Background (Encyclopedia of Global Warming)
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.
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Chemical Composition of the Earth’s Atmosphere (Encyclopedia of Global Warming)
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....
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Chemical Reactions in the Troposphere (Encyclopedia of Global Warming)
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...
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Chemical Reactions in the Stratosphere (Encyclopedia of Global Warming)
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...
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Atmospheric Chemistry and Global Climate Change (Encyclopedia of Global Warming)
Humans tend to be most aware of weather—that is, a local area’s short-term temperature and precipitation variations. Scientists such as atmospheric chemists tend to concentrate on climate, or a large region’s long-term variations in temperature, precipitation, and cloud cover. Because of discoveries revealing the extreme complexity of chemical reactions in the atmosphere, atmospheric chemistry has become a profoundly interdisciplinary field, depending on new facts and ideas found by physicists, meteorologists, climatologists, oceanographers, geologists, ecologists, and other scientists.
Paleoclimatologists have studied changes in Earth’s atmosphere over hundreds of millions of years, while other environmental and atmospheric chemists have focused on such pivotal modern problems as global warming. These studies have led to research aimed at understanding the causes of global warming and the development of theories to explain existing data. Particularly useful has been computerized modeling of Earth’s atmosphere, through which experiments can be performed to help scientists understand likely future effects of climate change. These theoretical predictions have placed pressure on various governments to make important changes in policy, such as taxing fossil-fuel use to motivate reductions in GHG emissions.
Atmospheric chemists have come to realize that the goal of their research on...
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Context (Encyclopedia of Global Warming)
Atmospheric chemists’ discoveries have had a major influence on how environmentalists and other scientists understand the gravity, interrelatedness, and complexity of atmospheric problems. Many atmospheric chemists educate their students and the public about issues relating to global climate change, while others have been carefully monitoring the changes in the Earth’s atmosphere. They have also participated in international discussions and agreements about controlling GHG emissions, developing substitutes for CFCs, and passing local and international laws that would lessen the likelihood of some catastrophic scenarios predicted by various computer models. Just as the many components and reactions in the atmosphere make a full understanding of these complexities very difficult, so, too, environmental chemists find themselves in an even more complex milieu in which they have to integrate their understanding with those of other scientists, industrialists, and government officials in both developed and developing countries. Therefore, though global climate change is, at root, a physical and chemical issue, to solve the problem of global climate change will require an integrated, multidisciplinary, and international approach that, though daunting, appears to be increasingly necessary.
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Further Reading (Encyclopedia of Global Warming)
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. Intended to be “many things to many different people,” including scientists, politicians, and the public, this book grew out of an international conference on atmospheric chemistry. Contains lists of concrete proposals to ameliorate harmful atmospheric changes. Appendixes and index.
Jacob, Daniel J. Introduction to Atmospheric Chemistry. Princeton, N.J.: Princeton University Press, 2007. Undergraduate textbook written by a Harvard professor; provides an overview of the new and rapidly growing field of atmospheric chemistry. Illustrations and index.
Makhijani, Arjun, and Kevin R. Gurney. Mending the Ozone Hole: Science, Technology, and Policy. Cambridge, Mass.: MIT Press, 1996. Called “the most comprehensive overview of the ozone-depletion problem,” this book, accessible to both scientists and general readers, analyzes the problem as well as various solutions. Helpful summaries of chapters, appendixes, notes, twenty-five pages of references, and an index.
Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: John Wiley & Sons, 1998. This massive volume is intended “to provide a rigorous,...
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Atmospheric Chemistry (World of Earth Science)
Man lives at the bottom of an ocean of air. We may ordinarily take the atmosphere for granted and focus much more concern on the weather. This ocean of air, however, has profound consequences for life on Earth.
The surface density of air is about 0.074 lb/ft3 (1.184 g/l) and surface pressure is about 14 lb/ft2 (1 atm). This mass of air presses downward at all times. At a higher altitude, however, both the pressure and the density of air decrease. This explains why passenger jets, which often fly near 40,000 ft (12,192 m) to take advantage of the thin or low-density air, require pressurized cabins. Without them, passengers would not be able to take in enough oxygen with each breath.
