Permanent Atmospheric Gases
Although the composition of the atmosphere is always changing, an analysis of a "snapshot" of the atmosphere can provide us with a fairly good representation of the average concentrations of the "permanent" gases. "Permanent" here means the concentration is virtually constant near the earth's surface. Our snapshot reveals that nitrogen gas (N2) makes up 78% of the concentration of the dry atmosphere (by volume). Oxygen gas (O2), the second most abundant, makes up about 21% of the volume of the dry atmosphere. "Dry atmosphere" simply refers to a theoretical atmosphere that contains no water vapor. Together, nitrogen and oxygen make up about 99% of the air we breathe. Natural biological processes within the earth-atmosphere system maintain the amounts of these two gases to near constant proportions from the earth's surface up to about 50 miles (80 kilometers) above the earth.
Nitrogen and oxygen are the only gases that exist in the dry atmosphere that have concentrations above one percent near the earth's surface. The third most abundant gas in the dry atmosphere is argon (Ar). Argon is also considered a permanent gas. The "permanent" concentration of Argon is just less than one percent (about 0.93% to be more exact).
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