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What is the Dobson Unit (DU)?

European Space Agency

page last modified:
18 August 2011
G.M.B. Dobson The "Dobson unit" is named after professor G.M.B. Dobson (1889 - 1976), who has from the 1920s onwards done research on the ozone layer. Around 1930 he built the first "Dobson spectrophotometer", with which reliable measurements of the ozone layer became possible.

The "Dobson unit" indicates how much ozone there is in the air above a certain point on Earth. A proper unit would thus be "Kilogram per square meter".

The unit introduced by Dobson arises as follows. Suppose that all the ozone in the air would be in a (gas) layer just above the ground, at standard pressure (1013.25 hPa) and at standard temperature (0.0 Celsius). The amount of ozone is then indicated by the thickness of this layer, expressed in 0.01 millimeter. (This is why the ozone layer is sometimes referred to as being "thick" or "thin".)

1 Dobson unit (DU) is:
  • 2.6867E+20 molecules per square meter
  • 4.4615E-04 MOL per square meter
  • 2.1415E-05 kilogram ozon per square meter
Averaged over the entire world the ozone column has a value of about 300 DU. For the Netherlands this is an average of 280 DU in autumn and 380 DU in spring. For images of the current total ozone column values measured by satellite, see this page, and for the global ozone distribution derived from that see this page.

During spring on the southern hemisphere, September-November, the so-called "ozone hole" develops, with ozone values (well) below 200 DU. See, for example, these graphs of the depth and size of the Antarctische ozone hole

The Dobson Unit is sometimes also used for other trace gases in the atmosphere.


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