Ozone Layer

Introduction

The ozone layer refers to ozone—a gas composed of three oxygen atoms—that resides in the stratosphere, which is the layer of Earth’s atmosphere between about 6 and 30 mi (10 and 48 km) above the surface of Earth. Over 90% of the total atmospheric ozone is found in the stratosphere.

The atmospheric ozone layer differs from the layer of ozone that can accumulate near the ground in areas with heavy air pollution arising from vehicle emissions. Ground-level ozone is undesirable because it can be irritating to the lungs when breathed. People with chronic respiratory ailments are particularly prone to discomfort and illness from ground-level ozone.

In contrast, the atmospheric ozone layer is beneficial, as it provides a vital shield for protecting Earth’s surface from harmful levels of ultraviolet (UV) radiation emitted from the sun.

Human activities have been responsible for the emission of a number of gases into the atmosphere that participate in the destruction of ozone. As a result, the amount of ultraviolet radiation reaching Earth’s surface has gradually increased throughout the twentieth century and into the twenty-first century. With recognition of the ozone depletion—graphically evident as a periodic and seasonal thinning of ozone over the Antarctic, which has been dubbed the ozone hole—efforts to diminish ozone depletion began in the 1980s.

Historical Background and Scientific Foundations

Ozone is a gas composed of three oxygen atoms (O₃). The form of oxygen that we breathe consists of a pair of atoms (O₂). O₂ comprises about 21% of the volume of the atmosphere, while ozone occupies much less volume (0.000004%). If all the atmospheric ozone was gathered together, the layer that encircled Earth would be only about one-eighth of an inch thick.

Most of the ozone in the atmosphere is not dispersed all through the atmosphere. Rather, it is confined to the uppermost layer of the atmosphere called the stratosphere, which begins 6 to 12 mi (10 to 19 km) above sea level and extends to nearly 30 mi (48 km) high, and tends to be thinnest above the equator and thickest at both poles. The stratosphere is not where clouds normally form and where there is a lot of air movement (these occur in a lower layer called the troposphere). This is an advantage, as the ozone remains relatively undisturbed.

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet (UV) radiation. At any time, the formation and consumption of ozone proceeds simultaneously. 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.

Ozone forms when the UV portion of sunlight splits an oxygen (O₂) molecule into two oxygen (O) atoms; one of these subsequently forms a chemical bond with another O₂ molecule to generate O₃. Even though UV light is required to form ozone, once the molecule has formed, it is capable of absorbing incoming UV light. This reduces the amount of UV radiation that passes through the atmosphere to Earth’s surface.

Reduction in UV ration at Earth’s surface is beneficial, as the high-energy UV waves are able to penetrate into the upper layers of skin. When they contact the genetic material of skin cells as well as other cells, the energy is sufficient to break one or both of the double strands of the cell’s deoxyribonucleic acid (DNA).

WORDS TO KNOW

ATMOSPHERE: The air surrounding Earth, described as a series of layers of different characteristics. The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases, acts as a buffer between Earth and the sun.

CLIMATE MODELS: Mathematical representations of climate processes. Climate models are computer programs that describe the structure of Earth’s land, ocean, atmospheric, and biological systems and the laws of nature that govern the behavior of those systems. Detail and accuracy of models are limited by scientific understanding of the climate system and by computer power. Climate models are essential to understanding paleoclimate, present-day climate, and future climate.

GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.

PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.

Sometimes the damage can be repaired by the cell. But sometimes the damage cannot be repaired, causing cell death, or is enough to cause the cell to function differently than before. The uncontrolled cell growth and division that is the hallmark of cancer can be a result of radiation-induced genetic change.

The depletion of the ozone layer was first observed in the mid-1980s. The basis for ozone destruction includes carbon, chlorine, and fluorine-containing compounds called chlorofluorocarbons (CFCs), hydrochlororfluorocarbons (which are very similar in structure to CFCs), bromine-containing hydrocarbons, and nitrogen oxide. CFCs were once widely used as a coolant in air conditioners and refrigerators, and to propel gas out of aerosol cans. Ironically, a main reason for their use was the assumption that they were chemically non-reactive, and so safe to use around people. Nitrogen oxide is a by-product of the burning of fuel and is released to the atmosphere in aircraft exhaust and other sources.

As the use of CFCs became more popular, their escape into the atmosphere accelerated. Subsequently, it was discovered that CFCs can persist in the stratosphere for up to 100 years, and that during that time incoming UV light can break the CFC molecule apart. This releases the chlorine component of CFC, which then can

destroy ozone. It has been estimated that one chlorine atom could destroy up to 100,000 ozone molecules.

Impacts and Issues

The reason that ozone depletion is of concern is because of ozone’s ability to absorb the genetically destructive wavelengths of ultraviolet radiation. As such, stratospheric ozone helps to protect humans and other organisms on Earth’s surface from some of the harmful effects of exposure to high-energy electromagnetic radiation from the sun. In fact, without the protective action of the stratospheric ozone layer, it is likely that life would not be possible on Earth’s surface, and that life in the ocean would be restricted to the depths where sunlight does not penetrate.

The most common effect of ultraviolet overexposure is a sunburn. Although this is seldom serious, other threats are. Basal cell carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These can usually be successfully treated if detected early enough. However, malignant melanoma, which accounts for about 5% of total skin cancers, is often ultimately fatal.

Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea of the eye, along with damage to the retina, suppression of the immune system, skin allergies, and accelerated aging of the skin.

As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals were curtailed. Their use as propellants in aerosol spray cans was banned in the 1980s. In 1987, the United Nations Environment Programme (UNEP) conducted a conference in Montreal, Quebec, Canada, which addressed the issue of CFCs and ozone depletion. The international agreement that was the culmination of the conference (the Montreal Protocol), pledged to alleviate ozone depletion by reducing the atmospheric release of CFCs. A 1990 revision of the protocol established more stringent timetables calling for the complete phase-out of global CFC use by the year 2000. The United States, Canada, Australia, and other developed countries have completely phased out production of CFCs. However, as

IN CONTEXT: LOWER ATMOSPHERIC OZONE AND CLIMATE CHANGE

In July 2007 researchers based at the University of Exeter in the United Kingdom published data in the science journal Nature that suggested that ozone (0³) played a greater potential role in climate change than scientists previously assumed. In addition to ozone’s role as a greenhouse gas in the upper atmosphere, data suggested that increased levels of ozone in the lower atmosphere damaged plant life by impairing photosynthesis and thereby decreased the ability of plants to act as a carbon sink that removes carbon dioxide (CO²) from Earth’s atmosphere.

of 2008, a complete global phase-out has not been achieved; developing countries now have until 2010 to meet this target.

See Also Air Pollution; Carbon Dioxide (CO₂) Emissions; Greenhouse Effect; Ozone Hole

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: Wiley Interscience, 2006.

Web Sites

NASA. “Ozone Hole Watch.” December 4, 2007. http://ozonewatch.gsfc.nasa.gov/ (accessed May 10, 2008).

Brian D. Hoyle