Dobson ozone spectrophotometer
Encyclopedia
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
Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere
. Ozone is tri-atomic oxygen, O3; ozone molecules absorb harmful UV light in the atmosphere before it reaches the surface of the earth. No UVC radiation penetrates to the ground as it is absorbed in the ozone-oxygen cycle
. However some longer-wave and less harmful UVB and most of the UVA
is not absorbed as ozone is less opaque to these frequencies, so they penetrate to the ground level of Earth in higher quantities. The sources of light
used may vary. Beside the direct sun
light, the light from the clear sky, moon
or star
s may be used.
A discussion of the instrument can be found here: ftp://ftp.cmdl.noaa.gov/dobson/Papers/dobson%20ozone%20spectrophotometer%20overview2.ppsx
A copy of the instrument handbook is available from the WMO: http://www.wmo.int/pages/prog/arep/gaw/documents/GAW183-Dobson-WEB.pdf
The Dobson Spectrometer measures the total ozone by measuring the relative intensity of the dangerous UVB radiation that reaches the Earth and comparing it to that of UVA radiation at ground level. If all of the ozone were removed from the atmosphere, the amount of UVB radiation would equal the amount of UVA radiation on the ground. As ozone does exist in the atmosphere, the Dobson Spectrometer can use the ratio between UVA and UVB radiation on the ground to determine how much ozone is present in the upper atmosphere to absorb the UVC radiation.
The ratio is determined by turning the R-dial, which can be rotated a full 300°, on the instrument. The spectrometer compares two different wavelength intensities, UVB (305 nm) and UVA (325 nm), in order to calculate the amount of ozone. When turned, the R-dial filters and blocks out the light of the UVC wavelength until the intensity of the two wavelengths of light are equal. The ratio of the two wavelengths at incidence can be deduced once the filtered intensities are the same. The results are measured in Dobson Unit
s, equal to 10 µm thickness of ozone compressed to Standard conditions for temperature and pressure
(STP) in the column. If all of the ozone in the atmospheric column one was measuring were compressed to STP, the thickness of the compressed atmosphere in mm would equal an answer in Dobson Units divided by 100.
The vertical distribution of ozone is derived using the Umkehr method
. This method relies on the intensities of reflected, rather than direct, UV light. Ozone distribution is derived from the change in the ratio of two UV frequencies with time as the sun sets. An Umkehr measurement takes about three hours, and provides data up to an altitude of 48 km, with the most accurate information for altitudes above 30 km.
The Dobson method has its drawbacks. It is strongly affected by aerosols and pollutants in the atmosphere, because they also absorb some of the light at the same wavelength. Measurements are made over a small area. Today this method is often used to calibrate data obtained by other methods, including satellites.
Some modernized versions of Dobson spectrophotometer exist and continue to provide data The Environment Canada (Alan Brewer) developed the Brewer Spectrophotometer still produced by Kipp & Zonen.
About 120 Dobsonmeters have been made, mostly by R&J Beck of London, of which about 50 remain in use today. The most famous ones are probably Nos. 31 and 51 with which Joe Farman
of the British Antarctic Survey
discovered the Ozone Hole in 1984.
The oldest instrument still in use is No.8 located at the roof of the Norwegian Polar Institute
at Ny Ålesund, Svalbard
. This instrument has the last reported data for 1997
The instrument D003, operated in Kunming, China
reported data to August 2009. The history of the stations and instruments can be found at the World Ozone and UV Data Centre
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...
. It was invented in 1926 by Gordon Dobson. A history of the development of the instrument is here
Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. Ozone is tri-atomic oxygen, O3; ozone molecules absorb harmful UV light in the atmosphere before it reaches the surface of the earth. No UVC radiation penetrates to the ground as it is absorbed in the ozone-oxygen cycle
Ozone-oxygen cycle
The ozone-oxygen cycle is the process by which ozone is continually regenerated in Earth's stratosphere, all the while converting ultraviolet radiation into heat. In 1930 Sydney Chapman resolved the chemistry involved. The process is commonly called the Chapman cycle by atmospheric scientists.Most...
. However some longer-wave and less harmful UVB and most of the UVA
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
is not absorbed as ozone is less opaque to these frequencies, so they penetrate to the ground level of Earth in higher quantities. The sources of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
used may vary. Beside the direct sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
light, the light from the clear sky, moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
or star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s may be used.
