Ozone layer is the most important for any type of the living organism on the earth when it is found in the stratosphere. Ozone is high reactive gas which has blue color. Ozone basically poisonous gas if inhaled can cause death. Ozone contains an extra atom of oxygen (O3).
Ozone is the protective layer which protects our earth from ultraviolet radiations. If there are 10 million molecules of air 2 million are the O2 and only 3 molecules are the ozone .Stratosphere is the earth’s upper atmosphere it extends from 10 km to 50 km from earth surface .There is high concentration of ozone in the stratosphere. Troposphere also contains a little bit amount ozone. Depletion of the ozone layer results the increase in the amount of the ultraviolet radiations.
In future if we could not control the use of chlorofluorocarbon & halogens the destruction of ozone layer increases day by day. In 1913 ozone layer was discovered by French physicists Charles Fabry and Henri Buisson. Properties of the ozone layer are explained by the British meteorologist G. M. B. Dobson and for that process he developed a spectrophotometer to measure the ozone layer from stratospheric to ground. Unit of ozone Dobson also by his name for the honor of Dobson .In 1970 chlorofluorocarbon’s discovered in the stratosphere which plays an important role in the destruction of the ozone layer. Formation of the ozone molecule is done when ultraviolet radiations hits the oxygen molecule and divided it in to two oxygen atoms. When one atom from these two oxygen atoms combined with oxygen molecule it formed the ozone molecule. The process in which ozone molecule is formed is called photolysis. Hydrogen, chlorine and nitrogen are naturally present in the stratosphere in a very low amount when these chemicals react with the sun light due to that reaction ozone breaks down naturally balance equilibrium between destruction and formation of ozone. Resulting of that process amount of ozone present in the stratosphere remains constant. At different altitudes the temperature and pressure vary due to that variation rate of destruction and rate of formation of ozone also vary according to the situation. Between 19-24 km there is highest concentration of ozone in stratosphere .Whether is the most important factor which effect the ozone concentration in stratosphere above the earth it changes day to day and there is a difference in concentration of the ozone between winter and spring season. Ozone layer over Canada thicker in winter and spring naturally and concentration of ozone change near about 25% from January to July.
There are two important breakdown mechanisms for the halocarbons, reaction with tropospheric hydroxyl radicals (OH) and photolysis especially by UV radiation in the stratosphere. The fully halogenated CFCs are entirely inert, and they therefore do not react with OH. They are removed exclusively by photolysis in the stratosphere.
Halons are the potent source of the ozone depleting molecules because it’s used as a chemical in refrigerants. It’s composed of Bromine chlorine and carbon and mostly Bromine in the atmosphere came from halons. Bromine is 50 times more effective than the chlorine in the destruction of the ozone.
When the natural balance of destruction and formation of the ozone layer is disturbed due to human activities and process of destruction is up side rather than formation then ozone layer is depleted. Man made compounds such as chlorofluorocarbons released the chlorine and bromine which is the main causing agents of ozone depletion now these days . Molina and Rowland (1974) proposed that stratospheric ozone could be depleted through catalytic cycles initiated by the photolytic release of chlorine radicals from chlorofluorocarbons (CFCs). Using a photochemical model, they estimated that the two most abundant CFCs, CFCl3 (CFC-11) and CF2Cl2 (CFC-12), would remain in the atmosphere for between 40 and 150 year but at this time scientist takes not seriously until the ozone hole discovered over Antarctica in . Chlorofluorocarbons’ are not washed out with rain also they can’t react with other chemicals. Ultraviolet radiations when strikes this chlorofluorocarbon due to that counters it released free chlorine. Chlorine reacts very fast with ozone and converts it in to oxygen. Scientists believe chlorofluorocarbons’ diffuse upwards into the earth’s stratosphere where the sun’s energy is powerful enough to break the tight bonds between the atoms of CFCs to yield atomic chlorine. The chlorine atoms participate in a set of chemical reactions that destroy ozone. These chemical reactions are presented below:
