According to the terminology that we introduced in Chapter 1, the troposphere is the layer of the atmosphere that extends from the earth's surface up to an altitude of about 10 km. Above this and extending to an altitude of about 50 km lies the stratosphere. Essentially all of the clouds and greenhouse gases in the earth's atmosphere are contained in the troposphere. That is why in our examination of the greenhouse effect we were able to regard the top of the troposphere as being the top of the atmosphere. In this chapter we will look at the question of the effect of human activities on the ozone that resides in the stratosphere. This ozone has very little effect as a greenhouse gas. What it does do is to protect the troposphere and the underlying earth from ultraviolet radiation that is harmful to life.
Figure 9.1. Ozone is produced in the high stratosphere, stored in the lower stratosphere, and destroyed in the troposphere.
We can quickly outline the natural life cycle of ozone with reference to the simple schematic of Figure 9.1. Ozone is a gas that is produced in the upper stratosphere by the action of solar ultraviolet (UV) radiation on molecular oxygen. As shown in Fig. 2.3, UV radiation is characterized by short wavelengths (less than .3 microns) compared to visible light (roughly .5 microns). From this production region ozone moves downward and accumulates in the lower stratosphere, in what we will call the ozone storage zone. The storage there is a temporary one because ozone is continually leaking out of the bottom of this reservoir into the troposphere. There it is caught up in downward air currents associated with weather systems and brought into contact with the earth's surface, where it is destroyed. The troposphere thereby serves as the sink of stratospheric ozone.
Figure 9.2. The distribution of ozone around the globe is uneven. More ozone molecules are located near the poles than near the equator.
The global distribution of ozone is illustrated by Fig. 9.2. The dots represent ozone molecules (there are, of course, many more than can be put onto the diagram). What we see here is the ozone storage zone. The troposphere below it (the ozone sink) appears as a clear region because it contains very little ozone. We see in this diagram another observed feature of the global ozone distribution: there is much more ozone over the polar regions than over the tropics. The diagram in Fig. 9.3 can help us to understand why.
Figure 9.3. Ozone is produced in the high stratosphere of the equatorial belt and transported poleward and downward by the prevailing wind currents.
Almost all of the ozone produced in the upper stratosphere is produced in the tropics, because this is where the solar radiation, including its UV component, is most intense. As the newly produced ozone moves downward to the ozone storage zone in the lower stratosphere, it also moves outward toward the polar regions of both hemispheres. That is why more of it is stored there than in tropical latitudes. To find out what is driving this downward and poleward transport of ozone, we refer you back to the discussion of the atmospheric heat engine in Section 3.1. In that context, the atmosphere tends to have rising motion in the tropics, poleward motion at high levels, and sinking motion in the polar regions as a result of the necessity of maintaining the energy balance of the climate system. Figure 9.3 depicts one of the deep convective clouds that are so common all through the year in the tropical troposphere. Inside these clouds is a very strong upward current of air that in many cases punches through the tropopause, as the division between the troposphere and stratosphere is called. (This is similar to the more dramatic but much rarer event of major volcanic eruption that we considered in Section 4.1.) The statistical effect of these convective cloud events over the whole of the tropics constitutes an intrusion of tropospheric air into the stratosphere. It is this constant pumping action from below that drives an upward air current that extends all around the equatorial belt. Air in this slow but persistent current moves upward in the equatorial lower stratosphere and reaches as high as the ozone production region before turning poleward and gradually sinking in both hemispheres. It is this air current, reminiscent of a huge fountain, which packs ozone into a storage zone over the polar regions.
Now we will consider some of the details of ozone production. In this discussion we can use the earlier simple diagram (Fig. 9.1) that illustrates the zones of production, storage, and disappearance of ozone. At the top of the production zone, there is some high energy UV radiation ( [[lambda]] < .190 micron) which attacks molecular oxygen throughout the ozone production zone. Here we use the wavelength ([[lambda]]) as a means of labelling radiation, just as we did earlier in the course when considering infrared radiation. The attack converts some of the molecular oxygen (O2) into atomic oxygen (O) as shown in Fig. 9.4. Using the chemical notation developed in Section 5.2, we can write this process symbolically as:
UV radiation + O2 -------> O + O
Figure 9.4. Oxygen molecules bombarded by UV radiation split apart into 2 oxygen atoms.
