Describe The Chemical Processes That Are Predominant In The Stratosphere
In Chapter 1, the gas-phase chemistry of the unpolluted stratosphere was explored. Since the late 1970s, however, the normal functioning of the stratosphere's ozone screen—and the protection it provides us—has been periodically upset by anthropogenic chlorine-containing chemicals in the atmosphere. Most famously, these substances now cause an ozone hole to open each spring season above the South Pole. Ozone levels in the stratosphere over the North Pole as well, and to some extent even that over our heads, have also been depleted. In this chapter, the extent of these stratospheric ozone losses are documented, and the special chemical processes that produce such destruction are described. We also document how knowledge of this chemistry led to action by humankind to prevent even more drastic loss of ozone, which should eventually heal the stratosphere.
We begin by describing how the amount of overhead ozone is reported and the history of how the ozone hole over the Antarctic was first discovered.
Dobson Units for Overhead Ozone
Ozone, 03, is a gas that is present in small concentrations throughout the atmosphere. The total amount of atmospheric ozone that lies over a given point on Earth is measured in terms of Dobson units (DU). One Dobson unit is equivalent to a 0.01-mm (0.001-cm) thickness of pure ozone at the density it would possess if it were brought to ground-level (1 atm) pressure and 0°C temperature.
On average, this total overhead ozone at temperate latitudes amounts to about 350 DU; thus if all the ozone were to be brought down to ground level, the layer of pure ozone would be only 3.5 mm thick. Because of stratospheric winds, ozone is transported from tropical regions, where most of it is produced, toward polar regions. Thus, ironically, the closer to the Equator you live, the smaller the total amount of ozone that protects you from ultraviolet light. Ozone concentrations in the tropics usually average 250 DU, whereas those in subpolar regions average 450 DU, except, of course, when holes appear in the ozone layer over such areas. There is natural seasonal variation of ozone concentration, with the highest levels in the early spring and the lowest in the fall.
The Annual Ozone Hole Above Antarctica
The Antarctic ozone hole was discovered by Dr. Joe C. Farman and his colleagues in the British Antarctic Survey. They had been recording ozone levels over this region since 1957. Their data indicated that the total amounts of ozone each October had been gradually falling each year, especially during the mid-September to mid-October period, with precipitous declines beginning in the late 1970s. This is illustrated in Figure 2-lb, where the average minimum daily amount of overhead ozone is plotted against the year. The period from September to November corresponds to the spring season at the South Pole and follows a period of very cold 24-hour nights common to polar winters. By the mid-1980s, the springtime loss in ozone at some altitudes over Antarctica was complete, resulting in a loss of more than 50% of the total overhead amount. It is therefore appropriate to speak of a "hole" in the ozone layer that now appears each spring over the Antarctic and lasts for several months. The average geographic area covered by the ozone hole has increased substantially since it began (see Figure 2-la) and now is comparable in size to that of the North American continent.
The seasonal evolution and decline of the Antarctic ozone hole in a recent year (2006) is illustrated in Figure 2-2. For reasons that will be explained later in the chapter, substantial ozone depletion does not start to occur until late August (Figures 2-2a, b) and begins to decline in November, as the stratospheric temperature rises (Figure 2-2c).
Initially it was not clear whether the hole was due to a natural phenomenon involving meteorological forces or to a chemical mechanism involving air pollutants. In the latter case, the suspect chemical was chlorine, produced mainly from gases that were released into the air in large quantities as a consequence of their use, for example, in air conditioners. Scientists had predicted that the chlorine would destroy ozone, but only to a small extent and only after several decades had elapsed. The discovery of the Antarctic ozone
FIGURE 2-1 Historical evaluation of the Antarctic ozone hole, (a) Area covered by the hole (average for September 7 to October!3), and (b) minimum overhead ozone (average for September 21 to October 16), Extreme ozone depletion occurred in 1998 and 2006, as indicated. No data were acquired during the 1995 season. [Source: NASA, at http://ozonewatch.gsfc.nasa.gov/]
hole came as a complete surprise to everyone. Subsequent research, however, confirmed that the hole indeed does occur as a result of chlorine pollution. The complicated chemical processes that cause ozone depletion are now understood and are discussed in this chapter. Based upon this knowledge, we can predict that the hole will continue to reappear each spring until about the middle of this century and that a corresponding hole may appear above the Arctic region.
