Dobson unit
Encyclopedia
The Dobson unit is a unit of measurement of atmospheric
ozone
columnar density, which is dominated by ozone in the stratospheric
ozone layer
. One Dobson unit refers to a layer of ozone that would be 10 µm thick under standard temperature and pressure. 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 molecule
s 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
.
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
columnar density, which is dominated by ozone in the stratospheric
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
. One Dobson unit refers to a layer of ozone that would be 10 µm thick under standard temperature and pressure. 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 molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s per square centimetre, or 2.69×1020 per square metre
Square metre
The square metre or square meter is the SI derived unit of area, with symbol m2 . It is defined as the area of a square whose sides measure exactly one 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
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
and bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
compounds.
The Dobson unit is named after Gordon Dobson, who was a researcher at the University of Oxford
University of Oxford
The University of Oxford is a university located in Oxford, United Kingdom. It is the second-oldest surviving university in the world and the oldest in the English-speaking world. Although its exact date of foundation is unclear, there is evidence of teaching as far back as 1096...
. In the 1920s, he built the first instrument to measure total ozone from the ground, now called the Dobson ozone spectrophotometer
Dobson ozone spectrophotometer
The Dobson spectrophotometer, also known as Dobsonmeter or Dobson spectrometer, is the earliest instrument used to measure atmospheric ozone. It was invented in 1926 by Gordon Dobson. A history of the development of the instrument is here...
.