CHAPTER 7 HOW DEEP IS THE OZONE HOLE? Questions Answered in this Chapter. Characteristics and Consequences of the Ozone Layer

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1 CHAPTER 7 HOW DEEP IS THE OZONE HOLE? The ozone hole has been in the news periodically since it was first discovered in Most scientists accept the explanations offered by atmospheric scientists for this seasonal phenomenon. As a result of appeals by these scientists, the production of chlorofluorocarbons (CFCs), compounds thought to be primarily responsible for the ozone hole, has been banned by international agreement. Has this ban worked? We explore in this chapter the nature of the evidence and the chemical reactions thought to be responsible for the production of the ozone hole as well as its future development. What are the facts about the ozone hole? Questions Answered in this Chapter 1. What is the specific meaning of the term ozone hole, and what are the conditions necessary for its formation? 2. What is the experimental evidence for the ozone hole? 3. In what ways are chlorofluorocarbons (CFCs) involved in ozone hole formation? 4. What are the major differences between the Arctic and Antarctic regions regarding ozone hole formation? 5.What progress has been made in finding CFC replacements? Characteristics and Consequences of the Ozone Layer The location of the ozone layer in the stratosphere and the proposed chemical reactions leading to the creation of the ozone layer were discussed in Chapter 3. Before studying the chemistry of the ozone hole, we need a better understanding of the importance of the ozone layer to life. The fragility of the ozone layer is shown by the following imaginary experiment. If all the ozone molecules in all parts of the atmosphere were relocated in a single uniform layer at ground level (instead of being primarily in the very low pressure stratosphere), covering the Earth so that the ozone alone had a pressure equal to atmospheric pressure, this layer would be only 3 millimeters thick (0.12 inches). It is this relatively small number of molecules that make land-based life possible by its efficient shielding of the Earth s surface against damaging ultraviolet radiation from the sun. The majority of these ozone molecules are not in the troposphere, but are instead located primarily in the very low pressure stratosphere Fragile nature of the ozone layer Chapter 7 How Deep is the Ozone Hole? Page 7-1

2 in a region given the somewhat misleading name of ozone layer. There are far more ozone molecules in the stratosphere than in the troposphere, even on the worst bad air smog day. As indicated in Chapter 3, half of the Earth s atmosphere is bathed in electromagnetic radiation emitted by the Sun. Table 7-1 indicates that O 2 and O 3 absorb sunlight in different sub-regions of the ultraviolet with different efficiencies. The combination of these two chemical compounds absorbs all of the sun s UVC and much of its UVB radiation. It is important that the majority of the UVB radiation from the Sun be absorbed before it reaches the Earth s surface because this radiation is damaging to critical DNA molecules in the body. Even though there are biochemical systems designed to repair DNA damage from UVB absorption, it is possible to overwhelm these natural repair systems to produce permanent DNA damage. UV damage is expressed as solar-induced sunburn, premature skin aging, and several types of skin cancers, one of them the difficult to cure melanoma. Cataracts (a clouding of the lens of the eye) can be caused by long term exposure to UVB radiation. UV absorption by the ozone layer Table 7-1 Regions of the Electromagnetic Spectrum Absorbed by O 2 and O 3 Wavelength Name Wavelength Subrange UV Absorption by: (nanometers) (nanometers) O 2 O 3 DNA 50 Vacuum ultraviolet } UVC complete heavy heavy 290 Ultraviolet } UVB none moderate moderate 320 } UVA none slight none Visible 700 Chapter 7 How Deep is the Ozone Hole? Page 7-2

3 The amount of UVB radiation reaching the ground is different in different parts of the world, depending on weather, time of day, season, and altitude. For example, in the US, Denver has ~10% more UVB radiation than Washington, DC. Philadelphia has four times as much UV in the summer than in the winter. All other things being equal, the further south in latitude, the higher the UV content of sunshine and the incidence of skin cancers other than melanoma show a rough correlation with the latitude. Those who are light-skinned are most vulnerable to solar-induced skin cancers. The probability of an individual contracting skin cancer increases with that individual s exposure to UV radiation from the sun. The incidence of the most deadly form of skin cancer, melanoma, is increasing faster than that of any other cancer, doubling every one or two decades in countries such as the US, UK and Australia. The exact cause of melanoma cancer is currently under debate. Some claim UVA, others claim UVB is the initiating factor. The ratio of UVA to UVB reaching the skin is about 20 to 1. Factors that determine UV exposure The dose of solar radiation depends on the angle at which sunlight enters the Earth. Early in the morning or late in the afternoon, the sunlight path through the atmosphere to the exposed individual is longer, and more UVB radiation is filtered out or is scattered by atmospheric dust than is filtered or scattered at noon. The UV content of sunlight in the summer is greatest between 10 AM and 2 PM. For every 1% reduction in atmospheric ozone concentration, the dose of UVB radiation received on the ground is increased from 2 to 4%. In 400 B.C., Greek athletes preparing for the Olympic Games rubbed themselves with olive oil and sand to prevent sunburn. The sand prevented light from burning the skin by scattering or absorbing it. In much the same way, today s sun blocks help scatter or filter the radiation received by the skin. Some of these lotions leave a film of chemical compounds on the skin that absorbs radiation in the UVB and preferably also in the UVA spectral region and that turns the absorbed energy into heat or reflects harmful radiation. There is evidence that unfiltered UVA radiation, which causes tanning, also adversely affects the human immune system and causes wrinkling and premature aging of the skin. An SPF (sun protection factor) of 15 means that the exposure dose of UVB is reduced by a factor of 15 after application of the sun blocker to the skin. In the U.S., there is a new four-star system for rating sun blocks, where the efficiency of filtering out UVA sun rays increases with the number of stars. This is based on tests of the sunscreen with living and non-living test systems. This is important since there is indirect evidence that the alarming increases in melanoma skin cancers are related to exposure to UVA rays. Repeated exposure to UVA during childhood, having red hair, and having many moles, are all thought to be correlated with increased risk of contracting melanoma. All sun blocks need to be applied liberally and often. Always read the directions! However, at present no sun block filters out the sun entirely and extended exposure to sunlight can have deadly consequences. Chapter 7 How Deep is the Ozone Hole? Page 7-3