FIGURE 2-2 Evolution of the 2006 Antarctic ozone hole, (a) Area covered by the hole in millions of square kilometers;
(b) minimum daily amount of overhead ozone in Dobson units; and
(c) minimum daily temperature in the lower stratosphere in degrees Kelvin. [Source: NASA, at http: ozonewatch.gsfc.nasa.gov/]
(b)1979-1995 i i i i i
-60 -30 Equator 30 60
-60 -30 Equator 30 60
FIGURE 2-3 Changes in average overhead ozone at different latitudes.
(a) Increases 1996-2005;
(b) decreases 1979-1995. [Source: E. C. Weatherland anc S. B. Anderson, Nature 441 (2006): 39.|
As a consequence of these discoveries, governments worldwide moved quickly to legislate a phase-out in production of the responsible chemicals. Thus the situation was not made much worse by the development of even more severe ozone depletion over populated areas, with the corresponding threat to the health of humans and other organisms that this increase would bring.
Ozone Depletion in Temperate Areas
Ozone was being depleted not just in the air above Antarctica but to some extent worldwide. The average overhead ozone loss at mid-latitudes amounted to about 3% in the 1980s. As indicated by the lengths of the vertical black bars in Figure 2-3b, the losses during the 1980s and early 1990s were greater the higher the latitude in both the Northern and Southern Hemispheres. However, this trend in ozone loss was reversed in the period from 1996 to 2005, the gains in the Northern Hemisphere in this period approximately canceling the earlier losses (Figure 2-3a). Although some of the recovery could be due to controls on emissions, much of it probably occurred because of natural trends in that period toward higher ozone levels due to the solar cycle and a lack of volcanic activity as well as to relatively warm Arctic winters.
The Ozone Hole and Other Sites of Ozone Depletion
As discussed previously, scientists discovered in 1985 that stratospheric ozone over Antarctica is reduced by about 50% for several months each year, due mainly to the action of chlorine. An episode of this sort, during which there is
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said to be a hole in the ozone layer, occurs from September to early November, corresponding to spring at the South Pole. The hole has been appearing since about 1979, as was shown in Figure 2-1, which illustrates the variation in the minimum September-October ozone concentrations above the Antarctic as a function of year. Extensive research in the late 1980s led to an understanding of the chemistry of this phenomenon. In this section, we discuss the peculiar process by which chlorine in the stratosphere becomes activated to destroy ozone and look at the detailed mechanism by which destruction occurs. We then consider the various measures of ozone-hole size, which allow us to investigate whether the hole above the Antarctic has been declining over time, whether a hole exists above the North Pole, and the effects of the holes on the amount of UV light to which we are exposed at ground level.
The Activation of Catalytically Inactive Chlorine
The ozone hole occurs as a result of special polar winter weather conditions in the lower stratosphere, where ozone concentrations usually are highest, that temporarily convert all the chlorine that is stored in the catalytically inactive forms HC1 and C10N02 into the active forms CI and CIO, all of which were discussed in Chapter 1. Consequently, the high concentration of active chlorine causes a large, though temporary, annual depletion of ozone.
The conversion of inactive to active chlorine occurs at the surface of particles formed by a solution of water, sulfuric acid (H2SO4), and nitric acid (HNO3), the latter formed by combination of hydroxyl radical (OH) with nitrogen dioxide (N02) gas. The same conversion reactions could potentially occur in the gas phase but are so slow there as to be of negligible importance; they become rapid only when they occur on the surfaces of cold particles.
