Ozone Layer

Ozone Layer
Orbiting above the Earth, an astronaut can look down on our home and see the thin blue ribbon that rims our planet. That transparent blanket – our atmosphere – makes life possible. It provides the air we breathe and regulates our global temperature. And it contains a special ingredient called ozone that filters deadly solar radiation.
The Atmosphere
The gaseous area surrounding the planet is divided into several spherical strata separated by narrow transition zones which is the atmosphere. (Graedel 1998) The upper boundary at which gases disperse into space lies at an altitude of approximately 1000 kilometers above sea level. (Graedel 1998) More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth’s surface. (Graedel 1998) Atmospheric layers are characterized in chemical compositions that produce variations in temperature. (Graedel 1998)
Here is a graph of the different layers in the atmosphere:

The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of the mass of the total atmosphere. (Garcia 1994) It is characterized by the density of its air and an average vertical temperature change of 6 degrees Celsius (C) per kilometer. (Graedel 1998)
Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet\'s surface. (Graedel 1998) The troposphere contains 99 % of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position. They are greatest above the tropics, where they may be as high as 3 %, and decrease toward the Polar Regions.
All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere. (Graedel 1998) Troposphere means "region of mixing" and is so named because of vigorous convective air currents within the layer. (Graedel 1998)
The upper boundary of the layer ranges in height from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons, highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. (Graedel 1998) Air temperature within the tropopause remains constant with increasing altitude.

The stratosphere is the second major stratum of air in the atmosphere. It resides between 10 and 50 km above the planet\'s surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. (Graedel 1998) Then it increases gradually to 200-220 degrees Kelvin (K) at the lower boundary of the stratopause (50 km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. (Graedel 1998) Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapor content within the layer is very low. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere. (Graedel 1998)
The ozone layer is located at an altitude between 20-30 km. Approximately 90 % of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in this region is about 10 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere. (Graedel 1998) Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet\'s surface would damage plant life and have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation would result in a host of biological effects, such as a dramatic increase in cancers.
Meteorological conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present. Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does production. (Graedel 1998)
The mesosphere, a layer extending from approximately 50 km to 80 km, is characterized by decreasing temperatures, which reach 190-180 K