4 Some plants and trees are susceptible to UVB radiation damage. Experiments have demonstrated that, although there are a number of UVB radiation resistant plant species, there are many plants that suffer when exposed to greater than normal amounts of UVB radiation. Soybeans and, to a lesser extent, corn and wheat, major agricultural crops, appear to be sensitive to UVB. UV damage to trees appears to be cumulative and may take many years to be observable. Figure 7-1 The natural cycle of ozone creation and destruction in the stratosphere. This is an alternative way of showing the cycle chemical equations to that shown in Chapter 3. Certain types of phytoplankton in the ocean surface are sensitive to UVB. Phytoplankton are small organisms at the bottom of the food chain in the ocean and are therefore critical to all marine life. It has been reported that biologically damaging UV penetrates the ocean as much as 20 to 30 meters during the occurrence of an ozone hole. Sensitivity of living species to UV Natural Ozone Formation and Destruction The natural ozone cycle Fig. 7-1 represents in graphical form the four reactions, introduced in Chapter 3 (equations 3-1, 3-2, 3-4 and 3-5) that explain the natural chemical cycle of stratospheric ozone production and destruction. O 2 absorbs UVC radiation to yield oxygen atoms (O). These oxygen atoms react with O 2 to produce O 3. Ozone then absorbs harmful UVB and UVC ultraviolet light, yielding molecular oxygen and electronically excited oxygen atoms (O*). These excited oxygen atoms must transfer some of their excess energy to a collision partner, M (any molecule that can carry away energy as increased vibrational and rotational motion), before the lower energy oxygen atoms can react with an oxygen molecule to again yield ozone in a cyclical process. The natural ozone cycle Chapter 7 How Deep is the Ozone Hole? Page 7-4

5 The net result of the absorption of two general classes of ultraviolet photons is the formation and destruction of ozone and the conversion of M into and excited M*, which ultimately causes a temperature increase in the region of ozone formation and destruction. Put more simply, the net result of the ozone layer formation is the conversion of harmful UV radiation into harmless thermal energy. This is why the stratosphere is warmer than the upper troposphere and this temperature difference is responsible for the temperature inversion that creates the stratosphere layer. Neither the oxygen atoms nor the ozone molecules in this cycle increase to very large concentrations in comparison with the concentration of oxygen molecules, because both O and O 3 are constantly being consumed and destroyed, resulting in relatively small, steady state concentrations. However, it is this small amount of ozone, the ozone layer, that is the critical protector of living matter on Earth. The bathtub analogy for ozone production and destruction A crude, but perhaps helpful, analogy of the above steady state process is that of a bathtub being filled and drained at the same time (Fig. 7-2). Let us first represent the situation in which there are no effects of human beings on the ozone layer. The bathtub (the stratosphere) is continually being filled with ozone (A) while at the same time it is being drained by a single drain (B). The drain (B) represents the natural destruction of ozone. Sunlight is responsible for determining both the rate of filling (A) of the ozone tub and the rate of draining (B). The concentration of ozone is small and constant, but every hour nearly all the ozone in the stratosphere is destroyed by UV light, while at the same time the same amount of ozone is being formed! Figure 7-2 The "ozone bathtub" analogy for understanding steady state concentrations of ozone that exist in the stratosphere. A represents the production of ozone and B represents the destruction of ozone. The level in the tub represents the steady state concentration of ozone at any given time. Figure 7-3 Same conditions as in Fig. 7-2 with the addition of ozone destruction aided by stratospheric CFC reactions (C). A lower steady state ozone concentration results. Chapter 7 How Deep is the Ozone Hole? Page 7-5

6 Let us now add a drain that arises from the activities of humans (drain C) to the ozone bathtub (Fig. 7-3). This would correspond to the role CFCs are proposed to play in the stratosphere by destroying ozone in addition to the ozone destroyed by sunlight. Because the ozone flow into the bathtub is constant, this additional drainage of the ozone must lower the ozone concentration level in the bathtub. This lowered ozone concentration allows more harmful UVB and UVC radiation through the stratosphere into the troposphere. Because the troposphere contains relatively little ozone, the harmful UV radiation reaches the Earth s surface and is absorbed by living matter containing DNA and other important biological molecules, causing biological damage. Thus, according to atmospheric scientists, CFCs are probably responsible for forming the anthropogenic drain in the ozone bathtub and lowering stratospheric ozone concentrations in areas where the ozone hole is found in the Antarctic. Additional ozone destruction reactions lower the steady state ozone concentration CFC Induced Destruction of Ozone Prediction of ozone losses by Rowland and Molina The proposal for a supersonic transport (SST) airplane fleet in the 1960s elicited warnings from the scientific community that nitrogen oxides from the SST exhaust might harm the ozone layer. Not too long thereafter, added fears were raised that chlorine released from the US Space Shuttle might harm the ozone layer. Rowland and Molina's warning (see box) switched the focus regarding ozone destruction to CFCs. Not long after this warning was published, CFCs were banned in the US as propellants in spray cans, but the manufacture of CFCs for other uses was not curtailed, and CFC concentrations in the atmosphere continued to rise. Rowland refused to let the issue drop and won a number of scientists to his side. The use of CFCs in refrigeration, manufacture of plastic foam materials, and cleaning of circuit boards continued unabated, and the fluorocarbon industry stubbornly resisted and even downplayed Rowland's arguments. However, following the discovery of the ozone hole (see below), CFC production was banned. Chapter 7 How Deep is the Ozone Hole? Page 7-6