In most parts of the world, even in winter, the stratosphere is cloudless. Condensation of water vapor into liquid droplets or solid crystals that would constitute clouds doesn't normally occur in the stratosphere since the concentration of water in that region is exceedingly small, although there are always small liquid droplets consisting largely of sulfuric acid present, as well as some solid sulfate particles. However, the temperature in the lower stratosphere drops so low ( —80°C) over the South Pole in the sunless winter months that condensation does occur. The usual stratospheric warming mechanism—the release of heat by the Oz + O reaction—is absent because of the lack of production of atomic oxygen from 02 and 03 when there is total darkness. In turn, because the polar stratosphere becomes so cold during the total darkness at mid-winter, the air pressure drops since it is proportional to the Kelvin temperature, according to the ideal gas law, PV = nRT. This pressure phenomenon, in combination with the Earth's rotation, produces a vortex, a whirling mass of air in which wind speeds can exceed 300 km (180 miles) per hour. Since matter cannot penetrate the vortex, the air inside it is isolated and remains very cold for many months. At the South Pole, the vortex is sustained well into the springtime (October). (The vortex around the North Pole usually breaks down in February or early March, before much sunlight has returned to the area, but recently there have been exceptions to this generalization, as discussed later.)
The particles produced by condensation of the gases within the vortex form polar stratospheric clouds, or PSCs. As the temperature drops, the first crystals to form are small ones containing water and sulfuric and nitric acids. When the air temperature drops a few degrees more, below -80°C, a larger type of crystal—consisting mainly of frozen water ice and perhaps also nitric acid—also forms.
Chemical reactions that lead ultimately to ozone destruction occur in a thin aqueous layer present at the surface of the PSC ice crystals. Upon contact, gaseous chorine nitrate, C10N02, reacts at the surface with water molecules to produce hypochlorous acid, HOC1:
C10N02(g) + HzO(aq)-» HOCl(aq) + HNQ3(aq)
Also in the aqueous layer, gaseous hydrogen chloride, HC1, dissolves and forms ions:
Reaction of the two forms of dissolved chlorine produces molecular chlorine, Cl2, which escapes to the surrounding air:
This process is illustrated schematically in Figure 2-4. Overall, when the steps are added together, the process corresponds to the net reaction
HCl(g) + C10N02(g)->Cl2(g) + HNOj(aq)
since the ions H and OHT re-form water. Similar reactions probably also occur on the surface of solid particles.
During the dark winter months, molecular chlorine accumulates within the vortex in the lower stratosphere and eventually becomes the predominant chlorine-containing gas. Once a little sunlight m0tecular chlorin7from"ina7tive foms^o7cWorine in the reappears in the very early Antarctic spring, or the winter and spring in the stratosphere in polar regions.
FIGURE 2-4 A scheme illustrating the production of
FIGURE 2-4 A scheme illustrating the production of air mass moves to the edge of the vortex where there is some sunlight, the chlorine molecules are decomposed by the light into atomic chlorine:
Similarly, any gaseous HOC1 molecules released from the surface of the crystals undergo photochemical decomposition to produce hydroxyl radicals and atomic chlorine:
Massive catalytic destruction of ozone by atomic chlorine then ensues.
Since stratospheric temperatures above the Antarctic remain below -80°C even in the early spring (Figure 2-2c), the crystals persist for months. Any of the CI that is converted back to HCl by the reaction with methane is subsequently reconverted to Cl2 on the crystals and then back to CI by sunlight. Inactivation of chlorine monoxide, CIO, by conversion to C10N02 does not occur, since all the N02 necessary for this reaction is temporarily bound as nitric acid in the crystals. The larger crystals move downward under the influence of gravity into thé upper troposphere, thereby removing N02 from the lower stratosphere over the South Pole and further preventing the deactivation of chlorine. This denitrification of the lower stratosphere extends the life of the Antarctic ozone hole and increases the ozone depletion.
