Composition of the Atmosphere

The earth’s atmosphere is a thin, gaseous envelope comprised mostly of nitrogen (N2) and oxygen (O2), with small amounts of other gases, such as water vapor (H2O) and carbon dioxide (CO2). Nestled in the atmosphere are clouds of liquid water and ice crystals.

• Although our atmosphere extends upward for many hundreds of kilometers, almost 99 percent of the atmosphere lies within a mere 30 km (about 19 mi) of the earth’s surface. 
• In fact, if the earth were to shrink to the size of a large beach ball, its inhabitable atmosphere would be thinner than a piece of paper. 
• This thin blanket of air constantly shields the surface and its inhabitants from the sun’s dangerous ultraviolet radiant energy, as well as from the onslaught of material from interplanetary space. 
• There is no definite upper  limit to the atmosphere; rather, it becomes thinner and thinner, eventually merging with empty space, which surrounds all the planets.

The various gases present in a volume of air near the earth’s surface. Notice that nitrogen (N2) occupies about 78 percent and oxygen (O2) about 21 percent of the total volume of dry air. If all the other gases are removed, these percentages for nitrogen and oxygen hold fairly constant up to an elevation of about 80 km (or 50 mi).

• At the surface, there is a balance between destruction (output) and production (input) of these gases. 
• For example, nitrogen is removed from the atmosphere primarily by biological processes that involve soil bacteria. 
• In addition, nitrogen is taken from the air by tiny ocean-dwelling plankton that convert it into nutrients that help fortify the ocean’s food chain. 
• It is returned to the atmosphere mainly through the decaying of plant and animal matter. Oxygen, on the other hand, is removed from the atmosphere when organic matter decays and when oxygen combines with other substances, producing oxides. 
• It is also taken from the atmosphere during breathing, as the lungs take in oxygen and release carbon dioxide. 
• The addition of oxygen to the atmosphere occurs during photosynthesis, as plants, in the presence of sunlight, combine carbon dioxide and water to produce sugar and oxygen.

*Radiant energy, or radiation, is energy transferred in the form of waves that have electrical and magnetic properties. The light that we see is radiation, as is ultraviolet light. 

**The abbreviation °C is used when measuring temperature in degrees Celsius, and °F is the abbreviation for degrees Fahrenheit.