The process is a statistical one in the sense that not all of the O2 molecules which find themselves in this environment of high energy ambient UV are victims. The newly formed O atoms can combine with other O2 molecules to form ozone molecules (O3), as illustrated schematically in Fig. 9.5 and in terms of a chemical equation by the following:
O + O2 -----> O3
Figure 9.5. An atom of oxygen may combine with a molecule of oxygen to form ozone.
Now, ozone turns out to be susceptible to destruction by slightly less energetic UV radiation (namely, that having wavelengths between .190 microns and .290 microns). We then have the following chemical reaction occurring:
UV radiation + O3 ------> O2 + O
Schematically, this process is shown in Fig. 9.6.
Figure 9.6. Ozone can be destroyed by UV radiation to form a molecule of oxygen and an atom of oxygen.
Again, the process is statistical: not all O3 which has been brought into this hazardous environment of UV will succumb.
The net result of all this is
i) to produce a mixture of O and O3 in the region between 25 and 50 km,
ii) to absorb all of the UV with [[lambda]] < .19 microns and much of the UV with [[lambda]] between .19 and .29 microns in the region between 25 and 50 km.
Now, as we have seen earlier in this section, there is a steady export downward below 25 km of O3 created in the production zone . The region into which it goes can be called the ozone storage zone, because there is so much of it there and because the UV that attacks ozone has been so depleted that any given molecule has a good probability of living on in peace until finally being exported to the troposphere, where it will quickly be swept down to the ground and destroyed through a variety of chemical reactions.
It is very risky to introduce chlorine into the ozone production region. If this happens, ozone can be depleted rapidly as a result of the chemical reaction shown in Fig. 9.7.
Figure 9.7. The presence of chlorine (Cl) atoms in the stratosphere can lead to the destruction of ozone through a sequence of chemical reactions.
We can describe this chain reaction as follows: in a mixture of ozone and atomic oxygen, chlorine will take one of each, form two oxygen molecules and be free to repeat the chain reaction again. This is an example of a catalytic reaction. We will refer to this reaction as the classic chlorine catalytic cycle. In chemical notation, we can write this cycle as:
Cl + O3 ------> ClO + O2
ClO + O ------> Cl + O2
It has been calculated that this reaction can occur 100,000 times before the chlorine atom bonds tightly to other molecules and stops the process. Two reactions commonly lead to this bonding and removal of chlorine as follows:
Methane (CH4) + Chlorine (Cl) ---> Hydrogen Chloride (HCl) + CH3
Chlorine Monoxide (ClO) + Nitrogen Dioxide (NO2) ---> Chlorine Nitrate (ClONO2)
The two compounds, hydrogen chloride and chlorine nitrate, are reservoirs of chlorine. They represent stable (non-reactive) compounds that contain chlorine as opposed to the highly reactive molecule, chlorine monoxide.
Chlorine is being introduced into the stratosphere by means of the man-made chemicals known as chloroflurocarbons (CFCs). As discussed in Section 6.3, the concentration of CFCs in the atmosphere has increased sharply over the past several decades. One example of a CFC is the compound CFCl3, in which a carbon atom, a fluorine atom, and three chlorine atoms are joined together. These CFC molecules can be altered only when bombarded by high energy UV, such as that found in the ozone production layer above 25 km. What happens is that a single chlorine gets stripped off by the UV radiation, and enters the classic chlorine catalytic cycle referred to above. However, the situation is not as bad as it may seem. It is true that chlorine released like this can and has decreased ozone in the ozone production zone. However, the vast majority of stratospheric ozone resides not in the production zone but lower in the storage zone. There is very little short wavelength UV radiation to strip chlorine off the CFCs at that level since most of the UV has been absorbed above in the ozone production zone. Even if there were a source of chlorine there, the classic catalytic cycle wouldn't take place because that cycle depends also on atomic oxygen being present, and that is found almost exclusively in the ozone production layer. Thanks to these mitigating factors, the worst case scenario has always held that chlorine from CFCs would never amount to more than a 5% reduction in the total amount of stratospheric ozone. This was the prevailing thinking at the time of discovery (in 1985) of the ozone hole.
