The atmosphere surrounds the Earth as a thin layer, about 100 km thick, and is composed of 78% N2 (nitrogen), 21% O2 (oxygen), 1% Ar (Argon) and 0.0.39% CO2 (carbon dioxide). The atmosphere has a number of different layers and each of these layers has different properties. Weather changes, clouds, rain and wind form in the lower atmosphere, a 10 to 15 km thick layer known as the Troposphere. Although the troposphere is deeper in tropical regions, reaching 20 km, in polar regions it can reach 7 km during winter. The troposphere constitutes 90% of the total mass of the atmosphere and almost all the water vapor and this is due to the processes of transpiration and evaporation. As altitude increases in the troposphere, the temperature decreases by approximately 6.5°C for each kilometer of altitude increase. The upper limit of the troposphere is the coldest and ends at a point called the “tropopause”, where the upper atmosphere, also known as the “stratosphere”, begins. The stratosphere, which extends about 50 km from where the troposphere ends, is stratified in terms of temperature, with the lower layers being colder while the temperature increases in the upper layers. The reason why the temperature of the stratosphere is stratified is because ozone (O3) absorbs high-energy ultraviolet C and ultraviolet B energy waves from the sun and is broken down into diatomic oxygen (O2) and atomic oxygen (O) . Therefore, the stratospheric ozone layer protects plants and animals from carcinogenic and lethal amounts of sun exposure. Although about 10% of the mass of the atmosphere is in the stratosphere, the air is very thin. The upper region of the atmosphere is known as the ionosphere and begins at about 85 km altitude and extends… to the center of the paper… particles that produce chloride free radicals from UV light. Chloride free radicals interact with the ozone layer O3 to break down the ozone layer: CL-CL –> CL. CFC chloride forms free radicals when UV light interacts with them: CL. + O3 –> CLO + O2. Chloride free radical interacts with ozone to produce O2 free radical: OO-> O. Oxygen free radical produced by UV light: O. –>O3 – O2 + O2. Oxygen radicals also attack the ozone layer to produce more oxygen radicals. In 2009, NASA released a computer simulation of what would happen if the use of CFCs was not restricted. The simulated two-thirds of the ozone layer would be destroyed by 2065 at the latest and, therefore, the amount of ultraviolet radiation in the country that comes into play would have to be increased by 650%. Ultraviolet radiation would have caused significant damage to all life on Earth.