Only when the PSCs and the vortex have vanished does chlorine return predominantly to the inactive forms. The liberation of HNOj from the remaining crystals into the gas phase results in its conversion to N02 by the action of sunlight:
More importantly, air containing normal amounts of N02 mixes with polar air once the vortex breaks down in late spring. The nitrogen dioxide quickly combines with chlorine monoxide to form the catalytically inactive chlorine nitrate. Consequently, the catalytic destruction cycles largely cease operation and the ozone concentration builds back up toward its normal level a few weeks after the PSCs have disappeared and the vortex has ceased, as illustrated in Figure 2-2. Thus the ozone hole closes for another year, though the ozone levels nowadays never quite return to their natural levels, even in the fall. However, before the ozone levels build back up in the spring, some of the ozone-poor air mass can move away from the Antarctic and mix with surrounding air, temporarily lowering the stratospheric ozone concentrations in adjoining geographic regions, such as Australia, New Zealand, and the southern portions of South America.
Reactions That Create the Ozone Hole
In the lower stratosphere—the region where the PSCs form and chlorine is activated—the concentration of free oxygen atoms is small; few atoms are produced there on account of the scarcity of the UV-C light that is required to dissociate 02. Furthermore, any atomic oxygen produced in this way immediately collides with the abundant 02 molecules to form ozone, O3. Thus, ozone destruction mechanisms based upon the O3 + O-> 2 02 reaction, even when catalyzed, are not important here.
Rather, most of the ozone destruction in the ozone hole occurs via the process called Mechanism II in Chapter 1, with both X and X' being atomic chlorine and with the overall reaction being 2 03-> 3 C2- Thus the sequence starts with the reaction of chlorine with ozone:
In Figure 2-5 the experimental CIO and 03 concentrations are plotted as a function of latitude for part of the Southern Hemisphere during the spring of 1987. As anticipated, the two species display opposing trends, i.e., they anti-correlate very closely. At sufficient distances away from the South Pole (which is at 90°S), the concentration of ozone is relatively high and that of CIO is low, since chlorine is mainly tied up in inactive forms. However, as one travels closer to the pole and enters the vortex region, the concentration of CIO suddenly becomes high and simultaneously that of 03 falls off sharply (Figure 2-5): Most of the chlorine has been activated and most of the ozone has consequently been destroyed. The latitude at which the concentrations both change sharply marks the beginning of the ozone hole, which continues through to the region above the South Pole. The anticorrelation of ozone and CIO concentrations shown in Figure 2-5 was considered by researchers to
be the "smoking gun," proving that anthropogenic chlorine compounds such as CFCs emitted into the atmosphere were indeed the cause of ozone-hole formation.
In the next reaction in the sequence, two CIO free radicals, produced in two separate step 1 events, combine temporarily to form a nonradical dimer, dichloroperoxide, ClOOCl (or C1202):
The rate of this reaction becomes important to ozone loss under these conditions because the chlorine monoxide concentration rises steeply due to the activation of the chlorine. Once the intensity of sunlight has risen to an appreciable amount in the Antarctic spring, the dichloroperoxide molecule ClOOCl absorbs UV light and splits off one chlorine atom. The resulting ClOO free radical is unstable, so it subsequently decomposes (in about a day), releasing the other chlorine atom:
Step 2b: ClOOCl + UV light —► ClOO + CI Step 2c: ClOO-*02 + CI
Adding steps 2a, 2b, and 2c, we see that the net result is the conversion of two CIO molecules to atomic chlorine via the intermediacy of the dimer ClOOCl, which corresponds to the second stage of Mechanism II:
By these processes CIO returns to the ozone-destroying form of chlorine, CI. If we add the above reaction to two times step 1 (the factor of 2 being required to produce the two intermediate CIO species needed in reaction 2a so that none remains in the overall equation), we obtain the overall reaction
Thus a complete catalytic ozone destruction cycle exists in the lower stratosphere under these special weather conditions, i.e., when a vortex is present. The cycle also requires very cold temperatures, since under warmer conditions ClOOCl is unstable and reverts back to two CIO molecules before it can undergo photolysis, thereby short-circuiting any ozone destruction. Before appreciable sunlight becomes available in the early spring, most of the chlorine exists as CIO and C1202 since step 2b requires fairly intense light levels; such an atmosphere is said to be primed for ozone destruction.
