By analysing the strength of absorption at these wavelengths, we can work out the concentration of the Ozone present. Measurements are taken in a wide variety of conditions to give us an insight into the normal composition of the stratosphere, but to measure a rate of change we require continuous measurements. Because of the random nature of balloon and plane flights, ground-based stations were setup to take measurements continuously by looking directly upwards into sky, using the suns rays as the light source.
These measurements were also verified by NASAs ozone monitor aboard one of its satellites, which works by measuring UV reflection ratio of the stratosphere directly from space. In addition to doing this we need to also look at the reactivity of the molecules in the stratosphere which react with ozone, to do this we would need to know the rate of reaction, what wavelength of radiation ozone absorbs and the types of reactions that take place. This is done by using flash photolysis (used because reactions are very quick and hard to observe).
Using flash photolysis, we are able to find the rate constant of the reaction. The rate constant depends on the temperature and pressure of which the reaction is happening and determines how fast it is going. By investigating reaction between different chemicals present in the stratosphere, a reference table is drawn up giving rate constants of all the known reactions. The alarm of Ozone depletion by CFC was raised in 1972. James Loverlock developed a method to detect CFCs in the troposphere and found that they are so stable that they will accumulate in the atmosphere.
Then, Sherwood Rowland found that CFCs will absorb the high energy ultra-violet radiation and release Cl in the stratosphere, which can react with 100,000 molecules of ozone. Ten years later, in 1987, Farman discovered the ozone hole by measurement. Scientist also flew over the hole and found that there is a drop in Ozone where ClO concentration is highest. This is shown below in the diagram which also shows the corresponding ClO concentrations. A satellite image showing a hole in the ozone layer over Antarctica (right) & Arctic (left), http://www. jpl. nasa. gov/archive/mpe. html.