The Earth's global temperature, as well as the amount of solar radiation reaching its surface, can be significantly influenced by changes in the concentrations of chemicals naturally present in the atmosphere, such as natural greenhouse gases, and by anthropogenic chemicals, such as chlorofluorocarbons.

The significance of the Earth's atmosphere is vastly disproportionate to its size. Although its thickness relative to Earth's sphere is comparable to an apple's skin, it is essential for life. It was not until the eighteenth century that scientists began to understand the role of atmospheric gases such as oxygen and carbon dioxide (CO₂) in plant and animal life, and it was not until the end of the nineteenth century that scientists grasped the details of how soil microorganisms utilized atmospheric nitrogen to create compounds necessary for the health of plants and animals. Throughout the twentieth century, climatologists, atmospheric chemists, and others gathered information about how such anthropogenic gases as CO₂, methane, and nitrous oxide were increasing Earth's greenhouse effect and elevating the planet's average global temperature. This enhanced greenhouse effect fosters climate changes that are potentially so devastating that some scholars have called climate change the most important issue of the twenty-first century.

Approximately three-quarters of the Earth's air mass is located in the troposphere, and dry air in this region is 78.1 percent nitrogen, 20.9 percent oxygen, and 0.93 percent argon by volume. The troposphere also contains trace amounts of many other gases, such as methane, various nitrogen oxides, ammonia, sulfur dioxide, and ozone, and these come from both natural and anthropogenic sources. Human activities have not changed the concentrations of the major gases in the atmosphere--nitrogen and oxygen--but scientific evidence accumulated over the past century indicates that human beings, particularly in advanced industrialized societies, are dramatically affecting the concentrations of certain trace gases. Examples of these include CO₂, methane, nitrous oxide, carbon monoxide, chlorofluorocarbons (CFCs), and sulfur dioxide. Some of these atmospheric trace gases, such as CFCs, result from certain industries and their products, such as refrigerants and aerosols. Others, such as CO₂ and sulfur dioxide, are produced by burning fossil fuels. Agricultural practices are also significant sources of such gases as methane and nitrous oxide.

Although the Earth's stratosphere contains much less matter than the troposphere, it contains similar proportions of such gases as nitrogen and oxygen. It differs markedly from the troposphere, however, in its concentrations of water vapor and ozone. Stratospheric water-vapor concentrations are only about one-thousandth of tropospheric concentrations, but ozone concentrations are much higher in the stratosphere. Ozone is localized in a layer ranging from about 15 to 35 kilometers above Earth's surface. This ozone layer, whose molecules are created when oxygen interacts with high energy solar radiation, prevents about 95 percent of the Sun's ultraviolet radiation from reaching Earth's surface, where it could damage living organisms. The ozone layer also prevents tropospheric oxygen from being converted to ozone, which, in the lower atmosphere, is a dangerous air pollutant.

Besides being home to such major gases as nitrogen and oxygen, the troposphere contains hundreds of other distinctive molecules, leading to myriad chemical reactions, some of which have an influence on climate change. Because oxygen is such a reactive species, many of these reactions are oxidations, and some scientists see these reactions as constituting a low-temperature combustion system. Fueling this combustion are chemicals released from both natural and artificial sources. For example, methane enters the troposphere in large amounts from swamp and bog emissions, termites, and ruminant animals. Human activities contribute a large number of organic compounds, and CO₂ and water are the end results of their oxidation. CO₂ and water vapor are powerful greenhouse gases (GHGs).

Atmospheric chemists have also been attempting to work out in detail the influence of chemical radicals on tropospheric gases. Such charged groups of atoms as the hydroxyl radical (composed of hydrogen and oxygen) play an important role in the daytime chemistry of the troposphere, and the nitrate radical (composed of nitrogen and oxygen) is the dominant nighttime oxidant. Fossil-fuel combustion is a significant contributor to tropospheric pollution. Particulates such as soot were a factor in some "killer smogs," and scientists have recently discovered that particulates contribute to global dimming, a lessening of sunlight's ability to penetrate particle-filled hazes and reach the Earth's surface. Sulfur dioxide, which is produced by the combustion of certain kinds of coal and oil, can be a primary air pollutant, since it is toxic to living organisms as well as damaging to buildings. It can also be a secondary air pollutant, because it reacts with water vapor to create sulfuric acid, which is an acid rain component, causing harm to various life forms, including trees and fish.

Just as in the lower atmosphere, chemical reactions in the upper atmosphere exhibit great variety, and some of these reactions have an important influence on climate change. Over the past decades, the chemical species that has received the most attention has been ozone. Scientists paid heightened attention to the chemical reactions in the ozone layer when, in the late 1980's, a hole was discovered in this layer above the Antarctic. During the 1970's scientists had found a threat to the ozone layer when they worked out the reactions between chlorine-containing radicals and ozone. These reactions changed ozone molecules into diatomic oxygen molecules, thus weakening the ability of the ozone layer to protect Earth's surface from high-energy solar radiation.

A primary source of these catalytic, chlorine-containing species turned out to be CFCs. General Motors had introduced CFCs in 1930, and they proved to be successful in such products as refrigerator and air-conditioning coolants, as well as aerosol propellants. Because of the widespread and accelerating use of CFCs, the tropospheric concentrations of these chemicals increased from the 1930's to the 1970's, when Mexican chemist Mario Molina and American chemist F. Sherwood Rowland showed that CFCs, although seemingly inert in the troposphere, became very reactive in the stratosphere. There, ultraviolet radiation split the CFCs into highly reactive radicals that, in a series of reactions, promoted the debilitation of the protective ozone shield.

The exhaust from aircraft and spacecraft also helped deplete stratospheric ozone. Despite attempts, such as the Montreal Protocol (1987), to reduce concentrations of CFCs and other ozone-depleting chemicals in the atmosphere, the Antarctic ozone hole continued to grow in the 1990's and early twenty-first century. This meant that countries near Antarctica began experiencing higher levels of ultraviolet solar radiation.

Bibliography:

1)         Birks, John W., Jack G. Calvert, and Robert E. Sievers, eds. The Chemistry of the Atmosphere: Its Impact on Global Change--Perspectives and Recommendations. Washington, D.C.: American Chemical Society, 1993.

2)         Jacob, Daniel J. Introduction to Atmospheric Chemistry. Princeton, N.J.: Princeton University Press, 2007.

3)         Makhijani, Arjun, and Kevin R. Gurney. Mending the Ozone Hole: Science, Technology, and Policy. Cambridge, Mass.: MIT Press, 1996.

4)         Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: John Wiley & Sons, 1998.

5)         Wayne, Richard P. Chemistry of Atmospheres. 3d ed. New York: Oxford University Press, 2000.

EssayEmpire offers you the best custom essay writing services, along with term paper, thesis paper, and research paper writing help. Our company employs professional essay writers who are fully qualified in a variety of academic fields. If you require a high quality writing service that is capable of writing authentic essays, term papers or research papers because you simply don't have the time or resources to do them yourself or maybe they seem too complicated and time consuming, you don't need to look any further. EssayEmpire is the perfect place for all your needs. If you need high quality Essay on Chemical Composition of the Earth's Atmosphere at affordable prices please use our essay writing services offered by EssayEmpire.