Ozone Layer Essay, Research Paper

Ozone Layer

Introduction

Orbiting above the Earth, an astronaut can look down on our home and see the thin blue ribbon that rims our planet. That transparent blanket – our atmosphere – makes life possible. It provides the air we breathe and regulates our global temperature. And it contains a special ingredient called ozone that filters deadly solar radiation.

The Atmosphere

The gaseous area surrounding the planet is divided into several spherical strata separated by narrow transition zones which is the atmosphere. (Graedel 1998) The upper boundary at which gases disperse into space lies at an altitude of approximately 1000 kilometers above sea level. (Graedel 1998) More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth’s surface. (Graedel 1998) Atmospheric layers are characterized in chemical compositions that produce variations in temperature. (Graedel 1998)

Here is a graph of the different layers in the atmosphere:

Troposphere

The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of the mass of the total atmosphere. (Garcia 1994) It is characterized by the density of its air and an average vertical temperature change of 6 degrees Celsius (C) per kilometer. (Graedel 1998)

Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet’s surface. (Graedel 1998) The troposphere contains 99 % of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position. They are greatest above the tropics, where they may be as high as 3 %, and decrease toward the Polar Regions.

All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere. (Graedel 1998) Troposphere means “region of mixing” and is so named because of vigorous convective air currents within the layer. (Graedel 1998)

The upper boundary of the layer ranges in height from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons, highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. (Graedel 1998) Air temperature within the tropopause remains constant with increasing altitude.

Stratosphere

The stratosphere is the second major stratum of air in the atmosphere. It resides between 10 and 50 km above the planet’s surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. (Graedel 1998) Then it increases gradually to 200-220 degrees Kelvin (K) at the lower boundary of the stratopause (~50 km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. (Graedel 1998) Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapor content within the layer is very low. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere. (Graedel 1998)

The ozone layer is located at an altitude between 20-30 km. Approximately 90 % of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in this region is about 10 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere. (Graedel 1998) Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm – 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet’s surface would damage plant life and have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation would result in a host of biological effects, such as a dramatic increase in cancers.

Meteorological conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present. Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does production. (Graedel 1998)

Mesosphere

The mesosphere, a layer extending from approximately 50 km to 80 km, is characterized by decreasing temperatures, which reach 190-180 K at an altitude of 80 km. (Graedel 1998) In this region, concentrations of ozone and water vapor are negligible. Therefore the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth’s surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. (Graedel 1998) At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation. (Graedel 1998)

Thermosphere

The thermosphere is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 1000-1500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. (Graedel 1998) At an altitude of 100-200 km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated.

Exosphere

The exosphere is the most distant atmospheric region from Earth’s surface. The upper boundary of the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is a transitional zone between Earth’s atmosphere and interplanetary space. (Graedel 1998)

Formation of the Ozone Layer

One billion years ago, early aquatic organisms called blue-green algae began using energy from the Sun to split molecules of H2O and CO2 and recombine them into organic compounds and molecular oxygen (O2). (Garcia 1994) This solar energy conversion process is known as photosynthesis. Some of the photosynthetically created oxygen combined with organic carbon to recreate CO2 molecules. The remaining oxygen accumulated in the atmosphere, touching off a massive ecological disaster with respect to early existing anaerobic organisms. (Garcia 1994)

6H2O + 6CO2 ———-* C6H12O6+ 6O2

As oxygen in the atmosphere increased, CO2 decreased. High in the atmosphere, some oxygen (O2) molecules absorbed energy from the Sun’s ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combined with remaining oxygen (O2) to form ozone (O3) molecules, which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light.

The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence 600 million years ago. At this time, the oxygen level was approximately 10% of its present atmospheric concentration. (Garcia 1994) Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on the land. Ozone played a significant role in the evolution of life on Earth, and allows life, as we presently know it to exist.

