﻿ Dobson unit

# 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:

Or

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.

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