We will show in class a transparency which gives the chronology of the rise and fall of concern over the effect of CFCs on stratospheric ozone up until the discovery of the ozone hole. Throughout this period it was widely believed by the "scientific community" that ozone in the ozone storage layer was immune from the effects of the chlorine that has been introduced into the stratosphere by means of the global human production of CFCs. The ideas which formed the basis for this belief are easy to understand, and we review them again here. First of all, for reasons already noted, the classic chlorine catalytic cycle is effective in mopping up ozone only in the ozone production region, where there is relatively little ozone present to begin with. The fate of the chlorine that is liberated from CFCs (after destroying a modest amount of ozone, which would amount to at most a decrease of 5% in the global total) is to become locked up in one of the two chlorine reservoir species (ClONO2 and HCl). Just like ozone, these compounds are exported downward from the ozone production zone and into the ozone storage zone. There it was thought that the reservoir species could coexist with ozone.
Now we must make explicit mention of what process it is that brings CFCs up from the troposphere and into the upper stratosphere where the chlorine they contain can be liberated. This mechanism is none other than the circulation which we analyzed in Section 9.1 and likened there to a huge fountain. There is sufficient UV radiation at the levels of the ozone production zone to break down the CFCs and release chlorine. This chlorine destroys ozone via the classic chlorine catalytic cycle. Eventually each chlorine atom will get locked up in one of the chlorine reservoir species. These species (HCL and CLONO2) then get carried down into the ozone storage zone and outward toward the polar regions by the circulation.
The term "ozone hole" or "hole in the ozone layer" refers to a region of very low ozone content that develops every year and lasts for a few months. The region where this occurs is centered on the South Pole and has an area comparable to that of the United States. The ozone hole is not evident in Fig. 9.2, which shows only the natural distribution of ozone. The hole first started forming in the 1970s, but was not detected until 1985. The following figure (Fig. 9.8) illustrates the point. It shows measurements of the total amount of ozone present in the atmosphere over an observing station (Halley Bay) in Antarctica. The unit of measurement of ozone content is the "Dobson unit." We won't worry here about how much ozone corresponds to a Dobson-unit's worth; what matters is the relative size of the numbers.
The top trace in Fig. 9.8 shows the variation of ozone with time of year
averaged over the years 1957-73. This was before the ozone hole started to
develop. The behavior of ozone during the period 1980-84 is shown by the
broken trace. We see that ozone starts to drop in August, and is dropping
rapidly by the time that Southern Hemisphere spring arrives on September 23.
The lowest ozone amount occurs in October. Thereafter comes a gradual rise to
almost-normal levels (as defined by the 1957-73 average). The lower trace is
for 1990-91. It shows that the ozone hole is worse than it was (on average) in
the years 1980 through 1984. Note in particular, that the ozone is not
recovering as well as it used to (1980-84) after the October minimum and
consequently takes a longer time to get back up to almost-normal levels.
Figure 9.8. The total amount of ozone observed historically (1957-73) at Halley Bay, Antarctica is shown by a solid line as a function of time of year. The amount of ozone observed during 1980-84 (dashed line) and 1990 (dashed-dot line) is also shown.
Figure 9.9. On October 6, 1993, an instrument that measures ozone found unprecedented ozone losses (white) over Antarctica.
An idea of the spatial configuration of the ozone hole is afforded by Fig. 9.9. This is a map obtained by ozone measuring instruments aboard a satellite on October 6, 1993. The central core of the hole, where ozone has fallen to 100 Dobson units, appears white in the figure. The outline of Antarctica is clearly visible. Over most of it, ozone content is less than 130 Dobson units, as indicated by the light gray shading surrounding the central core. Successively darker shades of gray mark the increase of ozone to levels in excess of 300 Dobson units at lower latitudes.
