Ozone Depletion and Ozone Holes

Ozone is one of the natural gases in the atmosphere. It absorbs ultraviolet radiation from the sun. Chlorofluorocarbons, once used as primary refrigerants, aerosol propellants, and in the manufacturing of certain plastics, react in the atmosphere and release chlorine atoms. These then react and destroy molecules of ozone. When ozone depletion continues, scientists warn that depletion of stratospheric ozone will allow incoming ultraviolet radiation from the sun to reach the surface, resulting in severe damage to all living organisms. The term “ozone hole” refers to the seasonal decrease in stratospheric ozone concentration occurring over Antarctica. Ozone-hole formation is evidence that human activities can significantly alter the composition of the atmosphere.

Concentration of Ozone in Atmosphere

Ozone, although only a minor component of the atmosphere, plays a vital role in the survival of life on Earth. Ozone molecules absorb incoming high-energy ultraviolet (UV) light from the sun. Absorption of ultraviolet light in the stratospheric ozone layer, a region that contains the maximum concentration of atmospheric ozone, though only about 12 parts per million, prevents most UV light from reaching the surface of the planet. If none of the sun’s ultraviolet radiation was blocked by the ozone layer, it would be difficult for most forms of life, including humans, to survive on land.

The concentration of ozone in the atmosphere is highly variable, changing with altitude, geographic location, time of day, time of year, and prevailing local atmospheric conditions. Long-term fluctuations in ozone concentration are also seen, some of which are related to the solar sunspot cycle. While long-term average ozone concentrations are relatively stable, short-term fluctuations of as much as 10 percent in total column abundance of ozone as a result of the natural variability in ozone concentration are often observed.

Beginning in the early 1970s, a new and unexpected decrease in stratospheric ozone concentration was first observed. The decrease was localized near Antarctica, and appears in early spring, or September in the Southern Hemisphere. The initial decrease in ozone was small, but by 1980, decreases in total column abundance of ozone of as much as 30 percent were being recorded, well outside the range of variation expected as a result of random fluctuations. This seasonal depletion of stratospheric ozone above Antarctica, which by 1990 had reached 50 percent of the total column abundance of ozone, was soon given the label “ozone hole.”

Role of CFCs

While it was initially unclear whether the formation of the Antarctic ozone hole stemmed from natural causes or from anthropogenic effects on the environment, extensive field studies combined with the results of laboratory experiments and computer modeling of the atmosphere quickly led to a consistent and detailed explanation for ozone-hole formation. The formation of the ozone hole has two principal causes: chemical reactions that occur generally throughout the stratosphere and special conditions that exist in the Antarctic region.

Under normal conditions, the concentration of ozone in the stratosphere is determined by an equilibrium balance between reactions that remove ozone and those that produce ozone. The removal reactions are mainly catalytic chain reactions, in which trace atmospheric chemical species destroy ozone molecules without themselves being consumed. In such processes, it is possible for one chain component to remove many ozone molecules before being itself removed. A single chlorine atom, for example, is estimated to remove as many as 100,000 ozone molecules through chemical chain reactions before it is, itself, removed by forming a nonreactive species. The trace species involved in ozone removal include hydrogen oxides and nitrogen oxides, formed primarily by naturally occurring processes, and chlorine and bromine atoms and their corresponding oxides.

A major source of chlorine in the stratosphere is the decomposition of a class of compounds called chlorofluorocarbons (CFCs). Such compounds are used as refrigerants in refrigeration and air conditioning applications, and were commonly used as aerosol propellants and solvents, freely released into the atmosphere. Their use and handling are now strictly regulated. Chlorofluorocarbons are extremely stable in the lower atmosphere, with lifetimes of several decades, due to their extreme lack of chemical reactivity. The main fate of chlorofluorocarbons in the atmosphere, however, is slow migration into the stratosphere, where they absorb ultraviolet light and fragment to release chlorine atoms. The chlorine atoms produced from the breakdown of chlorofluorocarbons in the stratosphere provide an additional catalytic process by which stratospheric ozone is destroyed. A similar set of reactions involving a class of bromine-containing compounds called halons, used in some types of fire extinguishers, leads to additional ozone destruction by similar photochemical processes. By 1986, the average global loss of stratospheric ozone caused by the release of chlorofluorocarbons, halons, and related compounds into the environment was estimated to be 2 percent.

Antarctic Conditions

While the decomposition and subsequent reaction of chlorofluorocarbons, halons, and other synthetic compounds can explain the slow general decline in ozone concentration observed in the stratosphere, additional processes are needed to account for the more massive seasonal ozone depletion observed above Antarctica. These processes involve a set of special conditions that, in combination, are unique to the stratosphere above Antarctica.

During daylight hours, a portion of the chlorine present in the stratosphere is tied up in the form of reservoir species—compounds such as hydrogen chloride and chlorine nitrate that do not react with ozone. This slows the rate of removal of ozone by chlorine. Processes that directly or indirectly involve the absorption of sunlight transform reservoir species and release ozone-destroying chlorine atoms. During the Antarctic winter, when sunlight is entirely absent, stratospheric chlorine is rapidly converted into reservoir species.