7 The original Rowland-Molina chemical mechanism proposed for CFC-induced stratospheric ozone destruction before the discovery of the ozone hole was the following relatively simple chain reaction: initiation of chain reaction: CFC + UV organic radical + Cl (7-7) reaction chain: Cl + O3 ClO + O2 (7-8) ClO + O Cl + O 2 (7-9) net chain reaction: O 3 + O 2 O 2 addition of (7-8) and (7-9) (7-10) The addition of reaction (7-8) and (7-9) give ClO + Cl on both sides of the summed reactions, so one can cancel these two radicals, yielding the net reaction (7-10). Reactions (7-7) through (7-9) illustrate the proposed chlorine atom-catalyzed destruction of ozone. Reaction (7-7) produces the destructive chlorine atom that is necessary to initiate the chain reaction represented by reactions (7-8) and (7-9). The organic radical product in (7-7) is the remainder of the CFC molecule after removal of a neutral chlorine atom. The organic radical product does not cause ozone destruction and is therefore neglected in the succeeding reactions. We designate the odd electron, neutral chlorine atom, Cl, as a free radical, because it has an unpaired electron (hence the following the Cl symbol). The chlorine free radical is also a catalyst because the chlorine atom is continually being regenerated during the chain reaction. Once the chlorine atom is produced, ozone destruction can occur. Steps (7-8) and (7-9) form a chain reaction because the Cl consumed in reaction (7-8) is recreated as a product in (7-9) and the ClO consumed in (7-9) is recreated as a product in (7-8). Thus the creation of one Cl can destroy thousands of ozone molecules by cycling through reactions (7-8) and (7-9). Although this series of three reactions is not thought to be responsible for the formation of the Antarctic ozone hole, it gives us a feeling for the ozone-destroying capabilities of chlorine atominduced chain reactions, and it is probably responsible for the destruction of ozone in parts of the world other than Antarctica. Rowland and Molina mechanism for ozone destruction Chapter 7 How Deep is the Ozone Hole? Page 7-7

8 From Reluctant Environmentalist to Nobel Laureate On Earth Day, 1970, F. Sherwood ("Sherry") Rowland, pressured by his family to become more active in environmental activities, decided to engage in environmental chemical research. He had just stepped down as Chair of the Chemistry Department that he had helped build at the University of California at Irvine. He also had much to be satisfied with in his own research career, in which he employed radioisotopes as a tool in gas phase chemical research financed by the US Atomic Energy Commission (AEC). Rowland's environmental decision led to attendance at a European conference, after which he met an AEC official, one of whose missions was to lure gifted researchers into the young field of environmental chemistry. Rowland was invited to a subsequent environmental conference, where Rowland learned about the recent discovery that atmospheric chlorofluorocarbon (CFC) concentrations were steadily increasing everywhere in the world. He decided to investigate the gas phase chemistry of CFCs when the next opportunity arose. That opportunity came in 1973, when Rowland presented a talented, Mexican-born postdoctoral fellow, Mario Molina, with a choice of research projects, one of which was the CFC problem. Rowland wanted to know why CFCs were building up so fast in the atmosphere, and Molina was interested in the new environmental field of atmospheric chemistry. The match between the two researchers was perfect. Though neither of them was an expert in the field of environmental chemistry, they both brought unique perspectives to this field from their previous research on chemical reactions in the gas phase. Molina first worked diligently to find a sink for CFCs in the troposphere, but could find none (OH radicals did not attack CFCs!). Both Molina and Rowland knew that CFCs were able to work their way slowly into the stratosphere. Both were also aware that chlorine-containing compounds would be decomposed into highly reactive chlorine atoms by short wavelength UV photons found only in the stratosphere. Both knew of the fragile nature of the ozone layer. Molina's ultimate goal was to integrate this knowledge together into one complex set of chemical equations and to calculate the potential damage to the Earth's ozone layer, given the current and projected future concentrations of CFCs in the stratosphere. Neither Molina nor Rowland were prepared for the results of the calculations, first performed by Molina, and then confirmed by Rowland. After rechecking these calculations one more time, Rowland went home, puzzled and disturbed. In answer to his wife's How did things go at the laboratory?, Rowland replied, The work is going well, but it looks like the end of the world! What his and Molina's calculations had predicted was the future destruction of anywhere from 20 to 40% of the world's ozone layer, implying a disastrous increase in harmful UV radiation from the sun. These calculations were a prelude to a very long, often discouraging, usually lonely, and often bitter fight that Rowland waged to stop the production of CFCs throughout the world. It was only after the discovery of the ozone hole in 1985 that Rowland and Molina were finally vindicated in their pioneering work and in their insistence on the need to keep CFCs and other similar compounds out of the stratosphere. For this research work, Rowland was awarded the 1989 Japan prize. In 1995 he and Molina shared with another atmospheric chemistry pioneer, Paul Crutzen, the highest honor a chemist can attain, the Nobel Prize in Chemistry. Theirs was the first Nobel Prize awarded in the field of environmental science. Chapter 7 How Deep is the Ozone Hole? Page 7-8

