![]() In contrast to nitrogen, oxygen is extremely reactive. The next most common gas, oxygen (O 2), is required for the respiration (breathing) of all animal life on Earth, from humans to bacteria. The most common atmospheric gas, nitrogen (chemical symbol N 2) is largely inert, meaning that it does not readily react with other substances to form new chemical compounds. Composition of the atmosphere Major gases Human activities may be affecting the levels of some important atmospheric components, particularly carbon dioxide and ozone. Oxygen, upon which all animal life depends, probably accumulated as excess emissions from plants that produce it as a waste product during photosynthesis. It is thought that the present atmosphere developed from gases ejected by volcanoes. The upper mesosphere and lower thermosphere are called the ionosphere, this region is important to radio communications, because radio waves can bounce off the layer and travel great distances. These two layers include regions of charged atoms and molecules, or ions. Above the stratosphere are the mesosphere and thermosphere. The next layer, the stratosphere, contains most of the atmosphere ’s ozone, which absorbs high-energy radiation from the sun and makes life on the surface possible. The layer closest to the surface is the troposphere, which contains over 80% of the atmospheric mass and nearly all the water vapor. The atmosphere can be divided into vertical layers determined by the way temperature changes with altitude. All of these trace gases have important effects on Earth ’s climate. The remainder, less than 0.1%, contains such trace gases as water vapor, carbon dioxide, and ozone. ![]() Earth ’s atmosphere is composed of about 78% nitrogen, 21% oxygen, and 0.93% argon.
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