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Name: Margie B.
Status: Student
Age: 18
Location: N/A
Country: N/A
Date: June 8, 2002


Question:
Why do the concentrations of the trace gases carbon dioxide, methane and oxides of nitrogen vary from place to place on the Earth's surface?



Replies:
The presence of these gases in any particular location depends upon both fixed and moving sources. Methane and CO2 are evolved from pasture land. Ammonia can be liberated from liquid ammonia fertilizer, CO2, CO, and nitrogen oxides are emitted by auto exhausts. In addition, CO2 is in equilibrium with ocean water and its solubility will change as the temperature of the water changes (solubility decreases with increasing temperature). There are many sources of various "trace" gases that can contribute to local variations.

Vince Calder


Margie,

Carbon dioxide, methane, and nitrogen oxides are produced and removed both naturally and by man's activities. In both cases, the production and removal are controlled or affected by land and vegetation type, by the density and type of industries present, by the number of people and cattle in an area, by the animal biology of the area, by whether it is a land area or over water (and dependent on the kind of water), by meteorology (temperature, precipitation, weather patterns and climate), and by lightning.

You can see that atmospheric chemistry is a complex system.

Let us look at each of these three gases separately.

Carbon dioxide is produced (called "sources") by people and animals breathing, by volcanoes, and by natural chemical processes, such as the out-gassing of wet limestone bedrock or soil. Carbon dioxide is also produced by the burning of fossil fuels by man. The latter has contributed to the rise of carbon dioxide in the atmosphere over the past 150 years or so. Plants and the oceans (called "sinks") remove carbon dioxide from the air. Where there are more sources, such as industry, or less sinks, such as forests, there will be more carbon dioxide. So, you would expect to see less carbon dioxide near the ground in a tropical forest than in an industrial area of the mid-western USA. Carbon dioxide is monitored at a high altitude on a volcano in Hawaii to avoid such influences and give scientists a true idea of the general atmospheric level of carbon dioxide.

Methane is produced (sources) primarily from the refining of oil, from fossil fuel wells and coal mines, by the deterioration of plant matter in swamps, and by animals such as termites and cattle that naturally convert plant matter into methane gas. Natural gas, such as that used in homes for heating and cooking, has a small amount of methane in it. Clearly, areas where many termites or cattle exist or where there are swamps or refineries will have higher levels of methane in the air. Methane is removed (sinks) by natural processes such as precipitation, by plants, and by water bodies.

Nitrogen oxides are produced (sources) naturally primarily by lightning (one of my areas of expertise). Around 10% of the nitrogen oxides in the Earth's atmosphere is produced by lightning. Volcanic activity contributes some as well.

Man's activities (fossil fuel burning - cars, industry, etc.) produce a large amount of nitrogen oxides also. Nitrogen oxides react naturally with other gases in the atmosphere, such as ozone, as well as natural particulates from trees and other plants, and can reduce atmospheric visibility or even create smog. A lot of nitrogen oxides are produced in cities, creating respiratory problems and smog. On a clear evening near a city (especially after a few days of "nice" weather) or as you fly in an airplane from or to a city you can see a brown tint as you look at the horizon. This is nitrogen oxides built up in the air under the stable, capping inversion over the city. This is not seen very often in rural areas, where there are few sources of nitrogen oxides. Nitrogen oxides are removed from the air by precipitation and by vegetation.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory



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