Dobson unit

The Dobson unit ( unit symbol: DU, Dobson unit of English ) is a measure of the strength of the ozone layer, expressed in mole of ozone over a unit area:

The unit is named after Gordon Dobson, who developed the first instrument to measure this size, the Dobson spectrophotometer.

Derivation and definition

To define the Dobson unit, the amount of ozone is first expressed as a volume of gas at standard conditions (STP):

And with the aid of the standard volume

That is, the thickness of the ozone layer will not be given as a molar amount per surface area, but as a volume per surface. This corresponds to the dimensional analysis ago in length, namely the hypothetical thickness of the ozone layer if they would focus as a pure ozone on the ground:

The definition is that a pure ozone layer with a thickness of 1 mm corresponds to exactly 100 DUs:


In the last two formulas, the following:

And thus

With 1 mmol = 1 millimole = 0.001 mol.

1 DU corresponds to

  • The mass per unit area ( by multiplying by the molar mass )
  • Of area number ( by multiplying by the Avogadro constant ).

Typical orders of magnitude

The average column height of the ozone layer is:

  • At higher latitudes in the summer sometimes over 500 DU
  • In temperate latitudes between 300 and 400 DU
  • Less at the equator ( because of the higher tropopause here the lower stratosphere regions where the ozone layer is mainly )
  • During the Antarctic spring, at the time of the ozone hole, far below 200 DU.

That would each having a thickness of pure ozone of only a few mm correspond, at a basis weight of ( In reality, the ozone layer is by several orders of magnitude thicker than a few millimeters, since they never present as pure ozone, but always finely distributed in the remaining ingredients the air. )

For comparison: (: desired UV filters at high altitude, near the ground irritation of the respiratory tract even in low concentrations, different effects of ozone ) the limit for ozone is smog.