A discussion of the instrument can be found here: ftp://ftp.cmdl.noaa.gov/dobson/Papers/dobson%20ozone%20spectrophotometer%20overview2.ppsx
A copy of the instrument handbook is available from the WMO: http://www.wmo.int/pages/prog/arep/gaw/documents/GAW183-Dobson-WEB.pdf
The Dobson Spectrometer measures the total ozone by measuring the relative intensity of the dangerous UVB radiation that reaches the Earth and comparing it to that of UVA radiation at ground level. If all of the ozone were removed from the atmosphere, the amount of UVB radiation would equal the amount of UVA radiation on the ground. As ozone does exist in the atmosphere, the Dobson Spectrometer can use the ratio between UVA and UVB radiation on the ground to determine how much ozone is present in the upper atmosphere to absorb the UVC radiation.
The ratio is determined by turning the R-dial, which can be rotated a full 300°, on the instrument. The spectrometer compares two different wavelength intensities, UVB (305 nm) and UVA (325 nm), in order to calculate the amount of ozone. When turned, the R-dial filters and blocks out the light of the UVC wavelength until the intensity of the two wavelengths of light are equal. The ratio of the two wavelengths at incidence can be deduced once the filtered intensities are the same. The results are measured in Dobson Unit
Dobson unit
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...
s, equal to 10 µm thickness of ozone compressed to Standard conditions for temperature and pressure
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
(STP) in the column. If all of the ozone in the atmospheric column one was measuring were compressed to STP, the thickness of the compressed atmosphere in mm would equal an answer in Dobson Units divided by 100.
The vertical distribution of ozone is derived using the Umkehr method
Umkehr effect
The Umkehr is the time variation of the ratio of the scattered intensity at two different wavelengths. The word means 'reversal' in German. The Umkehr effect is observed when measurements are made with ultraviolet spectrophotometer of the ratio of the zenith sky light intensities of two wavelengths...
. This method relies on the intensities of reflected, rather than direct, UV light. Ozone distribution is derived from the change in the ratio of two UV frequencies with time as the sun sets. An Umkehr measurement takes about three hours, and provides data up to an altitude of 48 km, with the most accurate information for altitudes above 30 km.
The Dobson method has its drawbacks. It is strongly affected by aerosols and pollutants in the atmosphere, because they also absorb some of the light at the same wavelength. Measurements are made over a small area. Today this method is often used to calibrate data obtained by other methods, including satellites.
Some modernized versions of Dobson spectrophotometer exist and continue to provide data The Environment Canada (Alan Brewer) developed the Brewer Spectrophotometer still produced by Kipp & Zonen.
About 120 Dobsonmeters have been made, mostly by R&J Beck of London, of which about 50 remain in use today. The most famous ones are probably Nos. 31 and 51 with which Joe Farman
Joe Farman
Joseph Charles Farman is the British geophysicist who, together with Brian Gardiner and Jon Shanklin, published the discovery of the ozone hole over Antarctica...
of the British Antarctic Survey
British Antarctic Survey
The British Antarctic Survey is the United Kingdom's national Antarctic operation and has an active role in Antarctic affairs. BAS is part of the Natural Environment Research Council and has over 400 staff. It operates five research stations, two ships and five aircraft in and around Antarctica....
discovered the Ozone Hole in 1984.
The oldest instrument still in use is No.8 located at the roof of the Norwegian Polar Institute
Norwegian Polar Institute
The Norwegian Polar Institute is Norway's national institution for polar research. It is run under the auspices of the Norwegian Ministry of Environment. The institute organizes expeditions to the Arctic and Antarctic regions and runs a research station at Ny-Ålesund...
at Ny Ålesund, Svalbard
Svalbard
Svalbard is an archipelago in the Arctic, constituting the northernmost part of Norway. It is located north of mainland Europe, midway between mainland Norway and the North Pole. The group of islands range from 74° to 81° north latitude , and from 10° to 35° east longitude. Spitsbergen is the...
. This instrument has the last reported data for 1997
The instrument D003, operated in Kunming, China
Kunming
' is the capital and largest city of Yunnan Province in Southwest China. It was known as Yunnan-Fou until the 1920s. A prefecture-level city, it is the political, economic, communications and cultural centre of Yunnan, and is the seat of the provincial government...
reported data to August 2009. The history of the stations and instruments can be found at the World Ozone and UV Data Centre