Composition of the Atmosphere
The gases in the atmosphere are composed of neutral, uncharged particles. Except for the noble gases, atoms in the gas phase share electrons with other atoms in chemical bonds so that their electron count can approach the more stable filled-shell configuration. The Earth's atmosphere consists of a mixture of noble gas atoms and many kinds of molecules.
Changes in CompositionEarth'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.
Molecular nitrogen and molecular oxygen are the most common gases in today's atmosphere. Others are present in small concentrations. The other more common gases are shown in the table below. There is a remarkable difference between the original, reducing atmosphere and the current oxidizing atmosphere.
The concentration of each gas is given in parts per billion (x/109) of all atoms or molecules present.
Note that helium in the atmosphere is derived from radioactive decay, loss of an alpha particle from some other nucleus. Many of the molecules are products of living things, oxygen from plants in photosynthesis for example. Chlorofluorocarbons, such as CF2Cl2, are made only through industrial processes.
PressureA key measure of gas-phase molecules is their pressure. For a gas in a container, the pressure of the gas is the force exerted by the gas particles hitting the surface of the container. There isn't really a container for our atmosphere so we need to think of pressure in a slightly different way.
The atmospheric pressure at the Earth's surface is given a unit of 1.0 atmosphere (atm). The SI unit for pressure is the pascal (Pa).
Layers of the AtmosphereThe atmosphere is composed of discrete layers. Atoms and molecules travel rapidly within a layer but only very slowly between layers. The layering results from temperature variations of the gas molecules.
In the ionosphere, there is a plasma. High energy solar radiation causes the atoms to ionize, separating free electrons from cations. The average kinetic energy is very high for the particles in the ionosphere but the gas density is very low.
Most of the UV radiation is filtered out by absorption in the stratosphere. Because the heat comes from the Earth, the temperature decreases as altitude increases in this layer.