Fact Sheet 4

Absorption Properties of Ozone and Oxygen

Relatice Absobtion

Data from C.W. Allen, Astrophysical Quantities 3ed
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The graph to the left shows the relative ultraviolet absorption coefficients for oxygen molecules and ozone molecules observed in the stratosphere at mid-latitudes in summer.

At the shortest wavelengths oxygen molecules dominate the absorption process, but the absorption can be seen to drop off dramatically beyond 170 nm.

At wavelengths longer than 170 nm ozone dominates the absorption of ultraviolet light.

The total absorption of all ultraviolet light is the combined effect of both oxygen and ozone.

Note that the depletion of ozone molecules effectively eliminates the absorption of ultraviolet radiation at wavelengths beyond 200 nm.

  • Peak ultraviolet absorption for O2 occurs at 145 nm
  • Peak ultraviolet absorption for O3 occurs at 250 nm

Measuring Ozone Concentration

The unit of measure for total ozone is called the Dobson Unit. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 Celsius) and pressure (101.3 kPa), or one atmosphere ("atm"), the column would be about 0.3 centimetres thick. To make the units easier to work with, the "Dobson Unit" is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 Dobson Units (DU).