Any theory (or explanation) of the ozone hole must address the facts that i) it
is confined to a region of the globe centered on the south Pole and ii) it
develops rapidly in September, reaches a peak in October, and starts filling up
again thereafter. A key relation between this timing and this geographical
location is illustrated in Fig. 9.10. It is that the region is in constant
darkness for several months before the ozone hole starts to form around the
first of September.
Figure 9.10. A view of the earth from a perspective above the South Pole. Note that the South Pole receives no sunlight from 21 March to 23 September.
We now outline the currently accepted theory for the formation of the ozone hole. First of all, it gets so cold during the Southern Hemisphere polar night that clouds form in the stratosphere. The most favorable place for these ice crystal clouds is right in the middle of the ozone storage region (roughly the layer between 15 and 25 km above the ground). In this layer we find not only ozone but also chlorine safely locked up (it was thought) in the reservoir species hydrogen chloride (HCl) and chlorine nitrate (ClONO2). Apparently, however, both of these species collect on the surfaces of the ice crystals in the clouds. These crystals are partly ice (H2O) and partly frozen nitric acid (HNO[-]3). On these surfaces the two reservoir species react with each other, and the net result of this is to liberate a chlorine molecule (Cl2) into the atmosphere. All of this takes place during the months of the polar night.
By the time the long polar night ends, a large fraction of the chlorine atoms (Cl) that had been locked up in the two reservoir species is now back floating around in the ozone storage region over Antarctica in the form of molecular chlorine (Cl2). But, in molecular form the two chlorine atoms keep each other busy, with the result that Cl2 does not react with ozone. But when the daylight comes back at the end of the polar night, solar radiation separates the chlorine molecule into two chlorine atoms.
The newly created chlorine atoms begin right away to destroy ozone. This time, the classic chlorine catalytic cycle is not the mechanism of destruction. That cycle requires the presence of atomic oxygen, and there is very little of that around in the ozone storage region (as compared to the ozone production region where, as we saw, the classic cycle works very well). Rather, there is a different catalytic cycle that comes into play. We do not need to go into the details of this new cycle. It is sufficient to note that, as in any catalytic cycle, the active ingredient can go around many times in the cycle before it finally gets locked up in some other compound.
Eventually chlorine atoms get used up and the ozone destruction stops, usually by the end of October. Then the ozone hole starts to fill in. Remember that the ozone hole is confined to an area roughly the size of the United States, but centered on the South Pole. The ozone that is filling up the hole is being brought from the equatorial regions (where ozone is produced) by the slow moving atmospheric current system that we discussed earlier. We note that this replacement of lost ozone is apparently not efficient enough to bring the ozone up to its normal level (see Fig. 9.8).
The ozone hole does represent a net loss of total atmospheric ozone over the year, and that is cause for concern. Fortunately, the hole is confined to a relatively small fraction of the globe where there is a very low human population (only the scientists). Also, it is fairly temporary, with the worst part only lasting for a couple of months each year. But that minimum is very drastic: more than a 50% reduction of ozone. This is a reduction ten times larger than the 5% reduction that was thought to be the very worst that stratospheric chlorine could possibly do.
1. Explain why ozone is produced and destroyed at different levels in the atmosphere.
2. Why is ozone stored more at the poles and less at the equator?
3. Why does little short wavelength ([[lambda]] < .19 microns) UV radiation reach the surface of the earth?
4. Describe briefly the classic chlorine catalytic reaction.
5. How do chlorine molecules enter the stratosphere?
6. Why does the ozone hole vary in intensity as a function of time of year?
7. What other molecules, besides chlorine and ozone, are required in the spring season over the South Pole to cause the destruction of ozone?
8. Which effect of the rising concentration of CFCs do you think is more important: (1) the greenhouse effect of the CFCs or (2) the depletion of stratospheric ozone? Why?