In the stratosphere, the CFC are broken down by UV radiations and release chlorine atom. Using CFC -12, the reaction is:
CF2Cl2 + hv → Cl + CF2Cl
CF2Cl + O2 → CF2O + ClO
The chlorine atom will then react with ozone and produce ClO:
Cl + O3 → ClO + O2
ClO + O → Cl + O2
The above reaction removes the oxygen radical and prevents it from recombining with an oxygen atom to form an ozone molecule. Thus the chlorine atom acts essentially as a catalyst. It is estimated that chlorine atom can destroy more than 1,00,000 ozone molecules before finally being removed from the stratosphere. Bromine also destroyed the ozone. Industrial halocarbons consist of bromine and chlorine. Chlorofluorocarbons are non-flame able and toxic and contain chlorine fluorine and carbon. The 5 main CFCs include CFC-11 (trichlorofluoromethane – CFCl3), CFC-12 (dichloro-difluoromethane – CF2Cl2), CFC-113 (trichloro-trifluoroethane – C2F3Cl3), CFC-114 (dichloro-tetrfluoroethane – C2F4Cl2), and CFC-115 (chloropentafluoroethane – C2F5Cl). CFCs mostly used in refrigerators ,electronic circuit boards ,air conditioner as a solvent in the cleaner, and as propellants in aerosols. Satellite observation of CFC abundance have allowed relative lifetimes to be derived using inter-species correlation in the lower stratosphere.Volk estimated life time of 96±12 year of CFC-12 and 112±31 for CFC-113 using an assumed lifetime for CFC-11 for 50 year .Lovelock measured CFC-11 using a gas chromatograph with an electron capture detector and estimated the lifetime of at least 10 years. Strictly rules and regulation for rocket launching to prevent the destruction of stratosphere ozone in future according to the new researcher study in California and Colorado. Ozone losses from unregulated rocket launches will eventually exceed ozone losses due to chlorofluorocarbons, or CFCs, which stimulated the 1987 Montreal Protocol banning ozone-depleting chemicals, said Martin Ross, chief study author from The Aerospace Corporation in Los Angeles. In future ozone layer depletion based on the expected growth of the space industry and known impact of rocket launches. If left the unregulated rocket launches by the year 2050 could result in more ozone destruction than was ever realized by chourflourocarbon. Current global rocket launches deplete the ozone layer by no more than a few hundredths of 1 percent annually, said Toohey. Space industry grows ozone depleting chemicals decline in the earth stratosphere. Ozone depletion from rocket launches is expected a major issue. Trace gas molecule known as radicals dominated stratosphere ozone destruction a single radical in the stratosphere can destroy 10000 molecules of ozone before its deactivation.
Natural refrigerant’s provided a number of alternatives of the CFC, HCFC and HFC. Natural refrigerants have zero ozone depletion potential ODP and also have low or zero global warming potential GWP. Natural refrigerants don’t have chlorine and fluorine atoms and can’t react with water .In Ammonia refrigerant Ammonia has the excellent thermodynamic and transport property it is better than the CFCs, HCFCs and HFCs. Hydrocarbons are the class of naturally occurring substances like propane, pentane and butane. Hydrocarbons are the safer alternatives of the CFC, HCFC and HFC. Hydrocarbons have zero ozone depletion potential. Hydrocarbons not combust spontaneously when contact with air .Water also used as refrigerant which is one of the ultimate natural refrigerants because of non-toxicity, non-flammability, zero-ODP, zero-GWP and very low cost. Water can be used as refrigerant in four ways like absorption chiller, desiccant dehumidification/evaporative cooling, adsorption chiller as well as compression chiller. CO2 is an excellent alternative among the natural refrigerants and carbon dioxide (CO2) which is neither flammable nor toxic. CO2 has a GWP of 1, but the net global warming impact when used as a technical gas.
RAO ADEEL UR REHMAN*, MUHAMMAD YAHYA1, ANDLEEB REHMAN2
, MUHAMMAD WASIM AKRAM3, MUHAMMAD QAISAR NAEEM KHAN4, SAFI RAZA BUKHARI5, ASAD HAMEED6
AUTHOR*;BSc(hons)Agriculture sciences, Department of soil and environmental sciences University college of Agriculture, University of Sargodha .
2 Co-author’s (hons.) Botany, Department of botany, University of Education okara campus
1&3,4 ,5,6 Co-author; Department of entomology, University of Sargodha.
Email id; [email protected] .