In the absence of additional chemical processes, the onset of spring in Antarctica and the return of sunlight convert a portion of the reservoir compounds into reactive chlorine species and reestablish the balance between ozone-producing and ozone-destroying processes. However, the extremely low temperatures occurring in the stratosphere above Antarctica during the winter months lead to the formation of polar stratospheric clouds, which, because of the extremely low concentration of water vapor in the stratosphere, do not form during other seasons or outside the polar regions of the globe. The ice crystals that compose the clouds act as catalysts that convert reservoir species into diatomic chlorine and other gaseous chlorine compounds that, in the presence of sunlight, re-form ozone-destroying species. At the same time, nitrogen oxides in the collection of reservoir species are converted into nitric acid, which remains attached to the ice crystals. As these ice crystals are slowly removed from the stratosphere by gravity, the potential for conversion of active forms of chlorine into reservoir species is greatly reduced. Because of this, when spring arrives, large amounts of ozone-destroying chlorine species are produced by the action of sunlight, and only a small fraction of this reactive chlorine is converted into reservoir species. The increased rate of ozone removal caused by the abundance of reactive chlorine present in the stratosphere leads to ozone depletion and formation of the ozone hole.

An additional process important in formation of the ozone hole is the unique air-circulation pattern in the stratosphere above Antarctica. During the winter and early spring, a vortex of winds circulates about the South Pole. This polar vortex minimizes movement of ozone and reservoir-forming compounds from other regions of the stratosphere. As this polar vortex breaks up in mid-spring, ozone concentrations in the Antarctic stratosphere return to normal levels, and the ozone hole gradually disappears.

Atmospheric Ozone Study and Interpretation

Researchers utilize a great diversity of devices and techniques in the study and interpretation of atmospheric ozone. One popular technique is the use of simulation models. A good model is one that simulates the interrelationships and interactions of the various parts of the known system. The weakness of models is that, often, not enough is known to give an accurate picture of the total system or to make accurate predictions. Most modeling is done on computers. Scientists estimate how fast chemicals such as CFCs and nitrous oxide will be produced in the future and build a computer model of the way these chemicals react with ozone and with one another. From this model, it is possible to estimate future ozone levels at different altitudes and at different future dates.

Similar processes appear to be at work in the Arctic stratosphere, leading to ozone depletion, as in the Antarctic. However, the National Oceanic and Atmospheric Administration (NOAA) Aeronomy Laboratory in Boulder, Colorado, reported a discrepancy between observed ozone depletion and predicted levels, based on models that account accurately for Antarctic ozone depletion. This report suggested that some other mechanism is at work in the Arctic. Thus, while good models can be very useful in studying new data, observed discrepancies highlight the need for better system models and modeling algorithms. There are two models favored by most scientists in this area. Some scientists put forth a chemical model that says the depletion is caused by chemical events promoted by the presence of chlorofluorocarbons created by industrial processes. Acceptance of this model was promoted by the discovery of fluorine in the stratosphere. Fluorine does not naturally occur there, but it is related to and can be formed photochemically from CFCs. The other model assumes that the ozone hole was formed by dynamic air movement and mixing. This model best fits data gathered by ozone-sensing balloons that sample altitudes up to 30 kilometers and then radio the data back to Earth. Ozone depletion is confined to the atmosphere at altitudes between 12 and 20 kilometers. While the total ozone depletion is 35 percent, different strata have shown various amounts of depletion from 70 to 90 percent. Surprisingly, about half the ozone was gone in twenty-five days. This finding does not fit the chemical model very well.

Besides ozone-sensing balloons, satellite survey data provide more direct measurements obtained over longer periods of observation time. The National Aeronautics and Space Administration (NASA) obtains measurements with satellites equipped with instruments such as the Ozone Mapping and Profiler Suite (OMPS). Ozone measurements made by satellites helped to develop flight plans for the specialized aircraft NASA has also deployed in ozone studies. A DC-8, operating during the same period, is able to survey the polar vortex, owing to its greater range. At the same time, the European Centre for Medium-Range Weather Forecasts implemented the Copernicus Atmosphere Monitoring Service to, in part, analyze ozone conditions. In addition, scientists utilize many meteorological techniques and instruments, including chemical analysis of gases by means of infrared spectroscopy, mass spectroscopy and gas spectroscopy combined, gas chromatography, and oceanographic analysis of planktonic life in the southern Atlantic, Pacific, and Indian oceans. As new research methods and techniques have become available, they have also been applied to this essential study.

Public Health Concerns

Stratospheric ozone provides global protection from the lethal effects of ultraviolet radiation from the sun. This ability to absorb ultraviolet radiation protects all life forms on Earth’s surface from excessive ultraviolet radiation, which destroys plant and animal cells. Between 10 and 30 percent of the sun’s ultraviolet B (UV-B) radiation reaches the ground surface. If ozone levels were to drop by 10 percent, the amount of UV-B radiation reaching Earth would increase by 20 percent.

