In contrast to almost all other environmental problems, such as global warm- ing/climate change (Chapters 5 and 6), international agreement on remedies to stratospheric ozone depletion was obtained and successfully implemented in a fairly short period of time. Invoking the precautionary principle to mini- mize possible harm to humans and the environment, the use of CFCs in most aerosol products was banned in the late 1970s in North America and some Scandinavian countries.
This decision was taken 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 predic- tion. 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 atmo- sphere 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. Develop- ing countries had been allowed until 2010 to reach the same goal. Figure 2-10 shows how the tropospheric concentrations of the most widely used CFCs have changed in recent decades. The level of CFC-11 (CFCl3), the average atmospheric lifetime of which is about 50 years, peaked about 1993, six years after its production started a precipitous decline. Its concentration has dropped slowly since then; the level of CFC-12 (CF2Cl2), which has a life- time of more than 100 years, did not peak until about 2002.
The production of carbon tetrachloride and methyl chloroform has been phased out. The atmospheric level of CH3CCl3 has dropped already to a small fraction of its peak in the 1990s (Figure 2-10, light green curve) but that of CCl4 has declined very slowly due to a lack of a tropospheric sink (Figure 2-10, dark green curve). Developed countries have agreed to end production of HCFCs by 2030, and developing countries by 2040, with no increases allowed after 2015. The atmospheric concentration of the widely used HCFC-22 rose during the early 2000s, but may have levelled off (Figure 2-10, black curve).
PROBLEM 2-6
The precautionary principle was discussed in Table 0-1.
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, China and Korea—
which, as developing countries, had until 2010 to terminate production—
increased their production of these chemicals in the 1990s. For these reasons, the atmospheric concentration of halons continued to rise, but seems now to have levelled off.
The other bromine-containing ODS is the pesticide gas methyl bro- mide, CH3Br. Scientifically, we do not yet have a good handle on atmo- spheric methyl bromide. In particular:
• Significant new natural emission sources of the gas to the atmosphere continue to be discovered. Consequently, even the approximate ratio of synthetic/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 currently declining, albeit slowly.
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, to have been reduced by 20% in 2005, and is to be completely eliminated by 2015. However, its phase-out has been strongly
0 200 400
100 600
Concentration in parts-per-trillion (ppt)
1985
1980 1990 1995 2000 2005 2010
Year
CFC-11 CFC-12
500
300
HCFC-22
CFC-113 CH3CCI3
CCI4 FIGURE 2-10 Tropospheric
concentrations of CFCs and other chlorine-containing ODSs. [Source: NOAA, at http://
www.esrl.noaa.gov/gmd/hats/]
The Chemicals That Cause Ozone Destruction 63
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 Case Study Strawberry Fields—
The Banning of Methyl Bromide on the website associated with this chapter.
Recently the soil fumigant methyl iodide, CH3I, was approved in the United States as a replacement for methyl bromide. Although not a threat to the ozone layer, the use of methyl iodide is very controversial because it is highly toxic and difficult to control.
As a direct result of the implementation of the gradual phase-out of ozone-depleting substances, the total tropospheric concentration of chlo- rine peaked in 1994, and had declined by about 10% by 2007. Much of the initial drop was due to the phase-out of methyl chloroform, which has a short atmospheric lifetime (Figure 2-10, light green curve); since it has now been almost eliminated, the overall rate of decline of tropospheric chlorine has slowed. The concentrations of CFCs are slow to decrease because they were used in many applications such as foams and cooling devices that have only slowly emitted them to the atmosphere, a process that continues even today.
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 2-9). 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 strat- ospheric 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 concentra- tion. Thus the Antarctic ozone hole probably will not continue to appear after the middle of the twenty-first century, that is, once the chlorine equiva- lent concentration is reduced back to the 2 ppb level it had in the years before the hole began to form (Figure 2-9). More recent projections predict the Antarctic hole area will start to decrease in about 2023, but the full recovery will not happen until about 2070.
Without the Montreal Protocol agreements, catastrophic increases in chlorine, to many times the present level, would have occurred, particu- larly 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. By 2030, the stratospheric
CaseStudies Case Studies
chlorine level probably would have reached 9 ppb, resulting in mid-latitude losses of about 10–15%. And with the increase in ozone depletion 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? ●
PROBLEM 2-8
No controls on the release of CH3Cl, CH2Cl2, or CHCl3 have been proposed.
