Earth's atmosphere is a layer of gases that surrounds the planet, reaching a thickness of about 300 mi (480 km). This same distance may be considerable to a human traveling by car, but relative to the infinite vastness of space, our atmosphere is vanishingly thin.
This thin blue line cocooning Earth, which is so apparent from orbiting spacecraft and which makes life on the planet possible, is susceptible to change and damage. Contamination of the atmosphere by noxious compounds has occurred ever since prehistoric peoples used fire. However, the tremendous growth in the use of machinery that began in the late eighteenth century spawned the growth of industry and increased air pollution.
A legacy of the twentieth century is the use of compounds such as chlorofluorocarbons (CFCs) in a variety of household and personal products. The escape of these compounds into the atmosphere has led to the depletion of atmospheric ozone (O3), which has increased the amount of damaging ultraviolet radiation reaching Earth's surface. These and other human activities far outweigh natural forms of atmospheric pollution.
Historical Background and Scientific Foundations
Atmospheric pollution can occur naturally. For example, the Hawaiian volcano Kilauea spews out approximately 2,000 tons (1,800 metric tons) of sulfur dioxide (SO2) every day during eruptions. Even more spectacularly, the series of massive eruptions of the Krakatoa Volcano on August 27, 1883 propelled ash 50 mi (80 km) into the atmosphere. The ash restricted the penetration of sunlight to such an extent that the average global temperature the next year was up to 2°F (1.2°C) below normal.
Nonetheless, these and other natural causes of atmospheric pollution are nowhere near as influential as human activities, which have occurred for hundreds of thousands of years, ever since prehistoric peoples learned to use fire for cooking and warmth. Examination of ice cores recovered from the Greenland ice sheet has revealed higher levels of lead, mercury, and nickel beginning
about 5,000 years ago, when mining and smelting of metal ores began in Europe.
Despite this long history of atmospheric abuse, the present problems with atmospheric pollution date from the end of the eighteenth century and early nineteenth century, the period that is known as the Industrial Revolution. Then, a number of technological advances including the introduction of the steam engine spurred the growth of manufacturing plants in or near major cities (a source of cheap and abundant labor for the factories). The concentration of industries and their use of coal as the power source instead of flowing water increased air pollution.
Although the air in the atmosphere is still made up predominantly of oxygen and nitrogen, atmospheric data collected since the Industrial Revolution have shown that the content of other, so-called trace gases has changed. Furthermore, new compounds have appeared. The best example are chlorofluorocarbons (CFCs), which are a product of twentieth century technology.
The atmospheric release of CFCs and hydrochlororfluorocarbons (which are used in air conditioners, refrigerators, and aerosol cans), halons (an ingredient of fire extinguishers), methyl chloroform, and methyl bromide have depleted the level of the atmospheric gas called ozone. Ozone consists of three oxygen molecules. The gas is found mainly in an upper layer of the atmosphere called the stratosphere.
Ozone is able to absorb the ultraviolet (UV) wavelengths of sunlight. UV light has sufficient energy to penetrate into the upper layers of skin, causing sunburn and, more ominously, to break apart the chains of genetic material inside cells. This genetic damage can lead to the development of some cancers. UV light can also damage vision. CFCs and the other compounds chemically destroy ozone, allowing more UV radiation to reach Earth's surface.
Scientists began to monitor atmospheric ozone levels in the 1970s. Over the next three decades, the declining levels of ozone were recognized. The decline is not evenly spread throughout the atmosphere. Rather, the depletion is more pronounced in some regions, in particular over the Antarctic as an “ozone hole.”
Since the time of the Industrial Revolution, trace gases such as carbon dioxide (CO2), nitrous oxide (N2O), and sulfur dioxide (SO2) have built up in concentration in the atmosphere. These gases have been dubbed greenhouse gases because, analogous to the way a greenhouse traps the sun's heat, they cause the heat from the sun to be retained by the atmosphere. The result has been the warming of the atmosphere that is popularly known as global warming.
Impacts and Issues
Human industrial activity is by far the greatest contributor to atmospheric pollution. According to the U.S. Environmental Protection Agency (EPA), approximately 6.5 billion lbs (3 billion kg) of toxic compounds (including 100 million lbs, or 45 million kg, of cancer-causing chemicals) are released to the atmosphere every year in the United States alone.
A recent illustration of the influence of human activity on atmospheric quality is Beijing, China. Satellite monitoring of China as part of the European Space Agency's Dragon Programme has revealed that the development of Beijing into an industrially important mega-city containing over 10 million people and nearly three million vehicles has been accompanied by an accumulation of the planet's highest levels of nitrogen dioxide. This gas is a respiratory irritant and, paradoxically to ozone depletion higher in the atmosphere, causes ozone build-up near the ground, which triggers smog. Similar findings have been found over developing areas of India.
