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O3 + hν → O + O2 This is the reaction that is responsible for the screening effect of ozone, since it absorbs ultraviolet radiation.
Chemists are able to calculate the concentration of ozone that should be in the stratosphere by working out how fast the rate of making and destroying occur, but when they actually measured the concentration it was a great deal less than expected. This tells us that there must be something else that is destroying the ozone other that the natural reactions.
There are other radicals in the stratosphere that can react with ozone. An example of these is the chlorine atom and the bromine atom. Chloromethane and bromomethane can get into the stratosphere naturally from the oceans and by burning of coal and vegetation. Most of the chloromethane and bromomethane react in the troposphere but some manage to get onto the stratosphere. Once in the stratosphere the chloromethane splits up due to solar radiation and then the chlorine atom is able to react with ozone. The reaction occurs in a catalytic cycle:
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Cl + O3 → ClO + O2 The ClO is another radical able to react again
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ClO + O → Cl + O2 So now there are two reaction competing to destroy ozone
1. Cl + O3 → ClO + O2
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O + O3 → O2 + O2
In the stratosphere there is a much lower concentration of Cl atom compared to O atoms, so it would seem that the reaction with Cl would be insignificant but chemists have measured the rate of reaction of Cl with ozone and have found it to be 1500 faster than the rate of reaction with O atoms. Also because Cl atoms are regenerated in a catalytic cycle they play a large part in removing ozone.
In the 1930’s scientist Thomas Midgley demonstrated a new refrigerant. He inhaled a lungful of dichlorodifluoromethane (CCl2F2) and used it to blow out a candle. He was showing two of the properties of the gas; its lack of toxicity and lack of flammability. CCl2F2 belongs to family known as chlorofluorocarbons (CFC’s), which contain chlorine, fluorine and carbon. Some other useful things about CFC’s is there low boiling points and low reactivity. CFC’s seemed to be the perfect answer to many problems. They had many uses such as propellant in aerosols, refrigerants, blowing agents for making expanded plastics and as cleaning solvents.
In the troposphere they remain unreactive and there estimated lifetime is about 100 years. This gives them plenty of time to reach the stratosphere where they are no longer unreactive. They are split up by the solar radiation giving out the fluorine and chlorine to react in the stratosphere destroying ozone.
In 1984 a group of British scientists discovered a ‘hole’ in the ozone layer over Antarctica due to CFC’s. Due to the scientists finding this hole there has been a worldwide ban on using CFC’s since 1990. A replacement to CFC’s are hydrochlorofluorocarbons (HCFC’s) and hydroflurocarbons (HFC’s). The difference with these is that they contain a C-H bond which is quickly broken down in the troposphere before they have time to reach to stratosphere. Unfortunately they are not the perfect solution as they contribute to global warming.
The Greenhouse effect occurs in the. Without it the Earth would be uninhabited. By trapping the Sun’s radiation the atmosphere keeps the temperature high enough to support life. Greenhouse gases are gases that trap the Sun’s radiation. Burning fossil fuels, therefore releasing CO2 can make more greenhouse gases effectively warming the Earth.
The Sun radiates energy mainly in the ultraviolet and visible. The part that is absorbed helps to heat the Earth, and the Earth the radiates energy back into space. Some of the energy radiated is absorbed by the troposphere heat the Earth. This is known as the greenhouse effect. If we release to many greenhouse gases this will in turn heat the Earth more causing sea levels to rise as the ice caps melt. The rise in sea level will leave low-lying coastal place to disappear under-water.
