We have seen that tropospheric or ground-level pollution has been shown to contribute to a number of problems in the natural world, to exacerbate asthma and breathing difficulties in humans, and to cause cancer. Now we turn to the effects of certain pollutants in the stratosphere that have a substantial impact on the health of humans and ecosystems. In the troposphere, ozone is an oxidant that can harm respiratory systems in animals and damage a number of structures in plants. However, in the stratosphere ozone forms a necessary, protective shield against radiation from the Sun; it absorbs ultraviolet light and prevents harmful ultraviolet radiation from reaching Earth.

Learning Objectives

After reading this module, you should be able to

The Sun radiates energy at many different wavelengths, including the ultraviolet range (see FIGURE 5.1 on page 45). The ultraviolet wavelengths are further classified into three groups: UV-A, or low-energy ultraviolet radiation, and the shorter, higher-energy UV-B and UV-C wavelengths. UV radiation of all types can damage the tissues and DNA of living organisms. Exposure to UV-B radiation increases the risk of skin cancer and cataracts, and suppresses the immune system in humans. Exposure to UV-B is also harmful to the cells of plants, and it reduces their ability to convert sunlight into usable energy. UV-B exposure can therefore harm entire biological communities. For example, losses of phytoplankton—the microscopic algae that form the base of many marine food chains—will cause the depletion of fisheries.

As we saw in Chapter 4, a layer of ozone in the stratosphere (see FIGURE 9.1 on page 106) absorbs ultraviolet radiation, filtering out harmful UV rays from the Sun. It is easy to confuse stratospheric ozone with tropospheric, or ground-level, ozone that we discussed earlier in this chapter because it is the same gas, O₃. However, stratospheric ozone occurs higher in the atmosphere where its ability to absorb ultraviolet radiation and thereby shield the surface below makes stratospheric ozone critically important to life on Earth. In this section we will examine the formation of stratospheric ozone and look at how it breaks down.

Formation of Stratospheric Ozone

When solar radiation strikes O₂ in the stratosphere, 16 to 50 km (10–31 miles) above Earth’s surface, a series of chemical reactions begins that produces a new molecule: ozone (O₃).

In the first step, UV-C radiation breaks the molecular bond holding an oxygen molecule together:

O₂ + UV-C → O + O

This happens to only a few oxygen molecules at any given time. The vast majority of the oxygen in the atmosphere remains in the form O₂.

In the second step, a free oxygen atom (O) produced in the first reaction encounters an oxygen molecule, and they form ozone.

O + O₂ → O₃

Both UV-B and UV-C radiation can break a bond in this new ozone molecule, forming molecular oxygen and a free oxygen atom once again:

O₃ + UV-B or UV-C → O₂ + O

Thus formation of ozone in the presence of sunlight and its subsequent breakdown is a cycle that can occur indefinitely as long as there is UV energy entering the atmosphere. Under normal conditions, the amount of ozone in the stratosphere remains at steady state.

Breakdown of Stratospheric Ozone

We rely on refrigeration to keep our foods safe and edible, and on air conditioning to keep us comfortable in hot weather. For many years, the same chemicals that made refrigeration and air conditioning possible were also used in a host of other consumer items, including aerosol spray cans and products such as Styrofoam. These chemicals, called chlorofluorocarbons, or CFCs, were considered essential to modern life, and producing them was a multibillion-dollar industry. CFCs were considered “safe” because they are both nontoxic and nonflammable. But it turned out that these chemicals had adverse effects in one part of the upper atmosphere, the stratosphere, by promoting the breakdown of ozone.

CFCs introduce chlorine (Cl) into the stratosphere. When chlorine is present, it can attach to an oxygen atom in an ozone molecule, thereby breaking the bond between that atom and the molecule and forming chlorine monoxide (ClO) and O₂:

O₃ + Cl → ClO + O₂

Subsequently, the chlorine monoxide molecule reacts with a free oxygen atom, which pulls the oxygen from the ClO to produce free chlorine again:

ClO + O → Cl + O₂

When we consider these reactions together, we see that chlorine starts out and ends up as a free Cl atom. In contrast, an ozone molecule and a free oxygen atom are converted into two oxygen molecules. A substance that aids a reaction but does not get used up itself is called a catalyst. A single chlorine atom can catalyze the breakdown of as many as 100,000 ozone molecules until finally one chlorine atom finds another and the process is stopped. In the process, the ozone molecules are no longer available to absorb incoming UV-B radiation. As a result, the UV-B radiation can reach Earth’s surface and cause harm to biological organisms.

