The Dobson spectrophotometer, also known as Dobsonmeter, is the earliest instrument used to measure atmospheric ozone. It was developed in 1924 by Gordon Dobson.

Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere.

Total ozone measurements are made by measuring the relative intensities of selected pairs of ultraviolet (UV) wavelengths. One wavelength (305 nm) is strongly absorbed by ozone and the other (325 nm) is not ^[1]. 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 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, e.g. the Brewer Spectrophotometer.

About 120 Dobsonmeters have been made, mostly by R&J Beck of London. Today about 50 remain in use. The most famous ones are probably Nos. 37 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.

References

• New Scientist, 20. Sept 2008

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