Dobson Ozone Spectrophotometer

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Dobson spectrophotometer is the earliest instrument used to measure 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 wavelengths. One wavelength is strongly absorbed by ozone and the other is is not. 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.