9 Chain-stopping reactions and storage compounds According to the above chain reaction (7-8) and (7-9), all of the ozone in the world could be destroyed by the release of a single chlorine atom. However, there are far more chemical reactions involving chlorine atoms than (7-8) and (7-9), some of which inactivate the highly reactive Cl and ClO free radicals by chemically reacting or combining with them. When methane (CH 4 ) and nitrogen dioxide (NO 2 ) are present in the stratosphere, reactions (7-11) and (7-12) remove Cl and ClO from the chain reaction (7-8) and (7-9). These two reactions break (stop) the chlorine atom-induced chain reactions through the formation of the storage compounds hydrogen chloride (HCl) and chlorine nitrate (ClONO2 ) and in doing so remove free chlorine atoms (Cl ) from the atmosphere. Cl + CH4 HCl (storage compound) +CH 3 (7-11) ClO + NO2 ClONO 2 (storage compound) (7-12) Chainstopping reactions save ozone layer Figure 7-4 Early experimental evidence indicating development of the ozone hole in the Antarctic. Data are taken from balloon (Halley Bay) and satellite (South Pole Ozone) measurements during the Antarctic springtime. Note the rough correlation, slightly delayed, between ozone decrease and increases in the concentration of stratospheric chlorine compounds. A Dobson Unit (DU) is a measure of ozone concentration. A Dobson unit is formally defined as a number of ozone molecules contained in a column one square centimeter in area and with a height of the entire atmosphere. One Dobson Unit equals 2.7 x ozone molecules. On average throughout the world, the ozone concentration is 300 DU with wide variations depending on geographic location and season of the year. (S. Solomon in Nature, Vol. 347, 1990, pp ) The storage compounds are chemically more stable than Cl or ClO, so their formation effectively halts both the ozone-destroying chain reaction steps. The Ozone Hole Discovery Figure 7-4 illustrates Antarctic springtime ozone data as well as data on the growth of anthropogenic chlorine in the atmosphere. This figure is worth careful study because it typifies the type of data with which atmospheric scientists must work. Scientists had to reexamine archived United States satellite data regarding ozone concentrations following a British announcement of their Antarctica (Halley Bay) data in Fig 7-4. The US Chapter 7 How Deep is the Ozone Hole? Page data demonstrate major seasonal losses of ozone in Antarctica

10 scientists discovered that a computer program had discounted and rejected data showing low ozone concentrations. A reanalysis of the complete set of data (labeled South Pole Ozone in Fig. 7-4) indeed showed losses over the entire Antarctic continent (10% of the area of the southern hemisphere) of the same magnitude as the observed Halley Bay data at 77 S showed during the same time periods. These data showed a 3% loss of the world s ozone in a 6-week period. This was the first verification of a seasonal ozone hole in the Antarctic. Accelerated increases in the manmade chlorine in the stratosphere and troposphere took place at the same time that ozone levels were decreasing over the springtime Antarctic. The phenomenon has continued since then and even intensified. Each succeeding year, stratospheric ozone levels have declined rapidly in August and September (Antarctic spring), creating a hole (absence of ozone) in the thickest part of the stratospheric ozone layer. The depth of the hole is deepest by late September to early October. The hole then fills in through mixing with surrounding ozone-rich air by late January and is ready for the next year s cycle. The Low Temperature Puzzle Weather balloon of the type used to make ozone measurements in the Antarctic. Scientists were surprised that the ozone hole was initiated during the Antarctic springtime immediately following the completely dark Antarctic night winter, the season where the coldest temperatures on Earth are recorded in the Antarctic. Theoreticians were initially stumped. Chemists normally observe that chemical reaction rates decrease when the temperature decreases. The solution to this unexpected theoretical problem demanded the combined knowledge and Why are losses so great at very low temperatures? Chapter 7 How Deep is the Ozone Hole? Page 7-10

11 research of chemists, meteorologists, and other scientists. The first clue was that the ozone hole was generally centered over the South Pole at the time when a wintertime phenomenon, the polar vortex, still occurred. A liquid vortex can be imagined by water swirling down a drain or toilet. The polar vortex is the same type of process, but forms from a large mass of swirling air (Fig 7-5b) over both the South and North Poles during their respective winters. During its winter season, the Antarctic region is completely dark at all times (Fig. 7-5a). The previously sun-warmed Antarctic region and the gases above it emit infrared radiation to the sky and therefore are continually cooled. Because there is no solar radiation to heat up these gases or the Antarctic surface, they continue to lose thermal energy, producing the lowest temperatures on Earth. The vast temperature difference between the cold polar region and the sunlit land adjacent to the polar region results in pressure differences between the two regions, causing strong winds to blow toward the pole because lower pressures predominate in the polar region. However, the forces of the spinning Earth deflect these winds, causing them to flow sideways, forming a vortex. One property of a gas with vortex flow is that relatively little mixing occurs between the air outside and inside of the vortex. The gases inside the polar vortex continue to lose their energy through infrared radiation losses to the dark sky, and the temperature becomes colder as the Antarctic winter progresses. Winter conditions in Antarctica isolate the region Figure 7-5 Note that the Earth is turned upside down, with the South Pole up and the North Pole down in this illustration (a) Illustration of the reason for the Antarctic night, which is responsible for the very low temperatures in the stratosphere: because of the tilt of the earth with respect to the sun's rays (parallel lines), the South Pole is in a dark shadow during the Antarctic winter; (b) illustration of the vortex winds in the Antarctic winter, which, when combined with the very low temperatures, cause formation of very reactive clouds of ice particles. The ozone hole forms on the inside of this vortex directly over the South Pole.. Chapter 7 How Deep is the Ozone Hole? Page 7-11

