Present Day Atmosphere

The atmosphere we breathe is a relatively stable mixture of several hundred types of gases from different origins.This gaseous envelope surrounds the planet and revolves with it.It has a mass of about 5.15 x 10E¹⁵ tons held to the planet by gravitational attraction.The proportions of gases, excluding water vapor, are nearly uniform up to approximately 80 kilometers (km) above Earth's surface.The major components of this region, by volume, are oxygen (21%), nitrogen (78%), and argon (0.93%).Small amounts of other gases are also present.These remaining trace gases exist in such small quantities that they are measured in terms of a mixing ratio.This ratio is defined as the number of molecules of the trace gas divided by the total number of molecules present in the volume sampled.For example, O₃, CO₂, and chlorofluorocarbons (CFCs) are measured in parts per million by volume (ppmv), parts per billion by volume (ppbv) or parts per trillion by volume (pptv).

Atmospheric temperature and chemistry are believed to be controlled by the trace gases.There is increasing evidence that the percentages of environmentally significant trace gases are changing because of both natural and human factors.Examples of man-made gases are the chlorofluorocarbons CFC-11 and CFC-12 and halons.Carbon dioxide, nitrous oxide, and methane (CH₄) are produced by the burning of fossil fuels, expelled from living and dead biomass, and released by the metabolic processes of microorganisms in the soil, wetlands, and oceans of our planet.

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