The chemicals in Earth's atmosphere give the sky its blue color and block out a majority of the sun's harmful radiation. This odorless and colorless mixture also gives life to all the living things on Earth and without it this planet would be as barren as the planet Mars. The air is thinner higher in the atmosphere, giving way to a slightly different concentration of certain chemicals compared to the air lower in the atmosphere.
Atmospheric Water Vapor
One of the biggest variants in the atmosphere's composition depending on altitude, and latitude, is the concentration of water vapor. The concentration of water vapor at higher altitudes is less than it is at lower altitudes. This causes drier climates in areas of the Earth such as mountain peaks and high deserts. Latitude plays an important role in the concentration of atmospheric water vapor because the troposphere - the layer of the atmosphere closest to the surface of the Earth and where water vapor is most abundant - is thicker and therefore higher at tropical latitudes than at arctic latitudes.
The Most Abundant Chemicals in the Atmosphere
The rest of the atmosphere that does not include water vapor, or the "dry" air, is mostly made up of three gases. The most abundant is nitrogen, which accounts for approximately 78.08% of the atmosphere's dry air. The second most abundant is oxygen, which accounts for approximately 20.95% of the atmosphere's dry air and the portion humans and other animal life rely on the most for respiration. The third most abundant is argon, which accounts for approximately 0.93% of the atmosphere's air. Eight gases make up the remaining 0.04% of the atmosphere's dry air.
Trace Chemicals in the Atmosphere
The eight gases that make up the remaining 0.04% of the atmosphere's dry air are called "trace" gases because they make up such a small portion of the atmosphere. These eight gases are carbon dioxide, neon, helium, methane, krypton, nitrous oxide, hydrogen and ozone. Initially, all gases in the atmosphere came from inside the Earth through volcanic degassing. However some gases, such as oxygen, do not originate from volcanoes but instead are created through chemical processes such as photosynthesis.
Photosynthesis is the process by which plants combine carbon dioxide and water to form carbohydrate compounds for energy - a byproduct of this process is oxygen. Chemical processes such as this, as well as many others, play a role in maintaining the delicate balance of chemicals in the atmosphere.
Source:
Skinner, Brian J., Porter, Stephen C, and Park, Jeffrey. Dynamic Earth - An Introduction to Physical Geology. NJ: John Wiley and Sons Inc., 2004.
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