12 When stratospheric temperatures reach -85 C, water and nitric acid contained in the stratosphere form a type of ice cloud containing one nitric acid (HNO 3 ) molecule for every three water molecules. This chemical composition gives the NAT clouds their name (NAT = nitric acid trihydrate). As the temperature goes down even further, ice clouds are formed consisting of pure water. These two types of clouds are named collectively polar stratospheric Polar stratospheric clouds (PSCs) of the type found during the Antarctic winter. clouds (PSCs). These PSCs (picture left) can adsorb on their surfaces any atmospheric molecules that come in contact with them during the Arctic night, including the chlorine-containing storage compounds HCl and ClONO 2. The highly reactive surface of the ice crystals accelerate the destruction of these storage compounds causing the formation of new active chlorine compounds Cl 2 and HOCl. These compounds are released from PSCs into the atmosphere as gases. When sunlight of the Antarctic spring returns, these active compounds absorb visible and UV photons from the sun and release Cl free radicals. These free radicals can then destroy ozone in a chain reaction that is proposed to be more complex than Rowland and Molina s. It was discovered that nearly all of the protective compound NO 2 present in the Antarctic atmosphere during this dark, cold winter period is transported into the troposphere in the form of nitric acid in descending ice clouds, and thus there is no interruption of the chain reaction to form storage compounds again in the stratosphere. Therefore, the chain reaction involving Cl destroys nearly all of the ozone present in the lower stratosphere over the Antarctic continent during a period of weeks during the Antarctic spring, resulting in the ozone hole. Experimental evidence for ozone hole mechanism The critical chemical reaction proposed as being responsible for ozone losses in the ozone hole is equation (7-8). Cl + O3 ClO + O2 (7-8) Polar stratospheric clouds (PSCs) According to this equation, there should be a negative correlation between ozone and the ClO radical. That is, for every ozone molecule destroyed, there will be one ClO radical produced. Therefore, as the ozone concentration goes down, the ClO concentration should increase in a given region of the ozone hole. Chapter 7 How Deep is the Ozone Hole? Page 7-12

13 Figure 7-6 Historic data acquired through aircraft monitoring of ClO and O 3 concentrations during an ozone hole period in the Antarctic spring. The airplane took off from the tip of South America, rose into the vortex, leveling off, and headed straight into the ozone hole, descending and returning to the vortex before landing. The data shown here are plots of the relative concentrations (mixing ratios in parts per billion by volume) of ozone and the ClO radical concentrations vs. latitude. Increasing latitude means flying directly toward the South Pole toward the ozone hole and crossing into the hole. The data from September 16 is the graph that was widely publicized as the "smoking gun" data that were used as confirmation of the role of CFCs, because there is an almost perfect one-to-one anti-correlation between ozone and ClO concentrations. Contrast this figure with other data taken before and after 16 September. (Anderson, Toohey, and Brune, Science, Vol. 251, 1991, pp ) U2 spy plane used to collect data on ozone concentrations during an ozone event. The most convincing experimental evidence (Fig. 7-6) in support of reaction (7-8) comes from aircraft-borne instrumental studies in the Antarctic during the development of an ozone hole. The majority of these studies come from the ER-2, a converted U-2 spy plane (picture left) capable of flying in the lower part of the stratospheric ozone layer. Measurements of ozone and ClO concentration, made during the development of the ozone hole, show the anticipated negative correlation between the loss of ozone and the increase in the ClO radical concentration. The evidence in Fig. 7-6 is said to be the smoking gun that convinced many skeptical scientists and policy makers of the role of chlorine atoms in ozone hole development. These data can best be explained by a chemical reaction in which a chlorine atom attacks ozone to give ClO. This links Chapter 7 How Deep is the Ozone Hole? Page 7-13 Anticorrelation found between ozone destruction and ClO

14 chlorine atoms directly to ozone destruction. Most scientists believe that the primary chlorine atom source is the UV radiation-induced CFC destruction in the stratosphere. When the Antarctic spring sunlight returns, the vortex winds become unstable, begin to oscillate, and the vortex breaks up completely, exchanging air between the Antarctic region and its surroundings. This breakup allows the flow of both ozone and chain-stopping NO 2 radicals into the Antarctic atmosphere. However, ozone-poor air from the Antarctic ozone hole is, at the same time, exchanged and transported into regions relatively near the Antarctic such as the southern tip of Argentina, New Zealand, and even as far north as Australia. Significant lowering of ozone levels have been observed in these regions in the period immediately following ozone hole breakup. Ultimately, ozone levels return to normal in all Antarctic regions following the breakup of the ozone hole. The effects of the ozone hole are felt for more than a month during the early spring in a region larger than the United States. Because of the above arguments, the majority of the world s nations have banned the production of CFCs. Non-CFC compounds that may aid in ozone hole formation According to atmospheric scientists, there is one other class of compounds that poses a serious threat to the ozone layer: bromine-containing compounds. Bromine and chlorine are in the same family of the periodic table, Group 7. Both have the same number of valence electrons and, therefore, should have similar chemical behavior. The naturally-occurring bromine compound methyl bromide (CH 3 Br) is produced in large quantities by smoldering fires of burning vegetation. CH 3 Br is also an important agricultural soil fumigant. Another important series of bromine-containing compounds are the halons. These compounds, similar in some properties to CFCs, contain carbon, bromine, chlorine and fluorine atoms and have been used to extinguish airplane and airport runway fires. CH 3 Br and halons have sufficiently long tropospheric lifetimes to work their way into the stratosphere in small quantities. In the stratosphere, these small quantities of CH3Br and halons decompose to give bromine atoms, Br and thus have a Lewis structures of Cl vs. Br Cl Br Radiation Alert Under Ozone Hole in Southern Science News October 9, 2000 PUNTA ARENAS, Chile (Reuters) - A wide swath of southern Chile was on alert Monday as dangerous levels of ultraviolet radiation hit peaks because of the depletion of the protective ozone layer over the Antarctic. Health authorities warned the 120,000 residents of this wool and fishing city -- one of the few populated areas beneath the ozone hole in the southern hemisphere -- not to go out in the sun during the day. stong ozone depletion The ozone hole over the Antarctic this year has reached its deepest since scientists began potential (ODP), measuring it 15 years ago, with more than 50 that is, a significant percent depletion being recorded throughout most potential to induce of the hole, United Nations experts said Friday. the destruction of ozone in the stratospheric ozone layer. However, because Chapter 7 How Deep is the Ozone Hole? Page 7-14 Brcontaining compounds also endanger the ozone layer

