How Earth got its ozone layer

Transcription

1 How Earth got its ozone layer The primordial Earth had an atmosphere quite different from today s atmosphere. Significant amounts of carbon dioxide were present as well as nitrogen, but there was little oxygen. Earth receives radiated energy from the sun. The beginnings of life eventually led to plants that could absorb energy from the sun and make the carbon dioxide into molecular oxygen. At present and for many millions of years, Earth s atmosphere has been about 18% oxygen. All that oxygen was manufactured by living plants. It is mixed evenly throughout the atmosphere. Some of the solar radiation that reaches Earth is high energy ultraviolet radiation (this light is of wavelengths less than the highest energy visible light, which has wavelength ~400 nm). (See Ch. 21 for a discussion of the meaning of wavelength.) In two specific wavelength ranges (or energy ranges, the two are interchangeable because E = hc/λ, where λ is the wavelength), there are reactions that produce single oxygen atoms, some of which combine with normal molecular oxygen, O 2, to make ozone, O 3. Some single oxygen atoms combine with ozone to make two normal oxygen atoms. All of these transformations are encapsulated in what is known as the Chapman cycle (after S. Chapman, who first described them in 1930). (4) The Chapman cycle is O 2 + solar energy of wavelength less than 242 nm 2O, O + O 2 O 3, O + O 3 2O 2, O 3 + solar energy of wavelength less than 336 nm O* + O 2. Here, O* is an excited state of oxygen. It can become de-excited through thermal collisions and become a single oxygen atom. It can be seen that light with wavelengths from 336 nm

2 Energy, Ch. 1, extension 1 Earth s ozone layer 2 on down will be absorbed. Only the lowest-energy ultraviolet radiation will reach the surface. How ozone is destroyed by nitrogen oxides In the 1970s, it was found by Crutzen that the dissociation of ozone also proceeds through reactions with oxides of nitrogen. (5) The Crutzen-identified nitrogen oxide cycle involves formation of NO 2 from NO: NO + O 3 NO 2 + O 2, NO 2 + O NO + O 2, NO 2 + solar energy NO + O. Clearly, the net effect of this cycle is to eliminate ozone: 2O 3 becomes 3O 2. Note that this cycle will continue until (somehow) the NO is removed. NO acts as a catalyst to destroy ozone. After a few kilometers penetration, most of the ultraviolet radiation has been absorbed by the oxygen present and ozone has been formed or destroyed. The thickness of the upper atmosphere containing the ozone produced is called the ozone layer. The key observation is that the ultraviolet radiation that makes and destroys ozone is prevented from reaching Earth s surface. This is how the ozone layer protects the living things on the planet s surface from the effects of ultraviolet radiation. How ozone is destroyed by chlorine oxides Once chlorine (or other halogen) is released from its carrier (CFC or halon) by ultraviolet radiation in the stratosphere, the major reactions are Cl + O 3 ClO + O 2,

3 Energy, Ch. 1, extension 1 Earth s ozone layer 3 ClO + O Cl + O 2. The net result of this reaction is O 3 + O 2O 2. As with the NO cycle, chlorine will continue its destruction until (somehow) it is removed from the stratosphere. It, too, is a catalyst for ozone destruction. Sources of methyl halides A few years ago, it was thought that all chlorine and bromine came directly from biological processes in the ocean. It had long been known that oceanic plankton (tiny plants) played a starring role in stratospheric processes through release of dimethyl sulfide. (6) Sulfur compounds work to cool the atmosphere. They also provide cloud condensation nuclei. Similar origins were believed to produce natural bromine and chlorine, but recently it was found that these sources can account for only about a tenth of the total annual flux of around 3.5 billion kg. (7,8) Several investigations have identified both biotic and nonliving (abiotic) sources of the natural chlorine and bromine. Only one of these biotic sources is related to human activity (rice cultivation). The chemical bond between bromine and chlorine is not quite as strong as the chlorine-carbon bond, so bromine compounds are easier for sunlight to tear apart if they reach the stratosphere (halons such as methyl bromide can be cleaned in the lower atmosphere by the hydroxyl radical (a negatively-charged ion made up of oxygen and hydrogen, written OH - ), but in spite of this enough remains to reach the stratosphere). Both ozone and the hydroxyl radical are part of the natural processes cleaning the atmosphere. The hydroxyl radical removes an estimated 3.65 trillion kilograms of pollutants (including those diminishing the ozone concentration in the stratosphere) annually. (9)

4 Energy, Ch. 1, extension 1 Earth s ozone layer 4 The halons (book Table 1.2, page 9) contain bromine atoms that form bromine oxide, BrO, and BrCl, which have characteristics similar to chlorine oxide. Bromine is more effective per atom in destroying ozone. (4) The World Meterological Organization estimates that the methyl bromide emissions total 97 to 298 kilotonnes (million kilograms) per year, with the best estimate around 170 kilotonnes per year. (10) A bit less than half this amount, around 75 kilotonnes, comes from human generation. Humans release large amounts of such compounds as a result of the chemical industry, at the present mostly from pesticides (for example, termite control), soil fumigants, (11) CFCs, and chemicals from fire extinguishers (see book Table 1.2). The CFC atmospheric concentrations are beginning to decline. (12) The others are expected to follow as the additions to the Montreal Protocol come into effect. Coastal salt marshes were found to produce chlorine and bromine from all vegetation zones, about twenty times as much chlorine as bromine. (13) Another group, (7) operating independently, found significant amounts of methyl chloride, CH 3 Cl, coming from warm coastal land (tropical islands) and common tropical plants (nearly a teragram a billion kilograms per year). Yokouchi et al. also note that measured atmospheric concentrations are larger in the tropics than farther north or south. Atmospheric mixing is believed to be responsible for transport to the temperate and polar regions. Rice paddies, in addition to emitting methane in copious amounts, also produce these compounds. About 1% of methyl bromide and 5% of methyl iodide come from rice cultivation. (14) The unplanted flooded fields emit as much as the cultivated fields. The inorganic source comes from reactions in organic-laden soils. (8) Reactions in such soils and sediments can produce halide ions through oxidation of organic matter by an electron acceptor (for example, a state of iron). The result is compounds such as CH 3 Cl,