What does that imply about their atmospheric lifetimes, compared to those
for CFCs, CCl4, and methyl chloroform? ●
2.16 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), and that some of it 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 mecha- nisms to diseases caused by bacteria, viruses, nematodes, and fungi. Hyper- sensitive 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 infec- tion. 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 to it will occur. In addition to using traditional pesticides to control the infestation of plants, more recently a second approach to this problem has been to develop geneti- cally altered plants. The DNA 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.
PROBLEM 2-7
PROBLEM 2-8
Review Questions 9–13 are based on material in the above sections.
65
Review Questions
Traditional pesticides are generally made by chemists employing lengthy chemical syntheses, which invariably create large quantities of waste, which are often toxic. In addition, the compounds (chemical feedstocks) from which the pesticides are produced are derived from petroleum. Approximately 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 harpin 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 0.0020.06 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 contami- nation and buildup in soil, water, and organisms and the fact that it leaves no residue in foods.
An added benefit of harpin is that it also acts as a plant growth stimu- lant. Harpin is thought to aid in photosynthesis and nutrient uptake, result- ing in increased biomass, early flowering, and enhanced fruit yields. Harpin is sold as a 3% solution in a product called Messenger.
Review Questions
1. What is a Dobson Unit? How is it used in relation to atmospheric ozone levels?
2. If the overhead ozone concentration at a point above the Earth’s surface is 250 DU, what is the equivalent thickness in millimeters of pure ozone at 1.0 atm pressure?
3. Describe the process by which chlorine becomes activated in the Antarctic ozone-hole phenomenon.
4. What are the steps in Mechanism II by which atomic chlorine destroys ozone in the spring over Antarctica?
5. Describe the reasons why the Antarctic ozone hole closes in late spring/early summer.
6. Explain why full-scale ozone holes have not yet been observed over the Arctic.
7. What are two effects to human health that scientists believe will result from ozone depletion?
8. Define what is meant by a tropospheric sink.
9. Explain what CFCs were and some of their uses. Did they have a tropospheric sink? Why did their emissions in air lead to an increase in stratospheric chlorine?
10. Explain what HCFCs are and state what sort of reaction provides a tropospheric sink for them. Is their destruction in the troposphere 100% complete? Why are HCFCs not considered to be suitable long-term replacements for CFCs?
11. What types of chemicals are proposed as long- term replacements for CFCs?
12. Chemically, what are halons? What was their main use?
13. What gases are being phased out according to the Montreal Protocol agreements?
Green Chemistry Questions
1. The development of carbon dioxide as a blowing agent for foam polystyrene won a Presidential Green Chemistry Challenge Award.
(a) Which of the three focus areas (see page xxviii) for these awards does this award best fit into?
(b) List two of the twelve principles of green chemistry (see pages xxiii–xxiv) that are addressed by the green chemistry of the carbon dioxide process.
2. What environmental advantages does the use of carbon dioxide as a blowing agent have over the use of CFCs and hydrocarbons?
3. Does the carbon dioxide that is used as a blowing agent contribute to global warming?
4. The development of harpin won a Presidential Green Chemistry Challenge Award.
(a) Which of the three focus areas (see page xxviii) for these awards does this award best fit into?
(b) List four of the twelve principles of green chemistry (see pages xxiii–xxiv) that are addressed by the green chemistry of the use of harpin.
5. Why is there little concern that pests will develop immunity to harpin?
6. Why is harpin not expected to accumulate in the environment?
Additional Problems
1. (a) Some authors use milliatmospheres centimeter (matm cm) rather than the equivalent Dobson Unit to express the unit for the amount of overhead ozone; 1 matm cm 1 DU. Prove that the number of moles of overhead ozone over a unit area on the Earth’s surface is proportional to the height of the layer, as specified in the definition of Dobson Units, and that 1 DU is equal to 1 matm cm.
(b) Calculate the total mass of ozone that is present in the atmosphere if the average overhead amount is 350 Dobson Units, and given that the radius of the Earth is about 6400 km. [Hints: The volume of a sphere, which you can approximate the Earth to be, is 4r3/3. You may assume that ozone behaves as an ideal gas.]