Ozone formation and ozone destruction have occurred for many years. But the use of CFCs as refrigerants starting in the 1920s and their increased use since that time led to the more rapid destruction of stratospheric ozone. After decades of CFC use, its effects became apparent.

Depletion of the Ozone Layer

In the mid-1980s, atmospheric researchers noticed that stratospheric ozone in Antarctica had been decreasing each year, beginning in about 1979. Since the late 1970s, global ozone concentrations had decreased by more than 10 percent. Depletion was greatest at the poles, but occurred worldwide. One study from Switzerland showed an erratic but clearly decreasing trend of ozone concentrations since 1970. The graph in FIGURE 49.1 shows the results of this study.

Researchers also determined that, in the Antarctic, ozone depletion was seasonal: Each year the depletion occurred from roughly August through November (late winter through early spring in the Southern Hemisphere).The depletion caused an area of severely reduced ozone concentrations over most of Antarctica, creating what has come to be called the “ozone hole.” A depletion of ozone also occurs over the Arctic in January through April, but it is not as severe, varies more from year to year, and does not cause a “hole” as in the Antarctic.

The cause of the formation of the ozone hole, which has received a great deal of media attention and has been studied intensively, is complex. It appears that extremely cold weather conditions during the polar winter cause a buildup of ice crystals mixed with nitrogen oxide. This in turn provides the perfect surface for the formation of the stable molecule Cl₂, which accumulates as atmospheric chlorine interacts with the ice crystals. When the Sun reappears in the spring, UV radiation breaks down this molecule into Cl again, which in turn catalyzes the destruction of ozone as described above. Because almost no ozone forms in the dark of the polar winter, a large “hole” occurs. Only after the temperatures warm up and the chlorine gets diluted by air coming from outside the polar region does the hole diminish. In contrast, the overall global trend of decreasing stratospheric ozone concentration is not related to temperature, but is caused by the breakdown reactions described earlier that result from increased concentrations of chlorine in the atmosphere.

Decreased stratospheric ozone has increased the amount of UV-B radiation that reaches the surface of Earth. A United Nations study showed that in mid-latitudes in North America, UV radiation at the surface of Earth increased about 4 percent between 1979 and 1992, although the increase is greater closer to the poles. For plants, both on land and in water, increasing exposure to UV-B radiation can be harmful to cells and can reduce photosynthetic activity, which could have an adverse impact on ecosystem productivity, among other things. In humans, particularly those with light skin, increasing exposure to UV-B radiation is correlated with increased risks of skin cancer, cataracts, and other eye problems, and with a suppressed immune system. Significant increases in skin cancers have already been recorded, especially in countries near the Antarctic ozone hole such as Chile and Australia.

In response to the decrease in stratospheric ozone, 24 nations in 1987 signed the Montreal Protocol on Substances That Deplete the Ozone Layer. This was a commitment to reduce CFC production by 50 percent by the year 2000. It was the most far-reaching environmental treaty to date, in which global CFC exporters like the United States appeared in some ways to prioritize the protection of the global biosphere over their short-term economic self-interest. More than 180 countries eventually signed a series of increasingly stringent amendments that required the elimination of CFC production and use in the developed world by 1996. In total, the protocol addressed 96 ozone-depleting compounds.

Because of these efforts, the concentration of chlorine in the stratosphere has stabilized at about 5 ppb (parts per billion) and should fall to about 1 ppb by 2100. The chlorine concentration reduction process is slow because CFCs are not easily removed from the stratosphere and in some recent years ozone depletion has continued to reach record levels. However, with the leveling off of chlorine concentrations, stratospheric ozone depletion should decrease in subsequent decades. The number of additional skin cancers should eventually decrease as well, although this effect will take some time due to the long time it takes for these cancers to develop.