Dobson ozone spectrophotometer

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Dobson ozone spectrophotometer
Gordon Dobson
First production 1924

The Dobson spectrophotometer, also known as Dobsonmeter, Dobson spectrometer, or just Dobson is one of the earliest instruments used to measure atmospheric ozone.

History

A Fery Spectrometer, using photographic plates, was the predecessor of the Dobson Spectrophotometer[1]

The Dobson spectrometer was invented in 1924 by British physicist and meteorologist

Gordon Dobson.[2] A history of the development of the instrument is here[3] and an example of one of Dobson's own instruments remains on display in the University of Oxford Department of Physics.[4]

Operation

Dobson

UVB and most of the UVA are 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 stars
may be used.

The Dobson spectrometer measures the total ozone by measuring the relative intensity of the 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),

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

UV light
. Ozone distribution is derived from the change in the ratio of the same UV-pair 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

pollutants
in the atmosphere, because they also absorb some of the light at the same wavelength. Measurements are made over a small area in the direction of the sun. Today this method is often used to calibrate data obtained by other methods, including satellites.

Instruments and manufacturers

Some modernized versions of Dobson spectrophotometer exist and continue to provide data.

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

Ozone Hole
in 1984. The "World Standard Dobson", No. 83, is owned and operated by the US Dept of Commerce's, NOAA, as is the secondary standard, No. 65.

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.[8]

The Environment Canada (Alan West Brewer) developed double- and single- monochromator spectrophotometers known as the "Brewer" Spectrophotometer produced by Kipp & Zonen.[9]

References

  1. PMID 20068600
    .
  2. ^ Dobson's original ozone spectrometer, 1926. - - Science Museum
  3. ^ G. M. B. Dobson - March 1968 / Vol. 7, No. 3 / APPLIED OPTICS (pps. 387 - 405). http://www.esrl.noaa.gov/gmd/ozwv/dobson/papers/Applied_Optics_v7_1968.pdf
  4. ^ History | University of Oxford Department of Physics
  5. ^ A discussion of the instrument can be found here: ftp://ftp.cmdl.noaa.gov/dobson/Papers/dobson%20ozone%20spectrophotometer%20overview2.ppsx[permanent dead link]
  6. ^ A copy of the instrument handbook is available from the WMO: http://www.wmo.int/pages/prog/arep/gaw/documents/GAW183-Dobson-WEB.pdf
  7. ^ "Dobson Spectrometer". Archived from the original on 2008-12-05. Retrieved 2008-10-05.
  8. ^ Home - World Ozone and Ultraviolet Radiation Data Centre (WOUDC) - [Meteorological Service of Canada - WOUDC]
  9. ^ Brewer Spectrophotometer - Kipp & Zonen Archived 2011-04-09 at the Wayback Machine

Further reading

  • New Scientist, 20. Sept 2008
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