2. The chemical formula for any CFC, HCFC, or HFC can be obtained by adding 90 to its code number. The three numerals in the result represent the number of C, H, and F atoms, respectively. The number of Cl atoms can then be
determined using the condition that the number of H, F, and Cl atoms must add up to 2n 2, where n is the number of C atoms. From this information, deduce the formulas for compounds with the following codes:
(a) 12 (b) 113 (c) 123 (d) 124 3. Using the information discussed in Problem 2 above, deduce the code numbers for each of the following compounds:
(a) CH3CCl3 (b) CCl4 (c) CH3CFCl2
4. Using the information in Problem 2, show that 134 is the appropriate label for CH2FCF3. Why is an a or b designation also required to uniquely characterize the latter compound? What would be the code numbers for the HCFs in R-410a, namely CH2F2 and CHF2CF3? Does the number 410 correspond to the code number for either of these compounds?
5. The chlorine dimer mechanism is not implicated in significant ozone destruction in the
67
Additional Problems
lower stratosphere at mid-latitudes even when the particle concentration becomes enhanced by volcanoes. Deduce two reasons why this mechanism is not important under these conditions.
6. When Mechanism II for ozone destruction operates with X Cl and X Br, the radicals ClO and BrO react together to reform atomic chlorine and bromine (see Problem 2-1). A fraction of the latter process proceeds by the intermediate formation of BrCl, which undergoes photolysis in daylight. At night, however, all the bromine eventually ends up as BrCl, which does not decompose and restart the mechanism until dawn. Deduce why all the bromine exists as BrCl at night, even though only a fraction of the ClO with BrO collisions yields this product.
7. Explain what changes are observed in the UV-B intensity at ground level during ozone hole episodes.
8. What would be the advantages of using hydrocarbons rather than HFCs or HCFCs as aerosol propellant to replace CFCs? What is their major disadvantage? What type of agent should be added to aerosol cans containing hydrocarbon propellants to overcome this disadvantage and make them safer?
9. Consider the following set of compounds:
CFCl3, CHFCl2, CF3Cl, and CHF3. Assuming that equal numbers of moles of each were released into the air at ground level, rank these four compounds in terms of their potential to catalytically destroy ozone in the stratosphere.
Explain your ranking.
3
The Chemistry of Ground-Level
Air Pollution
In this chapter, the following introductory chemistry topics are used:
m Ideal gas law
m Equilibrium concept, including redox reactions and their balancing
m Acid–base theory, including pH and weak acid calculations
Background from previous chapters used in this chapter:
m Excited states
m Photon energies, UV types (A, B, C)
m Gas-phase catalysis
m Sink concept
m Temperature inversions
As one travels from city to city in various parts of the world, the most obvi- ous environmental difference among them is often the extent of their air pollution. Some cities seem pristine, while others are blanketed by a haze that restricts visibility and induces coughing and tearing. As we shall see in this chapter and the next, the chemical nature of the air pollution, the origin of its reactants and the processes they undergo, and its effect on human health all vary considerably from place to place.
Although people often think industry must be the source of most air pollution—and the generation of electric power by coal can produce signifi- cant amounts of emissions—in modern times it is often exhaust from vehicles that is the main culprit. The most manifest sign of vehicular air pollution is the black smoke emanating from the tailpipes of diesel trucks and buses. This
Introduction
this most excellent canopy, the air,
look you, this excellent o’erhanging
firmament, this magestic roof fretted
with golden fire, why, it appears
no other thing to me than a foul
and pestilent congregation of vapours
Wm. Shakespeare, Hamlet, Act II, Scene 2
sight is more common now in developing countries, since such pollution has largely been controlled in developed nations. Indeed, over the past decades, as urban populations and vehicle densities have grown rapidly in developing countries, air pollution there has dramatically worsened. In general, serious regulation of air pollution is not attempted until a country has achieved a reasonably high degree of affluence.
One of the most important features of the Earth’s atmosphere is that it is an oxidizing environment, a phenomenon due to the large concentration of diatomic oxygen, O2, which it contains. Almost all the gases that are released into the air, whether they are “natural” substances or “pollutants,” are eventually completely oxidized in the atmosphere, and the end products subse- quently deposited on the Earth’s surface. The oxidation reactions are vital to the cleansing of the air.
In this chapter, the chemistry underlying the pollution of tropospheric air is examined. As background, we begin the chapter by discussing the con- centration units by which gases in the lower atmosphere are reported, and the constitution and chemical reactivity of clean air. The effects of polluted air upon the environment and upon human health are discussed in Chapter 4.