Science, AS-level, Case Study, Oaklands School
- Read the task in Box 1.
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Science, AS-level, Case Study, Oaklands School
This task, a case study, is part of the assessment for Year 12 students doing AS Science and worth 7.5% of the students’ overall grade. This is a year-only course, designed by OCR exam board primarily for A Level Arts and Humanities students who wish to take their scientific knowledge further. The course applies content from Biology, Chemistry and Physics to modern day scientific work. In this task, the case study task is on depletion of ozone in the atmosphere.
Students had two one-hour forty-minute lessons to prepare and begin the research for the task, followed by two weeks outside class time to complete the assignment. Lessons focused on how to conduct research, how to select useful materials and how to reference. The task instructions asked students to research specific aspects of the depletion of ozone and to write a report. The case studies were assessed on these 3 criteria (from OCR exam board): “the quality of selection and use of material”; “the quality of understanding of ethical, safe and skilful techniques” and finally, the ability to “explain and evaluate the results and impact of the work of other scientists” (1). It is important that students show in their writing that they have considered the reliability and validity of the data. This podcast focuses on how well students select and use sources.
Dobson the Scientist
Dobson was a scientist that devoted most of his life to the observation of the atmospheric ozone. The results of his devotion to the study of the atmospheric led to a great understanding of the structure and circulation of the stratosphere. He went to Oxford in 1920 to become a lecturer of Meteorology, “having previously been a Captain in the Royal Flying Corps and Director of the Experimental Department at the royal Aircraft Establishment, Farnborough, during World War 1.” He built a machine; it was the first spectrograph that he built in 1924 in his laboratory and workshop in a hut. His home was in Robin Wood, Boars Hill, near Oxford where he built the machine. The machine allowed relative intensity at two wavelengths to be measured directly. It was completed in 1927 0r 1928, the design being remarkably advanced for its day.
Scientists have been looking at the concentration of ozone in the atmosphere since the 1920’s. Instruments have become more advanced since then by using: balloons, aircraft, rockets and satellites. Figure two shows satellites observe the ozone concentration.
The work of G.M.B Dobson and other scientists in measuring concentrations of chloride and ozone in the atmosphere
G.M.B Dobson inferred correctly that the cause of the warm stratosphere was heating by the absorption of ultraviolet solar radiation by ozone, and he set out to make measurements of the amounts and their variability. (6)
During the 1920s, G.M.B. Dobson developed a spectrometer that could measure small concentrations of the ozone.
Dobson's first spectrograph employed a Fabry prism, an optical wedge consisting of gelatine and carbon black between quartz plates designed by T. Merton of the Clarendon Laboratory, and a filter consisting of a mixture of chlorine and bromine vapour to cut out unwanted solar radiation at longer wavelengths. A special tank was built to ensure consistent development of the photographic plates. (6)
The Dobson Spectrometer measures the total ozone by measuring the relative intensity of the dangerous UVB radiation that reaches the Earth and comparing it to that of UVA radiation at ground level. (8) As ozone does exist in the atmosphere, the Dobson Spectrometer can use the ratio between UVA and UVB radiation on the ground to determine how much ozone is present in the upper atmosphere to absorb the UVC radiation.(8)
Depletion of ozone in the atmosphere
Ozone is a gas that is found in the upper atmosphere which is the stratosphere . It contains 3 atoms of oxygen. The ozone molecule forms a gaseous layer which shields the earth from harmful ultraviolet radiation coming from the sun .
The ozone layer helps to keep the earth warm by absorbing infrared radiation and also protects us from ultraviolet radiation . Ozone plays an important role in absorbing radiation from the sun. If the radiation reaches the Earth surface it could cause damage to cells and produce mutation therefore causing cancer .
CFC is short for chlorofluorocarbon which contains chlorine, fluorine and carbon . It is a highly stable compound which is widely used as aerosol propellants, refrigerants and solvents . They are used because it is unreactive, non-toxic and stable liquids .
In 1980 it came clear that the main cause of depletion of the ozone layer was the release of man-made chemical which is the CFCs . The CFCs escape to the air and then slowly drift to the upper layer of the atmosphere to the stratosphere which affects the ozone layer . The ultraviolet radiation breaks down CFCs giving out chlorine. The chlorine then destroys large amounts of ozone . This was a problem because the ozone in the stratosphere protects the Earth from harmful radiation .
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