15 of the greater efficiency of bromine atom than chlorine atom reactions, the ODP of bromine-containing compounds is some 20 to 60 times more effective in destroying ozone per molecule than that of similar chlorine-containing compounds. Thus, even though far fewer bromine atoms are formed in the stratosphere than chlorine atoms, the greater efficiency of bromine atoms in ozone destruction has caused the use of halons to be phased out in the United States aviation industry, and in 1995 a freeze was placed on the production of methyl bromide. More recently, direct measurements of iodine oxide (IO) and bromine oxide (BrO) have been measured near the Antarctic surface. Both I and Br are responsible for ozone destruction, but the two act in synergy, quadrupling this destructive effect. The source of the I may be anthropogenic or from the ocean. The potential for an Arctic ozone hole During the late twentieth and early twenty-first centuries, no ozone hole occurred in the Arctic, despite its complete darkness and polar vortex winds during the Arctic winter. Average winter temperatures are generally F warmer in the Arctic than in the Antarctic. Polar stratospheric clouds occur more sporadically in the Arctic. The flow of air over the Himalayas and the Rocky Mountains and land-sea temperature contrasts generate very large atmospheric waves that produce disturbances in the stratosphere, leading to mixing of warmer mid-latitude air and cold Arctic air. Thus the Arctic vortex is warmer and less stable, and wanders or oscillates more than the Antarctic vortex. This wandering can take the vortex as far as the British Isles and continental Europe. The vortex generally breaks up during the Arctic winter, in contrast with the spring vortex breakup in the Antarctic. There are only sporadic reductions in protective stratospheric NO 2 radical concentrations during the Arctic Winter. Arctic conditions less favorable, but ozone losses have been significant Nevertheless, in their aircraft expeditions, scientists have been surprised to find relatively high ClO radical concentrations over Arctic regions. These ClO radical concentrations, hundreds of times higher than normal levels for the Arctic, in some cases even exceeded ClO concentrations measured in the Antarctic. It is also more difficult to detect ozone losses in the Arctic because the seasonal changes in ozone concentrations cause the normal ozone concentration to increase just at the time a potentital ozone hole could form. Thus, scientists must resort to statistical methods to determine whether there have been ozone losses in the recent past. It would appear that in recent years there have been significant ozone losses in the Arctic region during the months of January, February, and March. In recent years, the Actic winter polar vortex has strengthened and become more persistent, even during significant warming of the Arctic (Chapter 6). At the same time, scientists have measured overall losses of 30% of the Arctic ozone column in the springtime in some years, with losses as high as 70% in some altitudes. Because Chapter 7 How Deep is the Ozone Hole? Page 7-15

16 of the complexities of Arctic weather, it has not been possible to model the behavior responsible for these sporadic losses or predict future trends in Arctic springtime ozone losses. Ozone losses in non-polar and polar regions Analysis of long-term measurements from satellite data over the entire Earth shows a decrease in ozone with a possible recent stabilization. The ozone hole has more recently extended well beyond the Antarctic continent. Across the US, ozone concentrations have been as much as18% below normal. Over the South Pole, a record low of ozone concentration was observed. In 1997, there was no ozone observed at all between altitudes of 8.5 and 12 miles, and there was extensive ozone depletion at altitudes 12.5 miles and above, where no ozone depletion had been observed in earlier years. The area covered by the ozone hole was a record 9 million square miles. The 2000 ozone hole covered a larger area than previously recorded. Mt. Pinatubo volcanic aerosols may have been responsible for much of the ozone hole enlargement in 1993, but they don t explain O 3 losses in later years and at very high altitudes, where volcanic sulfate aerosols are not observed. The area covered by the ozone hole has increased rather than decreased. Non-polar region ozone losses measured The measurements of worldwide ozone concentrations have been taken over a relatively short time period and are probably still too limited to indicate a reliable trend. Because of constant atmospheric motions, it is difficult to disentangle ozone losses outside Antarctica, where the chemistry is best understood and where the losses are much more dramatic. There is as much as 25% variation in ozone concentrations throughout the world, especially at high latitudes. Scientists must resort to statistical methods to establish trends in ozone concentration in different regions of the world. Analyses must include seasonal variations as well as consideration of the 11-year solar cycle. However, all the studies accomplished to date point to the need for continuous, long-term studies of stratospheric ozone concentrations over the entire globe. Protection Against Ozone Layer Depletion Cessation of CFC production through international treaties An obvious first step in protecting the ozone layer in the stratosphere is to minimize the concentrations of CFCs and halons present in the stratosphere. This can be accomplished by stopping the manufacture and use of these compounds. This is already being accomplished through international treaties, namely the 1987 Montreal Protocol on Substances That Deplete the Ozone Layer and the Copenhagen and London Amendments of 1990 and The manufacture of CFCs in most of the Treaties banning production of CFCs are in force throughout world Chapter 7 How Deep is the Ozone Hole? Page 7-16

