Published by Australian Academy of Science
Activity 5 | Earth's sunscreen – the ozone layer

Graphing and analysing CFC-11 data

There are many different types of CFCs, usually referred to by number. The table shows the concentration of one of these types, CFC-11.

Year	1978	1980	1982	1984	1986	1988
Concentration of CFC-11 at Cape Grim, Tasmania (parts per trillion)	135	160	175	190	210	230

1. Graph the data given for CFC-11 concentrations from 1978 to 1988.

2. Assuming the line continues in the same fashion, estimate the concentration of the compound now. Try to find the latest figure and see how it compares with your extrapolated one. (Make sure that you check whether the current figure you look up is for CFC-11 and not all CFCs combined.)

3. Extrapolating backwards, estimate when the concentration of CFC-11 was effectively zero.

4. Write a paragraph explaining why CFCs in the atmosphere concern environmental scientists, even though their concentration is low.

Teachers notes

2. Students will probably estimate the current concentration of CFC-11 at 340 to 360 parts per trillion. In actual fact the concentration has stabilised and the 1996 to 2005 concentrations are closer to 260 parts per trillion.

3. Extrapolating backwards, students should estimate that the concentration of CFC-11 was effectively zero in about 1976. This reflects the explosion of use of CFCs in the 1970s.

4. Students should include in their answers the stable nature of CFCs and their ability to act as catalysts for ozone destruction.