The Dobson unit (DU) is a unit of measurement of the columnar density of a trace gas in the Earth's atmosphere. It originated, and continues to be widely used, as a measure of total-column ozone, which is dominated by ozone in the stratospheric ozone layer. One Dobson unit refers to a layer of gas that would be 10 µm thick under standard temperature and pressure, sometimes referred to as a 'milli-atmo-centimeter.' For example, 300 DU of ozone brought down to the surface of the Earth at 0 °C would occupy a layer only 3 mm thick. One DU is 2.69×1016 ozone molecules per square centimetre, or 2.69×1020 per square metre. This is 0.4462 millimoles of ozone per square metre.
A baseline value of 220 DU is chosen as the starting point for an ozone hole since total ozone values of less than 220 Dobson Units were not found in the historic observations over Antarctica prior to 1979. Also, from direct measurements over Antarctica, a column ozone level of less than 220 Dobson Units is a result of the ozone loss from chlorine and bromine compounds.
The Dobson unit is named after Gordon Dobson, who was a researcher at the University of Oxford. In the 1920s, he built the first instrument to measure total ozone from the ground, now called the Dobson ozone spectrophotometer.
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