Cfc's & Ozone Depletion



 

The term “ozone” has appeared in numerous magazine and newspaper articles and has been a subject of discussion on both radio and television. Despite all the publicity surrounding this term, however, many people are still confused by it. The confusion arises from the fact that ozone is both beneficial and harmful. We know that the ozone layer in the upper atmosphere acts as a filter for the sun's ultraviolet rays, reducing the amount of radiation that reaches the earth's surface. At ground level, though, high concentrations of ozone can be harmful. Ozone gas forms a layer all around the earth high in the stratosphere. It serves as a vital and effective protective barrier from the sun's ultraviolet rays. In recent years, scientists have sounded alarms internationally about the depletion of the ozone layer, citing chemical pollution as the major cause.
A specific class of chemical compounds called chlorofluorocarbons (CFCs) are most often identified as ozone destroyers. CFCs were once widely used in everything from air conditioner coolants to the propellant in aerosol cans but have now been banned in most developed nations, including the U.S.
Many scientists believe much more needs to be done to protect the ozone layer, and international efforts are ongoing.
 Ozone pollution is really an increase in the concentration of ozone in the air at ground level. Because sunlight has a critical role in its formation, ozone pollution is principally a daytime problem in the summer months. Ground-level ozone is produced when sunlight combines with hydrocarbons and nitrogen oxide, two compounds produced by cars, trucks, factories, and power-generating plants, and found wherever gasoline, diesel fuel, kerosene, oil, or natural gas are combusted. Urban areas with heavy traffic, and large industrialized communities, are the primary areas with ozone problems.
  The cause of ozone depletion is the increase in the level of free radicals such as hydroxyl radicals, nitric oxide radicals and atomic chlorine and bromine. The most important compound, which accounts for almost 80% of the total depletion of ozone in the stratosphere are chlorofluorocarbons (CFC). These compounds are very stable in the lower atmosphere of the Earth, but in the stratosphere, they break down to release a free chlorine atom due to ultraviolet radiation. A free chlorine atom reacts with an ozone molecule (O3) and forms chlorine monoxide (ClO) and a molecule of oxygen. Now chlorine monoxide reacts with an ozone molecule to form a chlorine atom and two molecules of oxygen. The free chlorine molecule again reacts with ozone to form chlorine monoxide. The process continues and the result is the reduction or depletion of ozone in the stratosphere.



  Ozone (O3) is found in two different parts of our atmosphere. Ground level ozone, a human health irritant and component of smog, is found in the lower atmosphere (troposphere) and has nothing to do with the "ozone hole." However, ozone in the stratosphere—the layer of atmosphere above the troposphere accounts for the vast majority of atmospheric ozone. Stratospheric ozone is protective of human health as it absorbs ultraviolet radiation from the sun, preventing the radiation from hitting Earth's surface and harming living organisms from this biologically dangerous radiation. 

The thickness of the polar stratospheric ozone layer depends on the rate of production of ozone in the tropical stratosphere, the movement of ozone from the tropics to the poles, the amount of ultraviolet radiation from the Sun, the polar stratospheric cloud cover, and the chemical reactions between the ozone and ozone- depleting substances. Each of these factors might be affected by climate change. Poleward motions in the stratosphere, which increase polar concentrations of ozone, as well as the strength of the polar stratospheric vortices, which decrease ozone via PSC formation, are both expected to increase as temperatures rise in the lower atmosphere. Yet temperatures in the lower stratosphere are decreasing as a result of increased carbon and other heat-trapping emissions. The reason for this apparent paradox—increasing temperatures at the Earth's surface and decreasing temperatures in higher parts of the atmosphere—can be explained using the blanket analogy. Carbon dioxide and other heat-trapping gases rise into the atmosphere, spread around the globe, and act like a blanket holding in heat around Earth. This blanket also protects the warm surface of the Earth from the cold air above it. As heat-trapping gas concentrations increase, the blanket thickness also increases. This further warms the Earth’s surface; heats the blanket itself; and traps more heat in the lower atmosphere. Heat that normally (i.e. before blanket thickening) would escape the lower atmosphere and enter the stratosphere no longer does so, leaving the stratosphere cooler. Cooling of the lower polar stratosphere enhances PSC formation, and thus contributes to ozone loss. It appears unlikely that the decrease in ozone-depleting substances will lead to restabilization of the pre-1980 stratospheric ozone layer because of the competing and uncertain effects of further climate change.