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Earth's atmosphere

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Minor components of air not listed above include:

GasVolumenitrous oxide0.5 ppmvxenon0.09 ppmvozone0.0 to 0.07 ppmvnitrogen dioxide0.02 ppmviodine0.01 ppmvcarbon monoxidetraceammoniatrace

Composition of the heterosphere

Above the turbopause (about 100 km), the Earth's atmosphere begins to have a composition that varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude, at a rate that depends on the molecular mass of the gas. Higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus, as the altitude increases in the heterosphere, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature3.

Biological significance

The Earth's atmosphere plays a vital role in sustaining life on this planet. Oxygen is needed for respiration by animals, plants, and some bacteria. Nitrogen is an inert gas that reduces the amount of oxygen available for the oxidation of natural materials, thus restricting spontaneous combustion (burning) of flammable materials and the corrosion of metals. Nitrogen is also used by "nitrogen-fixing" bacteria to produce compounds that are useful for plant growth. Plants that perform photosynthesis take up carbon dioxide from the air and release oxygen. Carbon dioxide and water vapor act as "greenhouse gases" that keep the Earth sufficiently warm to maintain life. Water vapor in the air is part of the water cycle that produces precipitation (such as rain and snow) that replenishes moisture in the soil. In addition, water vapor prevents exposed living tissue from drying up.

Moreover, several regions of the atmosphere exert their protective effect from a distance. For instance, the ozone layer absorbs UV radiation that can damage the tissues and genetic material of living organisms. The mesosphere, in which millions of meteors burn up daily, protects the Earth's surface from being continually bombarded by these falling objects. The magnetosphere, which extends well beyond the atmosphere, protects the Earth from the damaging rain of charged particles carried by the solar wind.

History of the Earth's atmosphere

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but one plausible sequence of events is given below.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere," to distinguish its chemical composition from two notably different earlier compositions. It is thought that the original atmosphere was primarily helium and hydrogen. Heat (from the still-molten crust and the Sun) dissipated this atmosphere.

About 3.5 billion years ago, the surface had cooled sufficiently to form a solid crust, still heavily populated with volcanoes that released steam, carbon dioxide, and ammonia. This led to formation of the "second atmosphere," composed primarily of carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. (Simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40 percent hydrogen 4). This second atmosphere had approximately 100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

During the next few million years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50 percent of the carbon dioxide was absorbed into the oceans.

Fossil evidence indicates that cyanobacteria were among the earliest types of bacterial life, existing approximately 3.3 billion years ago. They were the first organisms to carry out the photosynthetic conversion of carbon dioxide into oxygen, thus playing a major role in transforming the atmosphere from an anoxic state (a state without oxygen) to an oxic state (a state with oxygen). Oxygenation of the atmosphere is thought to have led to mass extinctions of species.

Later, photosynthesizing plants emerged and converted more carbon dioxide into oxygen. Over time, excess carbon became locked into fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to generate nitrogen. In addition, bacteria also converted ammonia into nitrogen.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first, the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere. With the appearance of an ozone layer, life forms were better protected from UV radiation. This oxygen-nitrogen atmosphere is the "third atmosphere."

Air pollution

Diagram of chemical and transport processes related to atmospheric composition.

Although technological advances have benefited humankind in numerous ways, they have been accompanied by adverse effects on the environment, including pollution of the air. Common air pollutants include carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), ozone, and particulate matter (PM). They are generally produced by such activities as (a) combustion (burning) of fuels for transportation and the generation of heat and electricity, and (b) industrial processes, including petroleum refining, cement manufacturing, and metal processing.

Carbon monoxide, a product of the incomplete combustion of fuels, is present at relatively high concentrations near roadways with heavy vehicular traffic. Inhalation of high levels of carbon monoxide can cause headaches, fatigue, respiratory problems, and (in extreme cases) death.

Among the various oxides of nitrogen, nitrogen dioxide (NO2) causes respiratory problems and contributes to acid rain. Nitrogen oxides also contribute to the formation of particulate matter in the air and nutrient overload in ponds and lakes, reducing water quality.

Various oxides of sulfur are formed by the burning of sulfur-containing fuels, including coal and oil, and also during the extraction of metals from ores and gasoline from oil. When dissolved in water, sulfur dioxide and sulfur trioxide form acids, contributing to acid rain. They also damage the respiratory system and contribute to the formation of particles that reduce visibility in the air.

In the presence of sunlight, nitrogen oxides can react with volatile organic compounds (such as gasoline vapors and chemical solvents) to produce ozone. Although ozone in the stratosphere exerts a protective effect by absorbing harmful UV radiation, ozone in the troposphere can irritate the lungs and cause inflammation, wheezing, coughing, and breathing difficulties. Repeated exposure to ozone can permanently damage the lungs. Ozone is a major component of city smog.

Particulate matter is a mixture of a variety of microscopic solids and liquid droplets, including metals, nitrates, sulfates, organic chemicals, and dust. The main concern is for small particles, with a diameter of 10 micrometers or less, as they can readily enter the lungs during the normal process of breathing. Particle pollution is linked to various problems, including difficulty breathing, chronic bronchitis, aggravated asthma, and irregular heartbeat.

Lead in the air is produced mainly by lead smelters, and also by waste incinerators, utilities, and lead-acid battery manufacturing plants. Exposure to lead can damage various body organs, including the brain, kidneys, and liver, and can cause osteoporosis and reproductive disorders.

To reduce the emission and accumulation of such pollutants, the governments of various nations have mandated measures such as the use of reformulated gasoline, catalytic converters in motor vehicle exhaust systems, and effluent traps for industrial wastes.

In addition to the problem of pollution, there is concern that global temperatures are rising as a result of increasing levels of greenhouse gases-such as carbon dioxide and methane-in the atmosphere. Greenhouse gases generally play a valuable role in that they absorb some of the Sun's heat and help keep the Earth warm and habitable. The concern is that a rise in the level of these gases is leading to the phenomenon of "global warming." Many scientists associate this rise with human activities such as the burning of fuels, land clearing, and agriculture. Others attribute global warming to increasing solar activity or other natural phenomena. In addition, the extent of warming caused by greenhouse gas emissions is being debated.

See also

References

  • Vercheval, J. The thermosphere: a part of the heterosphere. Retrieved June 23, 2015.

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