The Earth's Atmosphere

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between the different atmospheric layers:

• troposphere: From the Greek word "tropos" meaning to turn or mix. The troposphere is the lowest layer of the atmosphere starting at the surface going up to between 7 km (4.4 mi) at the poles and 17 km (10.6 mi) at the equator with some variation due to weather factors. The troposphere has a great deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which then rise to release latent heat as sensible heat that further uplifts the air mass. This process continues until all water vapor is removed. In the troposphere, on average, temperature decreases with height due to exspansive cooling.
• stratosphere: from that 7-17 km range to about 50 km, temperature increasing with height.
• mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.
• thermosphere: from 80-85 km to 640+ km, temperature increasing with height.
• exosphere: from 500-1000 km up to 10,000 km, free-moving particles that may migrate into and out of the magnetosphere or the solar wind.

Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by 50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).

Air pollution is a chemical, physical (e.g. particulate matter), or biological agent that modifies the natural characteristics of the atmosphere. stratospheric ozone dephletion due to air pollution has long been recognized as a threat to human health as well as to the earth's eco systems.

Worldwide air pollution is responsible for large numbers of deaths and cases of respiratory diseas. Enforced air quality standards, like the clean air act in the United States, have reduced the presence of some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions are actually mobile sources, principally the automobile. Gases such as carbon dioxide, which contribute to global warming, have recently gained recognition as pollutants.

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat from the still-molten crust, and the sun, plus a probably enhanced solar wind, dissipated this atmosphere.

About 4.4 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the early "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere, but as it cooled much of the carbon dioxide was dissolved in the seas and precipitated out as carbonates.

The later "second atmosphere" contained nitrogen, carbon dioxide, and very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggest that it may have had up to 40% hydrogen[4]. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide and methane, kept the Earth from freezing. In fact temperatures were probably very high, over 70 degrees C (158 degrees F), until some 2.7 billion years ago.

One of the earliest types of bacteria were the cyanobacteria. Fossil evidence indicates that bacteria shaped like these existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen) during the period 2.7 to 2.2 billion years ago. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.

Photosynthesising plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to release nitrogen; in addition, bacteria would also convert ammonia into nitrogen. But most of the modern day level of nitrogen are due mostly to sunlight-powered photolysis of ammonia released steadily over the aeons from volcanoes.

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, resulting in mass extinctions and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere". 200 - 250 million years ago, up to 35 percent of the atmosphere was oxygen (bubbles of ancient atmosphere were found in an amber).

This modern atmosphere has a composition which is enforced by oceanic blue-green algae as well as geological processes. O2 does not remain naturally free in an atmosphere, but tends to be consumed (by inorganic chemical reactions, as well as by animals, bacteria, and even land plants at night), while CO2 tends to be produced by respiration and decomposition and oxidation of organic matter.

Oxygen would vanish within a few million years due to chemical reactions and CO2 dissolves easily in water and would be gone in millennia if not replaced. Both are maintained by biological productivity and geological forces seemingly working hand-in-hand to maintain reasonably steady levels over millions of years.