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The atmosphere contains many gases, most in small amounts, including some pollutants and greenhouse gases. The most abundant gas in the atmosphere is nitrogen, with oxygen second. Argon, an inert gas, is the third most abundant gas in the atmosphere.
The atmosphere is concentrated at the earth’s surface and rapidly thins as you move upward, blending with space at roughly 100 miles above sea level. The atmosphere is actually very thin compared to the size of the earth, equivalent in thickness to a piece of paper laid over a beach ball. However, it is responsible for keeping our earth habitable and for producing weather.
The atmosphere is composed of a mix of several different gases in differing amounts. The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon. Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%. Gases like carbon dioxide, nitrous oxides, methane, and ozone are trace gases that account for about a tenth of one percent of the atmosphere. Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is. In the cold, dry artic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere. Water vapor content is very important in predicting weather.
Changes in Composition
Earth’s primordial atmosphere was probably similar to the gas cloud that created the sun and planets. It consisted of hydrogen and helium, along with methane, ammonia, and water. This was a reducing atmosphere. There was no molecular oxygen or other reactive oxides. Over time, some of this first atmosphere, particularly the lighter gases, outgassed and was lost. More water may have arrived with comets colliding on the surface of the planet. Volcanic activity in the early, Earth created major changes with release of water vapor, carbon dioxide, and ammonia along with small quantities of SO2, H2S, HCl, N2, NO2, He, Ar, and other noble gases. This produced the second atmosphere.
Comet impacts may have increased the amount of water. Water vapor formed clouds. These produced rain. Over a period of thousands of years, the liquid water accumulated as rivers, lakes, and oceans on the Earth’s surface. Bodies of liquid water acted as sinks for carbon dioxide. Chemical and biological processes transformed CO2 gas to carbonate rocks. The nitrogen and argon accumulated in the atmosphere. They do not react with water or other atmospheric components. Oxygen existed in only trace quantities before life began.
Living things created much of the third atmosphere, the one that now exists on Earth. Cyanobacteria were responsible for the rise in the atmospheric concentration of oxygen beginning 2.3 billion years ago. These bacteria, algae, and other plants produce oxygen by photosynthesis. Although most of this oxygen is used in respiration (biological oxidation) or in the atmospheric oxidation of the carbon-containing products, approximately 0.1 % of the organic matter is sequestered in sediments and that quantity of oxygen is added to the atmosphere. Over time, the excess oxygen has built up so that it is now makes up nearly 20% of the gases close to Earth.
| Composition of Earth’s Atmosphere | |
|---|---|
| Nitrogen | 78.1% |
| Oxygen | 20.9% |
| Argon | 0.9% |
| Carbon dioxide, Methane, Rare (inert) gases | 0.1% |