17 world was supposed to be phased out January 1, 1996 but, because of the expense of CFC replacements, there are CFC black markets, especially in South Asia. In a rare show of international concern for the environment, this series of treaties was signed to limit and, ultimately, cease production of CFCs and halons. Companies that manufactured CFCs voluntarily moved up phase-out dates. The swiftness and apparent success in implementing these treaties was a pleasant surprise to many of those like Rowland and Molina who remembered the reluctance to show concern for this problem before the discovery of the ozone hole. As a result of these treaties and other government regulations, a major commercial adjustment has been necessary because of the important roles of CFCs and halons in industry, the home, and transportation. Uses of and replacements for CFCs CFCs and halon substitutes have emerged from an intensive development program in the chemical industry. The largest use of CFCs has been in refrigeration and air conditioning. In both of these processes, the pressurized liquid CFC is suddenly expanded and transformed from a liquid to a gas. In expanding into the gaseous state, the CFC molecules cool because dipolar attractions between molecules cause them to slow down as they vaporize from the compressed liquid state into the gaseous state. This slowing results in a lower average speed in the resulting CFC gas, and this lower average speed corresponds to a lower gas temperature. This cool gas flows through refrigerator or air conditioner coils and picks up heat through a heat-conducting metal coil. The warmed gas then flows to a place outside the refrigerator compartment or air conditioned room and is compressed back into a liquid. Heat is released because of the formation of strong polar bonds during gas liquefaction. This heat is vented outside the refrigerator or air conditioned room. The compressed liquid coolant is again forced to vaporize and the refrigeration cycle repeats. Uses of CFCs CFCs also have been used as polyurethane and polystyrene foam blowing agents. These foams are employed in insulation for coolers, refrigerators, homes, etc. In this process, liquid polystyrene is filled with tiny CFC gas bubbles, so the final insulation product consists primarily of a thin solid polymer film surrounding many bubbles. When the polymer solidifies, the CFC blowing agent is trapped inside the rigid polymer. The foam filled with CFCs has a lower heat Chapter 7 How Deep is the Ozone Hole? Page 7-17

18 conductivity than a similar foam filled with other gases such as air or carbon dioxide and is therefore a good insulator. Industrial chemists are searching for drop-in replacements for CFCs, that is, new compounds with low ozone depleting potential (ODP) that can be substituted without modification of the equipment employing the CFCs. Such direct replacements have been very difficult to find. Desired properties for CFC replacements Ideally, CFC replacements should have physical and chemical properties similar to those of CFCs, for example, boiling point and heat conductivity. They need to be compatible with the gaskets and seals designed for equipment employing CFCs, to be thermally stable (i.e., will not break down at higher temperatures), non-flammable, to be non-toxic, and to have low ODP (ozone depletion potential) and greenhouse gas potential (Chapter 6). Any one of these requirements can be relatively easily accommodated. Finding a single compound that satisfies all of these specifications is a tall order! One conclusion is immediately apparent--cfcs are a unique class of chemical compounds that are hard to replace. Chemists can lower the atmospheric lifetimes of the CFC replacements by making the compounds more reactive in the troposphere. By lowering tropospheric lifetimes of these replacements, there is less likelihood that the compound will survive in the troposphere long enough to leak slowly into the stratosphere through the tropopause. One way to lower atmospheric lifetimes is by incorporating hydrogen atoms into CFC replacement molecules. Such hydrogen-containing compounds are designated as HCFCs (hydrochlorofluorocarbons), where the H stands hydrogen. We have seen that OH is a very reactive radical present in the troposphere, but is not very reactive with compounds containing only carbon-chlorine or carbon-fluourine bonds. One of the main chemical reactions of OH in the atmosphere is the abstraction of a hydrogen atom from compounds with which it collides to form HOH (water). A free radical is formed on the compound that loses the hydrogen atom. This free radical then reacts with oxygen in the air and, ultimately, forms a polar compound that is soluble in rain water and is washed out of the troposphere. Thus, HCFC substitutes have an efficient precipitation sink in the troposphere. However, the lifetimes of these substitutes are long enough that small amounts of these compounds can still work their way into the stratosphere and yield chlorine atoms from these chlorine-containing compounds. CFC replacements Use of fluorocarbons containing H atoms requires modification of refrigeration equipment because of the higher pressures required to liquefy HCFCs than CFCs. Because chlorine atoms are the major culprit in the ozone depletion chain reaction, one might simply eliminate the chlorine atom from the fluorocarbon. However, chlorine is essential to CFC properties for several reasons. Chlorine is a comparatively heavy atom (mass number 35 as compared with 19 for fluorine) and its electronegativity is high, resulting in significant dipole moments in chlorine-containing bonds. These two Chapter 7 How Deep is the Ozone Hole? Page 7-18