Ozone Production and Destruction

Stratospheric ozone is created and destroyed primarily by ultraviolet radiation. The air in the stratosphere is bombarded continuously with ultraviolet radiation from the Sun. When high energy ultraviolet rays strike molecules of ordinary oxygen (O2), they split the molecule into two single oxygen atoms. (Rowland 1989) The free oxygen atoms can then combine with oxygen molecules (O2) to form ozone (O3) molecules.

O2 + UV light -* 2 O

O + O2 + M -* O3 + M (where M indicates conservation of energy and momentum)

The same characteristic of ozone that makes it so valuable, its ability to absorb a range of ultraviolet radiation, also causes its destruction. When an ozone molecule is exposed to ultraviolet energy it may break back into O2 and O. During dissociation the atomic and molecular oxygens gain kinetic energy, which produces heat and causes an increase in atmospheric temperature. (Rowland 1989)

Ozone production is driven by UV radiation of wavelengths less than 240 nm. Ozone dissociation typically produces atomic oxygen that is stable when exposed to longer wavelengths, up to 320 nm, and shorter wavelengths of 400 to 700 nm. Longer wavelength photons penetrate deeper into the atmosphere, creating regions of ozone production and destruction. (Rowland 1989) When an ozone molecule absorbs even low energy ultraviolet, it splits into an ordinary oxygen molecule and a free oxygen atom.

O3 + UV, visible light -* O + O2

The free oxygen atom may then combine with an oxygen molecule, creating another ozone molecule, or it may take an oxygen atom from an existing ozone molecule to create two ordinary oxygen molecules. (Rowland 1989)

O + O2 -* O3 or O3 + O -* O2 + O2

Processes of ozone production and destruction, set off by ultraviolet radiation, are often referred to as “Chapman Reactions.”

Most O3 destruction takes place through catalytic processes rather than Chapman Reactions. Ozone is a highly unstable molecule that readily donates its extra oxygen molecule to free radical species such as nitrogen, hydrogen, bromine, and chlorine. (Rowland 1989) These compounds naturally occur in the stratosphere, released from sources such as soil, water vapor, and the oceans.

O3 + X -* XO + O2 (where X may be O, NO, OH, Br or Cl)

The Ozone Balance

Over Earth’s lifetime, natural processes have regulated the balance of ozone in the stratosphere. Scientists are finding that ozone levels change periodically as part of regular natural cycles such as seasons, periods of solar activity, and changes in wind direction. (Roan 1989) Concentrations are also affected by isolated events that inject materials into the stratosphere, such as volcanic eruptions.

Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirlpool of extremely cold stratospheric air forms over the region during the long polar night. (Roan 1989) The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds.

Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal surfaces on which the destructive reactions take place. (Rowland 1989) After the long polar winter, ozone within this extremely cold vortex is very vulnerable to the arrival of sunlight. As spring arrives, major ozone losses occur. In the southern hemisphere, the area of most severe ozone depletion is localized above Antarctica and is generally referred to as the ozone hole. The hole appears in the southern spring, following the continent’s coldest season and polar night.

Ozone depletion over the Arctic is not as well defined as in Antarctica. The rugged topography results in an air circulation pattern that is quite different from that of the South Pole, but expeditions have shown that the atmospheric chemistry of the two Polar Regions is very similar. (Roan 1989) In the Northern Hemisphere, the polar vortex is not as strong. It can break up and reform several times during the course of winter. One air mass after another enters the polar darkness and soon emerges back into low sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount from one parcel as in the southern hemisphere. (Roan 1989)

The end result is that we are losing ozone in both hemispheres. Ozone levels in the atmosphere have been monitored from the ground since the 1950s and by satellite since the 1970s. (Roan 1989) Regional total ozone levels measured from satellites over Antarctica have decreased 30-50% since their monitoring began. (Roan 1989)

Since ozone is created and destroyed by solar UV radiation, there is some correlation of ozone concentration with 11-year sunspot cycles. Sunspots emit high levels of electromagnetic radiation. The increased UV radiation contributes to ozone production. Sunspot variations only account for 2 to 4 % of the total variation in ozone concentrations. (Roan 1989) Natural cycles in ozone variation are also associated with the quasi-biennial oscillation in which tropical winds switch from easterly to westerly every 26 months. (Roan 1989) This cyclic change in wind direction accounts for approximately 3 % of the natural variation in ozone concentration.

