Dobson Unit: Definition
Total ozone is typically measured in Dobson units, a unit of measure with special physical significance that honors a pioneering scientist.
Prof. G. M. B. Dobson of Oxford University developed a technique to accurately measure the abundance of ozone using a spectrometer that quantifies the amount of ultraviolet radiation reaching the surface. Knowledge of the strength at which ozone absorbs radiation, coupled with the fact that ozone is the only atmospheric gas that attenuates radiation at these wavelengths, allows the total amount of ozone between the ground and the top of the atmosphere to be inferred in a straight-forward manner.
Ground-based Dobson spectrometers are still in use, around the world, to document the abundance of stratospheric ozone.
A Dobson unit represents the thickness of an atmospheric gas if it could be compressed to ground temperature and pressure. If we could magically isolate all the ozone molecules in the atmosphere, and take only these molecules and bring them to ground pressure and temperature, then a 1 Dobson unit (DU) layer of gas would occupy a layer only one-thousandth of a centimeter (cm). Since typical total ozone abundances are ~300 DU, the ozone layer (if only the ozone molecules could be collected and compressed) comprises a layer of gas only 0.3 cm, or one-tenth of an inch, thick!
Knowledge of the definition of a Dobson unit allows us to realize that ozone is a trace species, even in the stratosphere. Its relative abundance in the stratosphere is measured in "parts per million" (in other words, only several out every million stratospheric molecules happen to be ozone ... most molecules are nitrogen and oxygen). Despite the trace abundance of ozone, it is the primary absorber of biologically harmful ultraviolet radiation at many wavelengths (molecular oxygen completely absorbs lethal UV radiation at wavelengths below 240 nanometers). The minute abundance of ozone in our atmosphere lends credibility to the idea that its abundance could be perturbed by human activities.