The atmosphere is generally divided into four zones or layers. Starting at sea level and increasing in altitude, they are the troposphere (00 mi [06.1 km]), the stratosphere (100 mi [16.18.3 km]), the mesosphere (300 mi[48.36.6 km]), and the thermosphere (beyond 60 mi [96.6 km]). These altitudes are approximate and depend upon a variety of conditions, and are clearly distinct in both their physical properties (e.g., temperature) and their chemistry.
The troposphere is the region of air closest to the ground. It is where the clouds and storm systems are to be found, and where our weather occurs. The troposphere is in direct contact with effluent chemicals generated by living things. These can range from the carbon dioxide and water vapor we exhale to industrial or automotive pollutants. In the absence of such compounds, atmospheric chemistry is very simple. Since the splitting of both the nitrogen and oxygen molecules requires a great deal of energy, the atmospheric composition is fairly constant at sea level, and without interfering compounds.
Smog is the term applied to the mixture of nitrous oxides, spent hydrocarbons, carbon monoxide, and ozone that is generated by automobiles and industrial combustion. Smog is the thick brown haze that hovers over large populated areas. This combination of gases is reactive. The addition of water vapor or raindrops, for example, can result in the scrubbing of these compounds from the air but also the generation of nitrous, nitric, and carbonic acid. Ozone is a powerful oxidizing agent and results in the degradation of plastics and other materials. However, it is also capable of reacting with spent hydrocarbons to generate noxious chemicals.
Industrial pollutants, such as sulfur dioxide generated by coal-burning power plants, can generate acid rain as the sulfur dioxide is converted to sulfurous and sulfuric acid. Even forest fires contribute a large variety of chemical compounds into the atmosphere and induce chemical reactions. And, the largest of all natural disasters, a volcanic eruption, spews tons of chemical compounds into the troposphere where they react to produce acids and other compounds.
The stratosphere is the home of the ozone layer, which is misleading as it implies a distinct region in the atmosphere that has ozone as the major constituent. Ozone is never more than a minor constituent of the atmosphere, although it is a significant minor constituent. The concentration of ozone achieves its maximum in the stratosphere. It is here that the chemistry occurs that blocks incoming ultraviolet radiation.
The complete spectrum of radiation from the sun contains a significant amount of high energy ultraviolet light and the energy of these photons is sufficient to ionize atoms or molecules. If this light penetrated to Earth's surface, life as we know it could not exist as the ionizing radiation would continually break down complex molecules.
Within the ozone layer, this ultraviolet energy is absorbed by a delicate balance of two chemical reactions. The first is the photolytic reaction of molecular oxygen to give atomic oxygen, which subsequently combines with another oxygen molecule to give ozone. The second reaction is the absorption of another photon of ultraviolet light by an ozone molecule to give molecular oxygen and a free oxygen atom.
It is the combination of these two reactions that allows the ozone layer to protect the planet. These two reactions actually form an equilibrium with the forward reaction being the formation of ozone and the backwards reaction being the depletion.
The ozone concentration is thus at a constant and relatively low level. It occurs in the stratosphere because this is where the concentration of gases is not so high that the excited molecules are deactivated by collision, but not so low that the atomic oxygen generated can not find a molecular oxygen with which to react.
In the last half of the twentieth century, the manufacture of chlorofluorocarbons (CFCs) for use as propellants in aerosol sprays and refrigerants has resulted in a slow mixing of these compounds with the stratosphere. Upon exposure to high-energy ultraviolet light, the CFCs break down to atomic chlorine, which interferes with the natural balance between molecular oxygen and ozone. The result is a shift in the equilibrium and a depletion of the ozone level. The occurrence of ozone depletion was first noted over Antarctica. Subsequent investigations have demonstrated that the depletion of ozone also occurs over the Arctic, resulting in higher than normal levels of ultraviolet radiation reaching many heavily populated regions of North America. This is, perhaps, one of the most important discoveries in atmospheric chemistry and has lead to major changes in legislation in all countries in an attempt to stop ozone depletion.
Beyond the stratosphere, the energy levels increase dramatically and the available radiation is capable of initiating a wide variety of poorly characterized chemical reactions. Understanding all of the complexities of atmospheric chemistry is subject for much ongoing research.
See also Atmospheric composition and structure; Atmospheric pollution; Global warming