UV-B levels are responsible for the fading of paints and the yellowing of window glazing and for car finishes becoming chalky. These kinds of degradation will accelerate as the ozone layer is depleted. There could also be increased smog, urban air pollution, and a worsening of the problem of acid rain in cities. In humans, UV-B causes sunburn, snow blindness, skin cancer, and cataracts, and promotes aging and wrinkling of the skin. Skin cancer is one of the most common forms of cancer.

Ecological Concerns

Many other forms of life, from bacteria to forests and food crops, are adversely affected by excessive radiation. Ultraviolet radiation affects plant growth by slowing photosynthesis and by delaying germination in many plants, including trees and food crops. Scientists have a great concern for the organisms that live in the ocean and the effect ozone depletion may have on them. Phytoplankton, zooplankton, and krill (a shrimplike crustacean) could be greatly depleted if there were a drastic increase in ultraviolet A and B. The result would be a tremendous drop in the population of these free-floating organisms, which are extremely important because they are the foundation species of the global food chain. Phytoplankton use the energy of sunlight to convert inorganic compounds into organic plant matter. This process provides food for the next step in the food chain, the herbivorous zooplankton and krill. They, in turn, become the food for the next higher level of animals in the food chain. Initial studies of this food chain in the Antarctic suggested that elevated levels of ultraviolet radiation impair photosynthetic activity. Some studies have shown that a fifteen-day exposure to UV-B levels 20 percent higher than normal can kill off all anchovy larvae down to a depth of 10 meters. There is also concern that ozone depletion may alter the food chain and even cause changes in the organisms’ genetic makeup. An increase in the ultraviolet radiation is likely to lower fish catches and upset marine ecology, which has already suffered damage from human-made pollution.

International Response

The United Nations Environmental Program (UNEP) has continued working with governments, international organizations, and industry to develop a framework within which the international community can make decisions to minimize atmospheric changes and the effects they could have on Earth. In 1977, UNEP convened a meeting of experts to draft the World Plan of Action on the Ozone Layer. The plan called for a program of research on the ozone layer and on what would happen if the layer were compromised. In addition, UNEP created a group of experts and government representatives who framed the Convention for the Protection of the Ozone Layer. This convention was adopted in Vienna in March 1985, by twenty-one nations and the European Economic Community, and was subsequently signed by many more nations. The convention pledged that signatory nations would protect human health and the environment from the effects of ozone depletion. Action taken under the convention to protect the ozone layer included several countries restricting the use of CFCs or the amounts produced. The United States banned the use of CFCs in all nonessential applications in 1978. Some countries, such as Belgium and the Nordic countries, have, in effect, banned CFC production altogether. The group also worked with governments on a Protocol to the Convention that required signatory nations to limit their production of CFCs. It remained the hope and aim of these nations that such international cooperation would lead to a better global environment.

In 2017, it was announced that the ozone hole was the smallest it had been since 1988. However, this was mainly due to unusually warm temperatures, not to human efforts to reduce ozone-depleting chemicals. Scientists cautioned against seeing the smaller hole as being a sign of recovery. However, in January 2023, a United Nations–backed panel of experts announced the ozone was on track to recover within four decades, crediting the Montreal Protocol (1987) and the lowered use of ozone-depleting chemicals for the progress.

Principal Terms

catalyst: a substance that increases the rate of a chemical reaction without itself being altered in the process

chlorofluorocarbon (CFC): a group of chemical compounds containing carbon, fluorine, and chlorine, used in air conditioners, refrigerators, fire extinguishers, spray cans, and other applications

Dobson spectrophotometer: a ground-based instrument for measuring the total column abundance of ozone at a particular geographic location

food chain: the arrangement of the organisms of an ecological community according to the order of predation in which each consumes the next, usually lower, member as a food source

ozone layer: a region in the lower stratosphere, centered about 25 kilometers above the surface of Earth, which contains the highest concentration of ozone found in the atmosphere

ozone: the molecular form of oxygen containing three atoms of oxygen per molecule as O₃, as compared to elemental oxygen having the molecular formula O₂

phytoplankton: free-floating microscopic aquatic plants that use sunlight to convert carbon dioxide and water into food for themselves and for other organisms in the food chain

polar stratospheric clouds: clouds of ice crystals formed at extremely low temperatures in the polar stratosphere

polar vortex: a closed atmospheric circulation pattern around the South Pole that exists during the winter and early spring; atmospheric mixing between the polar vortex and regions outside the vortex is slow

stratosphere: the region of the atmosphere between 10 and 50 kilometers above the surface of Earth

total column abundance of ozone: the total number of molecules of ozone above a 1-centimeter-square area of Earth’s surface

Total Ozone Mapping Spectrometer (TOMS): a space-based instrument for measuring the total column abundance of ozone globally

ultraviolet solar radiation: electromagnetic radiation having wavelengths between 4 and 400 nanometers

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