Ozone Depletion

For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Here is a basic view of the chemical formation of a CFC.

Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other smaller applications. (Rowland 1989) Other chlorine-containing compounds include methyl chloroform, a solvent, and carbon tetrachloride, an industrial chemical. Halons, extremely effective fire-extinguishing agents, and methyl bromide, an effective produce and soil fumigant, contain bromine. All of these compounds have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere. (Rowland 1989) Because they release chlorine or bromine when they break down, they damage the protective ozone layer.

In the early 1970s, researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. They also examined the potential impacts of other chlorine sources. Chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. (Rowland 1989) Chlorine compounds from these sources readily combine with water and repeated measurements show that they rain out of the troposphere very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. (Rowland 1989) Over time, winds drive the CFCs into the stratosphere.

The CFCs are so stable that only exposure to strong UV radiation breaks them down. When that happens, the CFC molecule releases atomic chlorine. One chlorine atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created. To return to the analogy comparing ozone levels to a stream’s depth, CFCs act as a siphon, removing water faster than normal and reducing the depth of the stream. (Rowland 1989)

Large fires and certain types of marine life produce one stable form of chlorine that does reach the stratosphere. However, numerous experiments have shown that CFCs and other widely used chemicals produce roughly 85% of the chlorine in the stratosphere, while natural sources contribute only 15%. (Rowland 1989)

Large volcanic eruptions can have an indirect effect on ozone levels. Although Mt. Pinatubo’s 1991 eruption did not increase stratospheric chlorine concentrations, it did produce large amounts of tiny particles called aerosols (different from consumer products also known as aerosols). (Rowland 1989) These aerosols increase chlorine’s effectiveness at destroying ozone. The aerosols only increased depletion because of the presence of CFC- based chlorine. In effect, the aerosols increased the efficiency of the CFC siphon, lowering ozone levels even more than would have otherwise occurred. Unlike long-term ozone depletion, however, this effect is short-lived. The aerosols from Mt. Pinatubo have already disappeared, but satellite, ground-based, and balloon data still show ozone depletion occurring closer to the historic trend. (Rowland 1989)

One example of ozone depletion is the annual ozone “hole” over Antarctica that has occurred during the Antarctic Spring since the early 1980s. Rather than being a literal hole through the layer, the ozone hole is a large area of the stratosphere with extremely low amounts of ozone. (Prather 1996) Ozone levels fall by over 60% during the worst years.

In addition, research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America. Over the U.S., ozone levels have fallen 5-10%, depending on the season. (Prather 1996) Thus, ozone depletion is a global issue and not just a problem at the South Pole.

Reductions in ozone levels will lead to higher levels of UVB reaching the Earth’s surface. The sun’s output of UVB does not change; rather, less ozone means less protection, and hence more UVB reaches the Earth. (Prather 1996) Studies have shown that in the Antarctic, the amount of UVB measured at the surface can double during the annual ozone hole. (Prather 1996) Another study confirmed the relationship between reduced ozone and increased UVB levels in Canada during the past several years.

Laboratory and epidemiological studies demonstrate that UVB causes nonmelanoma skin cancer and plays a major role in malignant melanoma development. (Prather 1996) In addition, UVB has been linked to cataracts. All sunlight contains some UVB, even with normal ozone levels. It is always important to limit exposure to the sun. However, ozone depletion will increase the amount of UVB, which will then increase the risk of health effects. Furthermore, UVB harms some crops, plastics and other materials, and certain types of marine life.

