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## What is the Dobson Unit (DU)?

The concentration of a certain trace gas, for example ozone and SO2, in a column of air in the earth's atmosphere is often given in Dobson Units.

 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 of a given trace gas there is in the air above a certain point on earth. A proper unit in the International System of units would thus be "kilogram per square meter".

The unit introduced by Dobson is defined as follows. Suppose that all the trace gas in question 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 the trace gas, for example 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:
• 2.6867 x 1020 molecules per meter square
• 4.4615 x 10-4 mol/m2
• 2.1415 x 10-5 kg[O3]/m2
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. During spring on the southern hemisphere, September-November, the so-called "ozone hole" develops, with ozone values (well) below 200 DU.

For SO2, the typical background level concentration (i.e. away from emissions related to pollution and volcanic eruptions) is much less than 1 DU. Emissions related to pollution and small volcanic eruptions are of the order of 1 DU or a few DU. Strong and explosive eruptions may lead to concentrations well above 10 DU, even as high as 100 DU.

### Volume mixing ratio

Trace gas concentrations at a particular pressure level in the atmosphere are often given as a volume mixing ratio, or simply mixing ratio. This unit is defined as the ratio of the number density of the gas to the total number density of the atmosphere. In other words, the SO2 volume mixing ratio is the density of SO2 divided by the density of all constituents of the atmosphere in a unit volume (i.e. the number of molecules per unit volume).

Therefore, an SO2 mixing ratio of 10-9 means that the number density of SO2 is 10-9 times the total number density of air in a unit volume. Following the standard convention for the earth's troposphere and stratosphere, this mixing ratio equals 1 ppbv (parts per billion by volume).

For examples how to convert ozone volume mixing ratios into other units or vice versa, see this page elsewhere on the Web. 