Ozone Layer Depletion

Typically, stratospheric ozone (O₃) concentrations are about 0.2–0.4 ppm (parts per million), compared with about 0.03 ppm in unpolluted situations close to ground level in the troposphere. Stratospheric ozone concentrations are also measured in Dobson units (DU). A Dobson unit is equivalent to the amount of ozone that, if accumulated from the entire atmosphere and spread evenly over the surface of the earth at a pressure of one atmosphere and a temperature of about 68°F (20°C), would occupy a thickness of 10 mm (0.01 m or 0.4 in). Typically, stratospheric zone occurs at a concentration of about 350 DU, equivalent to a layer of only 3.5 mm (0.14 in).

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet radiation.

Molecular oxygen (O₂) interacts with ultraviolet radiation and splits into oxygen atoms (O) (reaction 1), which either recombine to form O₂ (reaction 2), or combine with O₂ to form O₃ (reaction 3). Once formed, the ozone can be consumed by various reactions, including a photodissociation involving ultraviolet radiation (reaction 4), or reactions with trace gases such as nitric oxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O), or with simple molecules or ions of the halogens, including chlorine (reaction 5), bromine, and fluorine. At any time, the formation and consumption of ozone proceed simultaneously. The actual concentration of ozone is a net function of the rates of reactions by which it is formed, and the rates of the reactions that consume this gas. The halogens catalyze the dissociation reaction; in other words, they increase the rate of ozone degradation without themselves being consumed in the chemical reaction. This means that they are available after one reaction to catalyze thousands of other such reactions. The increase of halogens present in the upper atmosphere has caused a shift in the equilibrium between the reactions toward an increased rate of ozone depletion.

The concentration of ozone in the stratosphere naturally varies with latitude and with time. Rates of ozone formation are largest over the equatorial regions of Earth because solar radiation is most intense over those latitudes. However, stratospheric winds carry tropical ozone to polar latitudes, where it tends to accumulate. On average, ozone concentrations are about 450 DU over sub-polar regions, and 250 DU over the tropics. Ozone concentrations can be as large as 600 DU during the wintertime maximum over the Antarctic. Daily variation in ozone can change by as much as 60 DU at higher latitudes. Seasonal variation at high latitudes can also be great, as much as 125 DU between spring and summer. Because ozone formation is driven by UV light, there is a small effect of the 11-year solar sunspot cycle on ozone. Intensified solar activity can also affect ozone concentration, and although these fluctuations can be intense, they usually do not persist. Although volcanic eruptions were once thought to contribute to ozone depletion, they are now considered a minor influence. Eruptions cause changes in the concentration of ozone by injecting aerosols into the atmosphere, which provide a surface on which ozone destruction reactions can occur quickly. These effects, however, are small and very short-lived. For example, the eruption of Mount Pinatubo in the Phillipines in 1991 injected 30 million tons of aerosol into the atmosphere, all of which was depleted in 11 months.

Human activities have resulted in large increases in emissions of some ozone consuming substances or their precursors into the atmosphere. As a result, there are concerns about potential changes in the dynamic equilibria among the stratospheric ozone reactions, which could result in decreases in ozone concentration.

The first concerns about depletion of stratospheric ozone were raised in the 1960s. At that time, a number of scientists suggested that emission of water vapor and various other chemicals from high-flying military jets and rockets might cause a consumption of stratospheric ozone. These discussions intensified during the early 1970s, when there were proposals to develop fleets of supersonic aircraft flying in the stratosphere. (Mostly for economic reasons, this capital-expensive commercial venture did not materialize.) Some scientists additionally suggested that emissions of oxides of nitrogen from vehicles and agricultural practices might also have some effect on the ozone layer, as could emissions associated with launchings of space shuttles and other spacecraft.

Since about the mid-to-late 1970s, there has been evidence of large decreases in the concentrations of stratospheric ozone at polar latitudes during the late winter to early springtime. The term used to describe these phenomena is ozone "holes." These seasonal occurrences are most noticeable over the Antarctic, where the ozone holes develop between September and November when the stratosphere is intensely cold, but sunlight is intense. The first convincing evidence of ozone holes was obtained over Antarctica in 1984, when the average ozone concentration in October was 180 DU, compared with 300 DU in the early 1970s. In November 1999, the ozone concentration was down to 165 DU. These sorts of observations stimulated a re-examination of earlier data from satellites and other observation systems, which suggested that the ozone holes have existed since at least the 1970s.

The Antarctic ozone holes typically develop at altitudes of 7.4–16 mi (12–25 km). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30–40%. However, in some years the decrease in ozone has been over 60%. In the worst years, the ozone concentration over Antarctica was only 120 DU. In October 1999, the ozone concentrations were less than 50% of what they were in the 1960s.

The immediate cause of the depletions of stratospheric ozone is thought to involve atoms of chlorine or simple compounds such as chlorine monoxide (ClO). However, these chemicals are thought to have an indirect origin through human activities, especially the emission of CFCs to the atmosphere. Once formed in the stratosphere by the degradation of a CFC molecule, a single chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere.

The occurrence of seasonal ozone holes is restricted to high-latitude regions, especially over Antarctica, and to a lesser degree over the Arctic. However, concentrations of stratospheric ozone can also be affected at lower latitudes, although the ozone depletion is relatively small. This happens during the late springtime, when the normal lower-latitude ozone concentrations are diluted by ozone-depleted polar air that becomes widely dispersed as the ozone holes break up and dissipate. Ozone layer depletions have been observed over North America, Europe, Asia, most of Africa, Australia, and South America. Ozone levels over the United States have fallen 5–10%, depending on the season. One study estimated that seasonal concentrations of stratospheric ozone over mid-latitudes of the Southern Hemisphere may have decreased by 3–8%. It has been hypothesized that global warming might be one cause of the seasonal ozone loss in the Southern Hemisphere.

An atmosphere with a relatively intact ozone layer (left) compared to one with an ozone layer compromised by chlorofluorocarbon (CFC) emissions. Illustration by Hans & Cassidy. Courtesy of Gale Group.