Box 3.2 How Is Ozone, O3, Made and Destroyed in the Stratosphere?
What Determines the Ozone Abundance?
Ozone is produced via the photolysis of molecular oxygen by solar radiation: . The oxygen atoms combine with O2 to make O3: . Ozone itself is photolyzed to make O atoms, but most of the O atoms recombine with O2 to make ozone. However, a small fraction of the O atoms react with ozone to destroy ozone: . So, there is a natural balance in the stratosphere between the formation of ozone and its destruction. This chemical scheme, first proposed by S. Chapman,1 is referred to as the Chapman mechanism.
In addition to O atoms, other reactants, many of which are naturally occurring, can also destroy ozone. For example, OH and HO2 radicals, which are present in the stratosphere, can catalytically destroy ozone: , . Notice that in this sequence of reactions, OH and HO2 are not lost, but two molecules of ozone are destroyed. This is the concept of homogeneous gas-phase free radical catalysis and the reason that a small abundance of reactive species can destroy a large amount of ozone.
As with OH and HO2, chlorine and bromine can also destroy ozone. Examples include:,, whose net result is the destruction of one ozone molecule and an O atom, which would have ended up as an ozone molecule. The most important catalytic reactions involving bromine and chlorine together are , , , which leads to a net reaction: . Another such sequence, which involves bromine with a naturally occurring species, is , , , , which also destroys two molecules of ozone. These reactions involving bromine are especially effective in the lower stratosphere, where much of the observed ozone depletion occurs. In addition to these catalytic cycles, there are a very large number of cycles involving nitrogen oxides, hydrogenated species, and halogen species.
The balance between production, via photolysis of oxygen, and loss via catalytic ozone destruction cycles, described above, determines the mean level of ozone in the atmosphere. Since the production rate of ozone is essentially constant, any enhancements in the loss processes, such as introduction of bromine compounds into the stratosphere, will lead to a lower level of ozone.
thought previously.6 The change in the vertical distribution of ozone has been measured and the trend deduced.
The ozone depletion observed in the upper stratosphere (Figure 3.2) is consistent with the Rowland and Molina hypothesis. For the lower stratosphere, where the majority of the ozone loss has occurred, it is now clear that such reactions also take place in/on sulfuric acid aerosols, which are always present at low levels in the stratosphere. Volcanic eruptions can greatly enhance the number of these droplets and increase the effectiveness of bromine and chlorine in destroying ozone. Thus, following the eruption of Mt. Pinatubo in 1991, there was a measurable decrease in stratospheric ozone abundance. The large loss of ozone during the 1990s in the lower stratosphere can be attributed, in part, to the eruption of Mt. Pinatubo, which increased the number of particles on which heterogeneous chemistry can occur and helped make the connection between the role of halogen chemistry and ozone changes.7 A semi-quantitative understanding of the entire ozone loss has emerged. The release of man-made chlorine and bromine compounds is the primary cause of ozone depletion.
The Antarctic ozone hole, formed in the Antarctic stratosphere during the springtime, caught the attention of the atmospheric sciences community in the mid-1980s.8 During August and September 1987—the end of winter and beginning of spring in the Southern Hemisphere—aircraft equipped with many different instruments for measuring a large number of chemical species were flown repeatedly over Antarctica.9 Among the chemicals measured were ozone and chlorine oxide, the reactive chemical identified in the laboratory as one of the participants in ozone-destroying chain reactions. On the first flights southward from the southern tip of South America, relatively high concentrations of ozone were measured everywhere over Antarctica. By mid-September, however, the instruments recorded low