10 How much ozone is in the layer, and what is a "Dobson Unit" ?
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This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
10 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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