Holes in our planet's "safe zone"
Ozone is a form of molecular oxygen containing three oxygen atoms. It plays a dual role in the atmosphere, sometimes good and sometimes bad. In the lower atmosphere known as troposphere, it is a noxious pollutant - a greenhouse gas and a major component of smog. At altitudes of 12 to 30 miles in the stratosphere, ozone acts as a savior by forming a protective blanket, shielding us from deadly solar ultraviolet (UV) radiation. In reasonable amounts, UV rays tan skin and stimulate vitamin D production in the skin. In large amounts they are carcinogenic. They would cause melanoma skin cancer, serious burns, and lethal mutations. Besides, they can cause the skin to thicken, wrinkle, develop dark spots, and become leathery. This will lead to premature skin aging, a phenomenon known as photoaging. Eyes are also damaged by UV rays. Harmful effects include cataracts, retinal injury, age-related macular degeneration, and snow blindness (temporary blindness caused by exposure to sunlight reflected from snow or ice.). They also suppress the immune system, reducing our ability to fight off these and other maladies. The UV rays could damage crops and aquatic ecosystems too. The amount of UV rays that will reach the Earth's surface is influenced by concentration of ozone in the stratosphere. When UV rays strike an ozone molecule, it is split into oxygen atom and oxygen molecule. The reaction products quickly recombine to form ozone and the harmful UV radiation is converted into thermal energy. Thus ozone is continually depleted and regenerated in a natural cycle, restoring the equilibrium concentration of the molecule. It is a renewable form of invisible filter that protects all forms of life from the Sun's harmful UV radiation. It is believed life would not have evolved the way it is, had it not been for ozone, our planet's “safe zone.†Human interference with the atmosphere is systematically reducing the concentration of ozone. The depletion is caused by the controversial chlorofluorocarbons (CFC). These were used in industrial solvents, refrigerators, air conditioners, coolants, anti-freeze, and as propellants in spray cans. The CFCs are extremely stable, inert, and insoluble in rain water. They quickly disperse into the atmosphere and eventually migrate to the stratosphere where UV rays dissociate them into chlorine oxide and other molecular fragments. Chlorine oxide reacts with ozone and destroys it, thereby creating the so-called “hole†in the ozone layer. Since ozone layer is our primary shield against UV rays from the Sun, less ozone means higher UV levels on the Earth's surface. To protect the ozone layer, production and use of CFCs have been phased out about two decades ago. Though a prudent move, it was too little too late. A sobering word is in order here. The CFCs last from 75 to 110 years. Thus the ones that were released before the phase-out are still alive and kicking. They are continuing with their ozone-destruction activity and will keep on doing so till they are neutralized by other reactions. While much attention has been focused on CFCs, little mention has been made of other gas molecules that have potential impact on the ozone layer. These gases, particularly different oxides of nitrogen with a lifespan from weeks to 170 years, when vented into the atmosphere are as potent an ozone-destroyer as the CFCs. In fact, they are now the largest ozone-depleting substance emitted through human activities, and is expected to remain the largest throughout the 21st century Natural sources of oxides of nitrogen are volcanoes, lightning, solar flares, and forest fires. Among the man-made sources, burning of fossil fuels and nitrogen based fertilizers are the principal ones. But emission from fertilizers is in balance with ozone replenishment. Other sources are the injection of nitric oxide from the exhaust of supersonic jets and military aircrafts flying at high altitudes. Another surprising and not oft discussed source is leftover nitric oxides produced during atmospheric nuclear tests in the 1950s. It is estimated that for each megaton of nuclear explosive power, 5000 tons of nitric oxide is produced. It seems depletion of the ozone layer is continuing unabated by the remnant CFCs and oxides of nitrogen. In fact, one of the biggest holes in the ozone layer was discovered in 2006! The good news is if emission of these oxides of nitrogen from human activities can be curtailed, then together with the ban on CFCs, the natural ozone production process will heal the ozone layer in about 75 years. “Earth without ozone is like a house without roof.â€
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