Ozone depletion

Ozone depletion, gradual thinning of Earth’s ozone layer in the upper atmosphere caused by the release of chemical compounds containing gaseous chlorine or bromine from industry and other human activities. The thinning is most pronounced in the polar regions, especially over Antarctica. Ozone depletion is a major environmental problem because it increases the amount of ultraviolet (UV) radiation that reaches Earth’s surface, which increases the rate of skin cancer, eye cataracts, and genetic and immune system damage. The Montreal Protocol, ratified in 1987, was the first of several comprehensive international agreements enacted to halt the production and use of ozone-depleting chemicals. As a result of continued international cooperation on this issue, the ozone layer is expected to recover over time.

History

In 1969 Dutch chemist Paul Crutzen published a paper that described the major nitrogen oxide catalytic cycle affecting ozone levels. Crutzen demonstrated that nitrogen oxides can react with free oxygen atoms, thus slowing the creation of ozone (O₃), and can also decompose ozone into nitrogen dioxide (NO₂) and oxygen gas (O₂). Some scientists and environmentalists in the 1970s used Crutzen’s research to assist their argument against the creation of a fleet of American supersonic transports (SSTs). They feared that the potential emission of nitrogen oxides and water vapour from these aircraft would damage the ozone layer. (SSTs were designed to fly at altitudes coincident with the ozone layer, some 15 to 35 km [9 to 22 miles] above Earth’s surface.) In reality, the American SST program was canceled, and only a small number of French-British Concordes and Soviet Tu-144s went into service, so that the effects of SSTs on the ozone layer were found to be negligible for the number of aircraft in operation.

In 1974, however, American chemists Mario Molina and F. Sherwood Rowland of the University of California at Irvine recognized that human-produced chlorofluorocarbons (CFCs)—molecules containing only carbon, fluorine, and chlorine atoms—could be a major source of chlorine in the stratosphere. They also noted that chlorine could destroy extensive amounts of ozone after it was liberated from CFCs by UV radiation. Free chlorine atoms and chlorine-containing gases, such as chlorine monoxide (ClO), could then break ozone molecules apart by stripping away one of the three oxygen atoms. Later research revealed that bromine and certain bromine-containing compounds, such as bromine monoxide (BrO), were even more effective at destroying ozone than were chlorine and its reactive compounds. Subsequent laboratory measurements, atmospheric measurements, and atmospheric-modeling studies soon substantiated the importance of their findings. Crutzen, Molina, and Rowland received the Nobel Prize for Chemistry in 1995 for their efforts.

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Human activities have had a significant effect on the global concentration and distribution of stratospheric ozone since before the 1980s. In addition, scientists have noted that large annual decreases in average ozone concentrations began to occur by at least 1980. Measurements from satellites, aircraft, ground-based sensors, and other instruments indicate that total integrated column levels of ozone (that is, the number of ozone molecules occurring per square metre in sampled columns of air) decreased globally by roughly 5 percent between 1970 and the mid-1990s, with little change afterward. The largest decreases in ozone took place in the high latitudes (toward the poles), and the smallest decreases occurred in the lower latitudes (the tropics). In addition, atmospheric measurements show that the depletion of the ozone layer increased the amount of UV radiation reaching Earth’s surface.

This global decrease in stratospheric ozone is well correlated with rising levels of chlorine and bromine in the stratosphere from the manufacture and release of CFCs and other halocarbons. Halocarbons are produced by industry for a variety of uses, such as refrigerants (in refrigerators, air conditioners, and large chillers), propellants for aerosol cans, blowing agents for making plastic foams, firefighting agents, and solvents for dry cleaning and degreasing. Atmospheric measurements have clearly corroborated theoretical studies showing that chlorine and bromine released from halocarbons in the stratosphere react with and destroy ozone.

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Antarctic ozone hole

The most severe case of ozone depletion was first documented in 1985 in a paper by British Antarctic Survey (BAS) scientists Joseph C. Farman, Brian G. Gardiner, and Jonathan D. Shanklin. Beginning in the late 1970s, a large and rapid decrease in total ozone, often by more than 60 percent relative to the global average, has been observed in the springtime (September to November) over Antarctica. Farman and his colleagues first documented this phenomenon over their BAS station at Halley Bay, Antarctica. Their analyses attracted the attention of the scientific community, which found that these decreases in the total ozone column were greater than 50 percent compared with historical values observed by both ground-based and satellite techniques.

As a result of the Farman paper, a number of hypotheses arose that attempted to explain the Antarctic “ozone hole.” It was initially proposed that the ozone decrease might be explained by the chlorine catalytic cycle, in which single chlorine atoms and their compounds strip single oxygen atoms from ozone molecules. Since more ozone loss occurred than could be explained by the supply of reactive chlorine available in the polar regions by known processes at that time, other hypotheses arose. A special measurement campaign conducted by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) in 1987, as well as later measurements, proved that chlorine and bromine chemistry were indeed responsible for the ozone hole, but for another reason: the hole appeared to be the product of chemical reactions occurring on particles that make up polar stratospheric clouds (PSCs) in the lower stratosphere.

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During the winter the air over the Antarctic becomes extremely cold as a result of the lack of sunlight and a reduced mixing of lower stratospheric air over Antarctica with air outside the region. This reduced mixing is caused by the circumpolar vortex, also called the polar winter vortex. Bounded by a stratospheric jet of wind circulating between approximately 50° and 65° S, the air over Antarctica and its adjacent seas is effectively isolated from air outside the region. The extremely cold temperatures inside the vortex lead to the formation of PSCs, which occur at altitudes of roughly 12 to 22 km (about 7 to 14 miles). Chemical reactions that take place on PSC particles convert less-reactive chlorine-containing molecules to more-reactive forms such as molecular chlorine (Cl₂) that accumulate during the polar night. (Bromine compounds and nitrogen oxides can also react with these cloud particles.) When day returns to Antarctica in the early spring, sunlight breaks the molecular chlorine into single chlorine atoms that can react with and destroy ozone. Ozone destruction continues until the breakup of the polar vortex, which usually takes place in November.

A polar winter vortex also forms in the Northern Hemisphere. However, in general, it is neither as strong nor as cold as the one that forms in the Antarctic. Although polar stratospheric clouds can form in the Arctic, they rarely last long enough for extensive decreases in ozone. Arctic ozone decreases of as much as 40 percent have been measured. This thinning typically occurs during years when lower-stratospheric temperatures in the Arctic vortex have been sufficiently low to lead to ozone-destruction processes similar to those found in the Antarctic ozone hole. As with Antarctica, large increases in concentrations in reactive chlorine have been measured in Arctic regions where high levels of ozone destruction occur.