Ozone is a gas in the atmosphere that protects everything living on the Earth from harmful ultraviolet (UV) rays from the Sun. Without the layer of ozone in the atmosphere, it would be very difficult for anything to survive on the surface. (Think of very bad sunburn, only much worse!) Plants cannot live and grow in heavy ultraviolet radiation, nor can the plankton that serves as food for most of the ocean life. The ozone layer acts as a shield to absorb the UV rays, and keep them from doing damage to the Earth’s surface. Or worse, think about skin cancer and eye cataracts! In the last thirty years, it has been discovered that stratospheric ozone is depleting as a result of anthropogenic pollutants. There are a number of chemical reactions that can deplete stratospheric ozone; however, some of the most significant depletion comes from the catalytic destruction of ozone by freed halogen radicals like chlorine and bromine.
The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) are emitted into the atmosphere. Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometers above the Earth’s surface. Strong UV light breaks apart the ODS molecule. CFCs, HCFCs, carbon tetrachloride, methyl chloroform, and other gases release chlorine atoms, and halons and methyl bromide release bromine atoms. It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere.
Ozone is constantly produced and destroyed in a natural cycle, as shown in the above picture, courtesy of NASA GSFC. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream’s depth at a particular location. Although individual water molecules are moving past the observer, the total depth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades. Large increases in stratospheric chlorine and bromine, however, have upset that balance. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up.
Therefore, ozone levels fall. Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more the depletion, the larger the increase in incoming UVB. UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem.
Chlorine and bromine are emitted to the atmosphere from both natural and human sources. These very stable human-made chemicals are not soluble in water and are not broken down chemically in the lower atmosphere. Thus, they survive long enough to reach the stratosphere. The CFCs and carbon tetrachloride are relatively unreactive in the lower atmosphere (the troposphere) and move unscathed into the stratosphere where they are decomposed by intense sunlight, releasing chlorine to catalyze the destruction of ozone molecules. Certain ozone-depleting chemicals (HCFC-22 and methyl chloroform) are more reactive in the troposphere and deliver less of their initial chlorine load to the stratosphere. Halons also are generally reactive in the troposphere and deliver only a fraction of their initial load of bromine to the stratosphere, but bromine is 40 times more efficient at destroying ozone than chlorine. Increasing attention is being focused on the ozone-depleting role of methyl bromide, which has three potentially major human sources (soil fumigation, biomass burning, and the exhaust of automobiles using leaded gasoline), in addition to a natural oceanic source.
U.S. production of ozone-depleting gases has declined significantly since 1988 and has now reached levels (measured by their ozone depletion potential) comparable to those of 30 years ago. Because of the international agreements to decrease production and ultimately to phase out production of CFCs and halons, total equivalent chlorine (total chlorine and bromine, with adjustments to account for bromine’s higher ozone depletion potential) in the troposphere peaked between 1992 and 1994 and has since decreased. Total chlorine abundance in the stratosphere is at or near peak; stratospheric bromine is likely still increasing. Increasing ozone losses are predicted for the remainder of the decade, with gradual recovery by the mid-21st century.
Ozone is a protective layer of the earth as it protects earth from harmful ultraviolet radiation coming from the sun. Destruction of ozone is a continuous process that happens by both natural and human-related resources. The natural process always has some other related process that replenishes the depletion made by these. But when humanly interferes with nature then irreparable losses damage the nature. Hence it is our moral duty to get knowledge about our nature and reduce the emission of such gases as are harmful to our environment.