Atmospheric Composition

ALL LIFE on Earth exists within a thin film of air, water, and soil about 9.3 mi. (15 km.) deep. This spherical shell of life is known as the biosphere. The biosphere can be divided into three layers: the atmosphere (air), the hydrosphere (water), and the lithosphere (rock and soil). The unique attributes of the Earth's atmosphere create a habitable place for humans, animals, and plants. It reaches over 348 mi. (560 km.) from the surface of the Earth. The atmosphere is a mixture of gases and particles that surround the planet. When seen from space, the atmosphere appears as a thin seam of dark blue light on a curved horizon. It is made of layers that surround the Earth like rings. About 99 percent of the Earth's atmosphere occurs within 31 mi. (50 km.) above the surface. The remaining one percent extends outward for several hundred km., fading gradually into interplanetary space.

The atmosphere serves several purposes: it provides habitants with the air to breathe, its gases retain the heat that warms the Earth, and its protective layer of ozone shields habitants from harmful ultraviolet rays emitted by the sun. The atmosphere also acts as a reservoir, or storehouse, for natural substances as well as emissions derived from human activities. Within the storehouse, physical and chemical actions and reactions take place. Many of these can affect climate and weather systems. Four distinct layers of the atmosphere have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. These are the: troposphere, stratosphere, mesosphere, and thermosphere. Beyond these atmospheric layers lies the exosphere.

Approximately 81 percent of Earth's atmosphere occurs in the troposphere, which extends about 3.710.5 mi. (6-17 km.) above the Earth's surface and is thickest at the equator. Temperatures in the troposphere generally decrease as altitude increases. Temperature in this layer is highest nearest the earth, in part because gases in the troposphere are warmed by heat radiated from the earth. The stratosphere extends beyond the troposphere to about 31 mi. (50 km.) above the Earth. Gases in the stratosphere are heated mainly by incoming radiation from the sun; temperatures in the stratosphere gradually increase as altitude increases. As a consequence of temperature differences between the troposphere and stratosphere, and the resulting circulation patterns, exchange of air between the two layers is slow.

The stratosphere is also known as the ozone layer. The distribution of ozone is closely linked to the vertical structure of the atmosphere. Approximately 90 percent of all ozone molecules are found in a broad band within the stratosphere. This layer of ozone-rich air acts as an invisible filter to protect life forms from over-exposure to the sun's harmful ultraviolet rays. Human activities have led to a depletion of the ozone layer observed since the mid-1980s.

The mesosphere starts just above the stratosphere and extends to 53 mi. (85 km.) beyond the stratosphere. In this region of atmosphere, temperatures fall as low as minus 135 degrees F (minus 93 degrees C). The thermosphere starts just above the mesosphere and extends 372 mi. (600 km.) beyond mesosphere. The thermosphere has two parts: the ionosphere (the inner part), and the exosphere (the outer part) that gradually merges into space.

Atmospheric Emission of Infrared Radiation

The composition of the atmosphere has changed over time. The original atmosphere primarily consisted of helium and hydrogen. Heat from the still-molten crust and the sun, and probably from enhanced solar wind as well, dissipated this atmosphere. The surface cooled about 4.4 billion years ago, but was still full of volcanoes that released steam, carbon dioxide, and ammonia. This led to the early second atmosphere, which was primarily composed of carbon dioxide and water vapors, with some nitrogen, but virtually no oxygen.

The second atmosphere had approximately 100 times more carbon dioxide than the third, or current, atmosphere. As the atmosphere cooled, much of the carbon dioxide either dissolved in the seas or precipitated as carbonates. Bacterial forms of life began approximately 3.3 billion years ago, and were probably were the first oxygen-producing phototro-pic organisms. They were responsible for converting the Earth's atmosphere from an anoxic to an oxic state during the period 2.7-2.2 billion years ago.

As life evolved on the planet and more plants appeared, the level of oxygen increased significantly, while levels of carbon dioxide dropped, eventually giving rise to the modern atmosphere, which is also known as the third atmosphere. This modern atmosphere has a composition enforced by oceanic blue-green algae, as well as geological processes. Thus, oxygen does not remain naturally free in atmosphere, but tends to be consumed by inorganic chemical reactions, animals, bacteria, and even plants at night. Carbon dioxide is produced by respiration and the decomposition of organic matter.

Dry air near the Earth's surface consists mainly of Nitrogen (78.1 percent by volume) and oxygen (20.9 percent) with a small amount of argon (about 0.9 percent) and carbon dioxide (about 0.035 percent). Air in the troposphere also contains water vapor and small amounts of "trace gases," such as methane, nitrous oxide, hydrogen, and ozone. Even though many of these gases are present in minute amounts, they cause the atmosphere to act like an insulating blanket around the planet. Without this atmospheric blanket, the Earth's surface would be too cold to sustain life. Naturally occurring atmospheric gases play a key role in determining the climate, and the increase in their concentration, attributed to natural and anthropogenic causes, results in more dramatic climate changes. In addition to these naturally occurring gases, there are other gases such as halocarbons (a result of industrialization) present in the troposphere. The chemical composition of the atmosphere is very important and influences the climate; the addition of gases changes the composition of the atmosphere, complicating climate-change processes.

sEE ALso: Atmospheric Vertical Structure; Mesosphere; Stratopause; Stratosphere; Thermosphere; Tropopause; Troposphere.

BIBLIogRAPHY. Edward Aguado and J.E. Burt, Understanding Weather and Climate (2006, Prentice Hall); C.D. Ahrens, Meteorology Today (Thomson Brooks/Cole, 2007); University of Tennessee at Knoxville Astrophysics, csep10 .phys.utk.edu/astr161 (cited November 2007).

Velma I. Grover Natural Resource Consultant

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