As much a concern as ozone depletion is, there is good news. The effect is reversible in the absence of the ozone-destroying compounds. More than 165 nations are signa-tories to the Montreal Protocol, an international agreement drafted in 1987 that commits them to phase out the use of ozone-depleting substances according to a timetable.
WORDS TO KNOW
CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.
INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.
OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.
STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth's surface.
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Atmospheric pollution (also commonly called air pollution) is derived chiefly from the spewing of gasses and solid particulates into the atmosphere. Many pollutants—dust, pollen, and soil particles—occur naturally, but most air pollution, as the term is most commonly used and understood, is caused by human activity. Although there are countless sources of air pollution, the most common are emissions from the burning of hydrocarbons or fossil fuels (e.g., coal and oil products). Most of the world's industrialized countries rely on the burning of fossil fuels; power plants heat homes and provide electricity , automobiles burn gas, and factories burn materials to create products.
Air pollution is a serious global problem, and is especially problematic in large urban areas such as Mexico City, Mexico, and Athens, Greece. Many people suffer from serious illnesses caused by smog and air pollution in these areas. Plants, buildings, and animals are also victims of a particular type of air pollution called acid rain . Acid rain is caused by airborne sulfur from burning coal in power plants and can be transported in rain droplets for thousands of miles. Poisons are then deposited in streams, lakes , and soils, causing damage to wildlife. In addition, acid rain eats into concrete and other solid structures, causing buildings to slowly deteriorate.
Scientists study air pollution by breaking the particulates into two different categories of gasses: permanent and variable. The most common of the stable gasses are nitrogen at 78%, and oxygen at 21% of the total atmosphere. Other highly variable gasses are water vapor, carbon dioxide , methane, carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone , ammonia, and hydrogen sulfide.
Output of variable gasses increases with the growth of industrialization and population. The benefits of progress cost people billions of dollars each year in repairing and preventing air pollution damage. This includes health care and the increased maintenance of structures such as the Great Pyramids of Egypt that are crumbling, in part due to air pollution.
The effects of air pollution have to be carefully measured because the build-up of particulates depends on atmospheric conditions and a specific area's emission level. Once pollutants are released into the atmosphere, wind patterns make it impossible to contain them to any particular region. This is why the effects of pollution from major oil fires in the Middle East are measurable in Europe and elsewhere. On the other hand, terrestrial formations such as mountain ridges can act as natural barriers. The terrain and climate of a particular area (e.g., Denver, Houston, and Los Angeles) can also help promote or deflect air pollution. Specifically, weather conditions called thermal inversions can trap the impurities and cause them to build up until they have reached dangerous levels. A thermal inversion is created when a layer of warm air settles over a layer of cool area closer to the ground. It can stay until rain or wind dissipates the layer of stationary warm air.
The United States government plays an active role in establishing safe and acceptable levels of clean air. In 1967, Congress passed the Air Quality Act that set forth outlines for air quality standards. The Environmental Protection Agency released the first nationwide survey on air pollution in 1989 after Congress passed a law requiring the report. In most cases, it is up to individual states, however, to enforce air pollution controls and meet federally mandated goals. In addition, states may set their own clean air standards that are more strict than those established at the federal level. For example, in 1989, California adopted a radical air pollution reduction plan that essentially requires each region to drastically reduce current levels of air pollution. Even as early as 1970, California adopted more stringent standards for motor-vehicle emissions.
Government regulations have shown moderate success. Since 1970, emissions of sulfur oxide, carbon monoxide, lead , and hydrocarbons have decreased by approximately 30% while nitrogen oxide output has been reduced by approximately 10%. Cars are now required to have pollution-control devices called catalytic converters, and most power plants are equipped with filters called scrubbers to remove sulfur oxides.
In addition to atmospheric pollution, indoor air pollution also poses special hazards. Some man-made sources of indoor air pollutants include asbestos particulates and formaldehyde vapors—once common building materials now thought to cause cancer. Lead paint is also a problem in older buildings, but its use has been phased out. Other sources of man-made indoor air pollution include improperly vented stoves and heaters, tobacco smoke, and emissions or spillage from pesticides, aerosol sprays, solvents, and disinfectants.
See also Atmosphere; Geochemistry; Global warming; Greenhouse gases and greenhouse effect; Petroleum, economic uses of; Rate factors in geologic processes; Weathering and weathering series