5 Energy, Ch. 1, extension 1 Earth s ozone layer 5 C 2 H 5 Cl, C 3 H 7 Cl, C 4 H 9 Br, CH 3 Br, C 2 H 5 Br, C 3 H 7 Br and C 4 H 9 Br, and CH 3 I, C 2 H 5 I, C 3 H 7 I and C 4 H 9 I for soils containing chlorine, bromine, and iodine, respectively. These abiotic processes could therefore be a large source of methyl halides. The halons (Table 1.1) contain bromine atoms that form bromine oxide, BrO, and BrCl, which have characteristics similar to chlorine oxide. Methyl bromide, a pesticide, is heavily used in agriculture and is responsible for much of the stratospheric bromine. Generally, iodine-containing compounds react more readily in the lower atmosphere and so do not cause ozone depletion because they do not survive long enough in the atmosphere to reach the stratosphere. (11) However, convective clouds could inject localized high iodine concentrations into the stratosphere occasionally, where it would destroy ozone just as its lower-mass cousins. How ozone is destroyed at the poles Ozone destruction is discussed clearly in references 4 and Ozone is destroyed through catalytic cycles involving reactive nitrogen (NO y ), Cl and Br ( halons are compounds containing bromine), (19) and hydrogen species (HO x ). (20) In the upper atmosphere, chlorine is found in reservoirs, in combination with nitrogen compounds. The CFCs, which were stable in the lower atmosphere, break apart under the action of sunlight and release chlorine to the reservoir molecules, (21) where chlorine (or bromine) is inactivated. In winter and spring, ozone destruction is mostly due to Cl and Br because at low temperatures, the reservoir molecules don t reform after they break apart. In summer, the halon reservoirs return through reactions involving nitric acid. However, when the sunlight is available in the summer, NO 3 is photolyzed within seconds. It can t form into N 2 O 5,

6 Energy, Ch. 1, extension 1 Earth s ozone layer 6 the inactive form of nitrogen oxide, so ozone is also destroyed at high rates in summer by active nitrogen oxides. Fig. E Total Ozone Mapping Spectrometer (TOMS) satellite data for ozone concentrations in the Antarctic for selected years. (NASA) Polar stratospheric clouds play a crucial role in the cycle of ozone destruction. In polar stratospheric clouds, nitric acid can be held away from contact with chlorine, allowing it to do its dastardly deeds. (15) The ozone-depleting reactions can take place on ice crystals, thought to be primarily nitric acid trihydrate (NAT; HNO 3 3H 2 O), frozen nitric acid with water. The clouds help destroy ozone by mixing chlorine nitrate and hydrochloric acid to form nitric acid and molecular chlorine. (21) When winter comes to polar regions, there is no sunlight and temperatures drop. When the temperature is low enough, clouds of nitric acid and water condense on sulfuric acid drops. The ice crystals can form only in extreme cold, around -78 C, in the extremely dry air. The ice is not pure water ice (which forms at -83 C) but is NAT. The NAT removes nitrogen from the region of the cloud; (21) the nitrogen

7 Energy, Ch. 1, extension 1 Earth s ozone layer 7 compounds are covered with water, freeze, and eventually fall as snow, especially those on the largest ice crystals. (15) This depletion of ozone is apparent in the Total Ozone Mapping Spectrometer (TOMS) data of Fig. E and in the more complete yearly evolution shown in Fig. E Fig. E TOMS October averages beginning in (NASA satellite data) By spring, most nitrogen has been removed by the clouds; the nitrogen concentration is so low that the chlorine does not get to recombine with nitrogen, but forms chlorine oxide, ClO. The ClO combines with another ClO molecule to form the chlorine oxide dimer (ClO) 2. Without nitrogen around, the (ClO) 2 breaks up through the action of sunlight into free chlorine atoms, keeping the cycle of ozone destruction going. (21)

8 Energy, Ch. 1, extension 1 Earth s ozone layer 8 The largest Antarctic ozone hole on record as of this writing occurred in the Antarctic Spring of 2000 (Fig. E01.1.3). (22) A comparison with the depletion shown in Fig. E shows how much bigger the 2000 depletion was. Fig. E TOMS image of the largest ozone hole on record, Sept. 6, (NASA satellite data) With the success of the Montreal protocol in controlling chlorofluorocarbon emissions, it is to be hoped that the ozone holes in the Arctic and Antarctic will begin to shrink. (23,24) Given the complexity of the interaction of chlorine concentrations and weather, the shrinkage may exhibit fits and starts, but the stratospheric concentration of chlorine is expected to begin declining in the early 2000s. There is a lag time between emissions reduction and polar chlorine concentration because it takes some years for the lower atmospheric chlorofluorocarbons to reach the stratosphere and be broken up through action of ultraviolet solar radiation. The best estimate for the lifetime of CFC-11, for example, is about 50 years. (24)