Dobson unit
The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer. The Dobson Unit is defined as the thickness (in units of 10 µm) of that layer of pure gas which would be formed by the total column amount at STP - standard conditions for temperature and pressure. This is sometimes referred to as a 'milli-atmo-centimeter.' A typical column amount of 300 DU of atmospheric ozone therefore would form a 3 mm layer of pure gas at the surface of the Earth if its temperature and pressure conformed to STP. The Dobson unit is named after Gordon Dobson, a researcher at the University of Oxford who in the 1920s built the first instrument to measure total ozone from the ground, making use of a double prism monochromator to measure the differential absorption of different bands of solar ultraviolet radiation by the ozone layer. This instrument, called the Dobson ozone spectrophotometer, has formed the backbone of the global network for monitoring atmospheric ozone and was the source of the discovery in 1984 of the Antarctic ozone hole.
The Dobson Unit is not part of the SI International System of Units. To address this shortcoming, a brief study in 1982 examined a number of alternative SI-based units suitable for column amounts of not only ozone but any gas in any planetary atmosphere and proposed the use of the unit of mole per square metre for all cases. Examples range from Earth's trace gases at levels of micro moles per square meter to Venus's carbon dioxide at mega moles per square meter. Typical values of total ozone in the Earth's atmosphere are conveniently represented in millimoles per square metre (mmol m-2.) One DU is equivalent to 0.4462 mmol m-2. One DU is also equivalent to 2.687×1020 molecules per square metre.
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