The World’s Reaction

In 1973, two scientists from the University of California at Irvine, Mario Molina and F. Sherwood Rowland, first discovered that man-made substances called chlorofluorocarbons (CFCs) could play a major role in the destruction of stratospheric ozone. (EPA 2000) Since that time there has been much controversy surrounding the subject of ozone depletion. Researchers have struggled to understand the nature and severity of the problem through numerous scientific studies. Nations from all over the world have come together and agreed to establish international industrial regulations in hope of protecting the ozone layer. (EPA 2000)

Because of uncertainty about how global environmental systems work, and because the people affected will probably live in circumstances very much different from those of today and may have different values, it is difficult to predict how present-day actions will affect future generations. To project or forecast the human consequences of global change at some point in the relatively distant future, one would need to know at least the following:

-the future state of the natural environment

-the future of social and economic organization

-the values held by the members of future social groups

-the proximate effects of global change on those values

-the responses that humans will have made in anticipation of global change or in response to ongoing global change (EPA 2000)

International Agreements

Even the value systems, and technological advancements, of present day nations are extremely different. Nonetheless, efforts to predict and protect are underway. Despite their differences, the international community made significant progress in addressing ozone depletion as a serious global environmental problem. Through the 1985 Vienna Convention for the Protection of the Ozone Layer, the 1987 Montreal Protocol on substances that deplete the ozone layer, and the 1990 London Amendments to the protocol, members from nations around the world have committed to phasing out the production and consumption of CFCs, and a number of related chemicals, by the year 2000. (EPA 2000)

Ozone depletion control started in the early 1970s, when the United States, along with a handful of other Western countries, expressed concern over emissions from supersonic transport (SST) aircraft and aerosol spray cans. (EPA 2000) Environmental groups organized opposition to the development of the SST and to the extensive use of aerosols. Public response led to a sharp drop in the sales of aerosol products. The U.S. Congress, prodded by government studies supporting the CFC ozone depletion theory and its links to skin cancer, approved the Toxic Substances Control Act of 1976, which gave the Environmental Protection Agency (EPA) authority to regulate CFCs. (EPA 2000) In 1978, the United States became the first country to ban the nonessential use of CFCs in aerosols. However, the EPA ruled that other uses of CFCs, such as refrigeration, were essential and lacked available substitutes.

Ozone depletion emerged as a major international issue in the 1980s. This occurred primarily as a result of initiatives by the United Nations Environmental Programme and actions of the international scientific and environmental communities. A United Nations Environment Program to protect the ozone layer was signed in Vienna in 1985, and a protocol outlining proposed protective actions followed. (EPA 2000) The Vienna convention of 1985 embodied an international environmental consensus that ozone depletion was a serious environmental problem. (EPA 2000) However, there was no consensus on the specific steps that each nation should take. The Montreal Protocol, signed in September 1987, stated that there would be a 50 % cut back in CFC production by 2000. (EPA 2000) The United States ratified the Montreal Protocol in 1988. The 1990 London Amendments to the protocol state that production of CFCs, CCl4, and halons will be completely halted by the year 2000. The phase out schedule for other compounds was accelerated by 4 years by the 1992 Copenhagan agreement.

All human activity potentially contributes directly or indirectly to global change. Earth’s atmosphere consists of a delicate balance of gases essential to life. Throughout the history of the planet, the atmospheric gases have been influenced by Earth processes and by the living organisms from both the oceans and land, and natural changes have occurred in the type of gases and their concentrations. (EPA 2000) Anthropogenic activities are now believed to be causing rapid changes in atmospheric composition on an accelerated time scale. (EPA 2000) Due to extended human life expectancies and greater population densities, the influence of humans will continue to grow.

Scientists are now confident that stratospheric ozone is being depleted worldwide. However, how much of the loss is the result of human activity, and how much is the result of fluctuations in natural cycles, still need to be determined. To understand global atmospheric changes, we need to understand the composition and chemistry of Earth’s atmosphere and how they are affected by human activity. To create accurate models, scientists must account for all of the factors affecting ozone creation and destruction, and conduct simultaneous, global studies over the course of many years. (EPA 2000)

Conclusion

The ozone layer must continue to be protected and we, as individuals, need to take the right steps into preserving our atmosphere and environment by finding new ways to prevent the ongoing destruction that has been done to it. If everyone does his or her part to help, no matter how big or small, we can ensure that this problem will be solved.

Bibliography

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