 2015. However, its phase-out has been strongly resisted by some U.S. farmers, and planned reductions have been deferred. The pros and cons of implementing the Montreal Protocol controls on this controversial chemical are discussed in the online Case Study Strawberry Fields—The Banning of Methyl Bromide on the website associated with this chapter.

As a direct result of the implementation of the gradual phase-out of ozone-depleting substances, the tropospheric concentration of chlorine peaked in 1993-1994, and had declined by about 5% by 2000. Much of the initial drop was due to the phase-out of methyl chloroform, which has a short atmospheric lifetime. The concentrations of CFCs are slow to drop because they were used in many applications such as foams and cooling devices that only slowly emit them to the atmosphere. The stratospheric chlorine-equivalent level was predicted to have peaked, at less than 4 ppb, at the turn of the century, with a gradual decline predicted thereafter (see Figure 2-10). Observations in 2000 indicated that the actual chlorine content in the stratosphere had peaked, but the bromine abundance was still increasing. The slowness in the decline of the stratospheric chlorine level is due to

• the long time it takes molecules to rise to the middle or upper stratosphere and to then absorb a photon and dissociate to atomic chlorine,

• the slowness of the removal of chlorine and bromine from the stratosphere, and

• the continued input of some chlorine and bromine into the atmosphere.

Because ozone is formed (and destroyed) in rapid natural processes, its level responds very quickly to a change in stratospheric chlorine concentration. Thus the Antarctic ozone hole probably will not continue to appear after the middle of the twenty-first century, that is, once the chlorine-equivalent concentration is reduced back to the 2-ppb level it had in the years before the hole began to form (Figure 2-10). Without the Montreal Protocol agreements, catastrophic increases in chlorine, to many times the present level, would have occurred, particularly since CFC usage and atmospheric release in developing countries would have increased dramatically. A further doubling of stratospheric chlorine levels would probably have led to the formation of a substantial ozone hole each spring over the Arctic region. And with significant ozone depletion over temperate areas would have come a catastrophic increase in skin cancers.

PROBLEM 2-7

Given that their C—H bonds are not quite as strong as those in CH4, can you rationalize why ethane, C2H6, or propane, C3H8, is a better choice than methane to inactivate atomic chlorine in the stratosphere?

Chapter 2 The Ozone Holes PROBLEM 2-8

No controls on the release of CH3C1, CH2C12, or CHC13 have been proposed. What does that imply about their atmospheric lifetimes, compared to those for CFCs, CC14, and methyl chloroform?

Green Chemistry: Harpin Technology-Eliciting Nature's Own Defenses Against Diseases

Earlier in the chapter, we learned that methyl bromide is used as a pesticide (more specifically, as a soil fumigant), some of which finds its way into the stratosphere, where it becomes involved in the destruction of the ozone layer. An interesting development, which offers an alternative to methyl bromide, is known as harpin technology. This technology was developed by EDEN Bioscience Corporation in Bothell, Washington, for which it was awarded a Presidential Green Chemistry Challenge Award in 2001.

Harpin is a naturally occurring bacterial protein that was isolated from the bacteria Erwinia amylovora at Cornell University. When applied to the stems and leaves of plants, harpin elicits the plant's natural defense mechanisms to diseases caused by bacteria, viruses, nematodes, and fungi. Hypersensitive response (HR), which is induced by harpin, is an initial defense by plants to invading pathogens that results in cell death at the point of infection. The dead cells surrounding the infection act as a physical barrier to the spread of the pathogen. In addition, the dead cells may release compounds that are lethal to the pathogen.

Pests often build up immunity to pesticides. However, since harpin does not directly affect the pest, it is unlikely that immunity will occur with it. In addition to using traditional pesticides to control infestations in plants, more recently a second approach to this problem has been to develop genetically altered plants. The DMA in such plants has been altered to provide the plant with a means to ward off various pests. Although this approach is often quite successful, it is not without its critics, especially in Europe, where genetically altered plants face serious restrictions. In contrast, harpin has no effect on the plant s DNA: It simply activates defenses that are innate to the plant.

Traditional pesticides are generally made by chemists employing lengthy chemical syntheses, which invariably create large quantities of waste, which is often toxic. In addition, the compounds (chemical feedstocks) from which the pesticides are produced are derived from petroleum. A pproximately 2.7% of all petroleum is used to produce chemical feedstocks, and thus the production of these compounds is in part responsible for the depletion of this nonrenewable resource. In contrast, harpin is made from a genetically altered benign laboratory strain of the Escherichia coli bacteria through a fermentation process. After the fermentation is complete, the bacteria are destroyed and the hatpin protein is extracted. Most of the wastes are biodegradable. Thus the production of harpin produces only nontoxic biodegradable wastes and does not require petroleum.

Harpin has very low toxicity. In addition, it is applied at 0.002—0.06 kg/ acre, which represents an approximately 70% reduction in quantity when compared to conventional pesticides. Harpin is rapidly decomposed by UV light and microorganisms, which is in part responsible for its lack of contamination and buildup in soil, water, and organisms as well as the fact that it leaves no residue in foods.

An added benefit of harpin is that it also acts as a plant growth stimulant. Hatpin is thought to aid in photosynthesis and nutrient uptake, resulting in increased biomass, early flowering, and enhanced fruit yields. Harpin is sold as a 3% solution in a product called Messenger.

Continue reading here: Green Chemistry Questions

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  • bonnie benedict
    Does C2F6 have the potential to increase destruction of the ozone in the stratosphere?
    8 months ago
  • Chilimanzar Longhole
    Do flourine atoms responsible for the destruction of the ozone layer?
    2 years ago
  • alanna
    Which chemicals have the potential to increase the destruction of ozone in the stratosphere?
    2 years ago
  • tytti
    Who were the two chemists that showed how cfcs could act to destroy ozone?
    3 years ago
  • nile
    Can you deduce why they dont occur, given that cf bonds are much stronger than of bonds?
    3 years ago