| Themes > Science > Earth Sciences > Hydrology, Meteorology, Climatology > Meteorology / Climatology > The Ozone Layer > How much ozone is in the layer, and what is a "Dobson Unit" ? |
A Dobson Unit (DU) is a convenient scale for measuring the total amount of ozone occupying a column overhead. If the ozone layer over the US were compressed to 0 degrees Celsius and 1 atmosphere pressure, it would be about 3 mm thick. So, 0.01 mm thickness at 0 C and 1 at is defined to be 1 DU; this makes the average thickness of the ozone layer over the US come out to be about 300 DU. In absolute terms, 1 DU is about 2.7 x 10^16 molecules/cm^2. The unit is named after G.M.B. Dobson, who carried out pioneering studies of atmospheric ozone between ~1920-1960. Dobson designed the standard instrument used to measure ozone from the ground. The Dobson spectrophotometer measures the intensity solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not [Dobson 1968b]. These instruments are still in use in many places, although they are gradually being replaced by the more elaborate Brewer spectrophotometers. Today ozone is measured in many ways, from aircraft, balloons, satellites, and space shuttle missions, but the worldwide Dobson network is the only source of long-term data. A station at Arosa in Switzerland has been measuring ozone since the 1920's (see http://www.umnw.ethz.ch/LAPETH/doc/totozon.html) and some other stations have records that go back nearly as long, although many were interrupted during World War II. The present worldwide network went into operation in 1956-57. |
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