About three-quarters of the ozone destruction in the Antarctic ozone hole occurs by the mechanism set forth above, in which chlorine is the only catalyst. This ozone destruction cycle contributes greatly to the creation of the ozone hole. Each chlorine destroys about 50 ozone molecules per day during the spring. The slow step in the mechanism is step 2a, which is the combination of 2 CIO molecules. Since the rate law for step 2a is second order in CIO concentration (i.e., its rate is proportional to the square of the CIO concentration), it proceeds at a substantial rate, and the destruction of ozone is significant, only when the CIO concentration is high. The abrupt appearance of the ozone hole is consistent with the quadratic rather than linear dependence of ozone destruction upon chlorine concentration by the C1202 mechanism. Let us hope that there are not many more environmental problems whose effects will display such nonlinear behavior and similarly surprise us!
A minor route for ozone destruction in the ozone hole involves Mechanism II with bromine as X' and chlorine as X (or vice versa). The CIO and BrO free-radical molecules produced in these processes then collide with each other and rearrange their atoms to eventually yield 02 and atomic chlorine and bromine. Write out the mechanism for this process, and add up the steps to determine the overall reaction.
Suppose that the concentration of chlorine continues to rise in the stratosphere but that the relative increase in bromine does not rise proportionately. Will the dominant mechanism involving dichloroperoxide or the "chlorine plus bromine" mechanism of Problem 2-1 become relatively more important or less important as the destroyer of ozone in the Antarctic spring?
Why is the mechanism involving dichloroperoxide of negligible importance in the destruction of ozone, compared to the mechanism that proceeds by CIO + O, in the upper levels of the stratosphere ?
In the lower stratosphere above Antarctica, an ozone destruction rate of about 2% per day occurs each September due to the combined effects of the various catalytic reaction sequences. As a result, by early October almost all the ozone is wiped out between altitudes of 15 and 20 km, just the region in which its concentration normally is highest over the South Pole. This result
0 5 10 15
Ozone abundance (mPa)
FIGURE 2-6 The typical vertical distribution of ozone over Antarctica in mid-spring (October) in 1962-1971 (black curve, before the ozone hole started), in the 1991-2001 period (dashed curve), and in 2001 (green curve). Ozone partial pressure is in millipascals, [Source: WMO/UNEP Scientific Assessment of Ozone Depletion 2006, Figure Q11-3.]
0 5 10 15
Ozone abundance (mPa)
is illustrated in Figure 2-6, which shows the measured ozone partial pressure as a function of altitude over the Antarctic in mid-spring in the years preceding the ozone-hole formation (black curve) and in 2001 (green curve). Notice that the depletion from 1.3 to 19 km was more complete in 2001 than on average in the preceding years (dashed curve).
In summary, the special vortex weather conditions in the lower stratosphere above the Antarctic in winter cause denitrifica-tion and lead to the conversion of inactive chlorine into Cl: and HOC1. These two compounds produce atomic chlorine when sunlight appears. The chlorine atoms efficiently destroy ozone via Mechanism II. Once the vortex disappears in the late spring, the ice particles on which the activation of chlorine compounds occurs disappear, the chlorine returns to inactive forms, and the hole heals.
The Size of the Antarctic Ozone Hole
Because (as explained later) the stratospheric concentration of chlorine continued to increase until the end of the twentieth century, the extent of Antarctic ozone depletion increased from the early 1980s at least until the late 1990s. There are several relevant measures of the extent of ozone depletion.
• One measure is the surface area covered by low overhead ozone; Figure 2-la shows the area within the 220-DU contour line for the mid-September to mid-October period as a function of year. This area grew rapidly and approximately linearly during the 1980s; the size of the hole in maximum depletion years (1998, 2006) is somewhat larger than in the 1980s, though smaller holes have appeared in some recent years.
• Similarly, the sharp decrease in the minimum amount of overhead ozone in the spring that occurred from 1978 to the late 1980s has been replaced by a slower decline, which now may have largely ceased (see Figure 2-lb).
• The average length of time during which ozone depletion occurs has also increased in recent years. Some reduction in ozone levels is now usually seen both in mid-winter (at least in the outer portions of the continent where
Ozone destruction step
Atomic chlorine reconstitution Mid-stratosphere Ozone hole/low stratosphere
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