19 properties allow liquid formation in CFCs at relatively low pressures. CFCs are ideal compounds for refrigeration purposes because they are easier to liquefy near room temperature and they take up large quantities of energy when they vaporize. Refrigerators and air conditioning equipment have been designed specifically for use with CFCs that were developed as a replacement for the much more dangerous refrigerant ammonia (NH 3 ). Special seals and gaskets have been designed for particular CFCs to prevent them from escaping from the pressurized system. The use of HCFCs in the same refrigeration equipment requires redesign of these seals and gaskets, and sometimes even redesign of the basic equipment. As a stopgap measure to meet the phaseout of the manufacture of CFCs, there was a move to manufacture HCFCs that still had chlorine atoms, but also contain hydrogen atoms to reduce their lifetimes in the troposphere. Ultimately only HFCs (hydrofluorocarbons containing only hydrogen, carbon, and fluorine, but no chlorine atoms) will be used, because their ODP is zero and their greenhouse gas potential is lowest of the three alternative compound types, CFCs, HCFCs and HFCs. The naming of these compounds reveals the chemical composition of the compounds, if you know the code (see box). CFCs, HCFCs, and HFCs are all very efficient greenhouse gases (Chapter 6). However, HCFCs and HFCs have shorter lifetimes in the atmosphere than the CFCs they replace. Thus, there is a net reduction in the concentration of greenhouse gases as CFCs and HFCs are phased out. Another meeting in Montreal 20 years after the original Montreal Protocol has produced another landmark international agreement to phase out HCFCs. A transitional fund of $150 was established to aid developing countries in achieving the HCFC phaseaout goal. This agreement has double value because it phases out compounds that adversely affect both the Earth s climate and its ozone layer Naming of CFCs and their replacements In the early years of their production, DuPont scientists devised a code for naming CFCs that masked their identity. This code can be broken using the following rules: 1. Add 90 to the number in the common name (e.g., for CFC-11, add 90 to get 101) 2. The first digit of the sum obtained in #1 above represents the number of carbon atoms in the molecule (e.g. 1 in 101 for CFC-11) 3. The second digit in the sum obtained in #1 represents the number of hydrogens in the molecule (e.g., 0 or none in CFC-11) Rules for naming CFCs, HCFCs, and HFCs Chapter 7 How Deep is the Ozone Hole? Page 7-19

20 4. The third digit in the sum obtained in #1 represents the number of fluorine atoms in the molecule (e.g., 1 in CFC-11) 5. Because every carbon atom has four single bonds (there are no double or triple bonds in CFCs, HCFCs, or HFCs), the remaining unspecified atoms are chlorine atoms (e.g. for CFC-11 the structure is CFCl 3 ). Thus, CFC-12 is CF 2 Cl 2 and HCFC-123 is C 2 HF 3 Cl 2, one of whose structures is shown below: F H F C C Cl F Cl There are other possible structures, called isomers, for HCFC-123 in which the same atoms are rearranged to form a similar, but not identical compound, for example: F H F C C Cl Cl F Structure of HCFC isomers In this second isomer, one chlorine is attached to each carbon atom, whereas in the first isomer both chlorines are attached to the same carbon atom. Both compounds have exactly the same atomic composition, i.e. two carbons, two chlorines, three fluorines, and one hydrogen. However, they are different isomers, and have different physical and chemical properties. These different isomers are designated by small letters following the number of the compound, e.g., HCFC-142b. Exercise 7-3 Formulas of CFCs and CFC replacements Write the chemical formulas for each of the following compounds. Write out a two dimensional structure such as those above for each of these compounds: (a) CFC-11; (b) CFC-114; (c) HCFC-124; (d) HFC-134a; (e) HFC-134b. Which of the above types of compounds has the highest ODP, and which has the lowest ODP? According to US law, the last person in the disposal chain for refrigerators and air conditioners is required to recapture the CFC in that device and recycle it, if possible. This requires special equipment, and many small businesses cannot afford to buy Chapter 7 How Deep is the Ozone Hole? Page 7-20

21 such equipment. However, the cost of CFCs is rising, and this may help act as an incentive not to vent CFCs to the atmosphere. The anticipated future development of the ozone hole It would appear that people around the world have taken the CFC threat to the ozone layer seriously enough to make changes at the national and international levels. CFC production has effectively ceased and there are already signs that atmospheric CFC concentrations are beginning to respond to these cutbacks. HCFCs are being manufactured as a stopgap measure, and there are strong pressures to cut their use to a minimum and phase them out earlier than planned. HFCs are already in use and more are being introduced commercially. Alternatives to the use of chlorofluorocarbons are already in place in the electronics industry. Thus, it would seem that the world has responded to the challenge first posed by Rowland and Molina. Future of the ozone hole? The lifetimes of the CFCs in the stratosphere are on the order of 50 years, less for the HCFCs. It is very difficult to predict the demise of the ozone hole, but there are already indications that the Montreal Protocol is working as intended. In 2006, the ozone hole hit a new record of covering 29 million square kilometers (much larger than Europe). Antarctic temperatures were also the lowest since 1979, which are thought to be responsible for this. In 2005, based on new models, scientists extended the lifetime of the Antarctic ozone hole phenomenon from 2050 to Although production of CFCs have been banned since 1996, stores of these compounds, which are larger than previously estimated, will be continued to be emitted until they are exhausted. It is probable that when the Antarctic ozone hole has recovered the ozone levels will stabilize at a new level because of the changed atmospheric conditions around There is evidence that ozone levels are beginning to recover in mid latitude non-polar regions, which offers hope that world-wide recovery is on track. However, Arctic ozone losses have increased with very low temperatures. It is probably too early to say that the world has turned the corner completely in ozone layer recovery. Nevertheless, there will still be releases of ozone-depleting chemicals in the next decade or so because of the release of CFCs from existing refrigerators, automobile, and home air conditioners. Unfortunately, the multiple decade lifetime of the CFCs already present in the troposphere and those released in the future will prevent the elimination of the ozone hole in the near future. According to projections, the ozone hole may be with us for more than a half century, unless there is some clever way found to prevent its formation. There have been a number of suggestions. For example, couldn t the excess ozone from smog in our cities be pumped into the stratosphere? This will not work because the ozone concentration levels of our most polluted cities are still much lower than the concentrations in the ozone layer itself. Chapter 7 How Deep is the Ozone Hole? Page 7-21