Gordon Dobson

Gordon Miller Bourne Dobson (February 25th, 1889 - March 11th, 1976) was a Physicien and British Météorologue of the Université of Oxford which made important research on the Ozone.

Biography

He invented in 1924 the first ozone measuring instrument starting from the ground: the Dobson spectrophotometer. Moreover, Dobson was also member of the Royal Society and accepted the Médaille Rumford in 1942, in reward for its work.

By studying meteorites, he noticed that the atmospheric profile of Température beyond the Tropopause was not constant as one believed it at the time (from where the name of Stratosphère). He showed that the temperature in stratosphere increases with the Altitude (the reverse from what occurs in the Troposphère). Dobson explained this phenomenon by the warming of stratospheric ozone due to the ultraviolet rays ; from there came also the concept of ozone Couche.

Measure ozone by the Dobson method

The Dobson method at the same time makes it possible to measure the ozone column above the detector, and to trace a vertical profile of distribution of ozone.

The principle for the measurement of the ozone column is based on the absorption of the solar radiation Ultraviolet ranging between 200 Nm and 300 Nm by the ozone molecules. One measures the relative intensity of selected pairs of ultra-violets radiation. For a Wavelength of 280 Nm for example, there is absorption of the radiation by the Molécules of ozone, there is thus little of this radiation which arrives at terrestrial surface. On the other hand, if there is formation of a hole of ozone above the detector, this same radiation is attenuated less. The problem is that there is not only the attenuation due to the ozone which comes into play: indeed, the particles present in the atmosphere diffuse the Lumière and consequently reduce also the intensity of the radiation. To eliminate this factor from diffusion by the particles, it is enough to measure the intensity of the radiation to another wavelength less absorptive by ozone, but rather near to the first so that the other phenomena of absorption are identical or at least interpolables. With this intention, one chooses for example the wavelength 310 Nm. The difference in intensity between these two radiations is only due to absorption by ozone. The disadvantage of this method is that it gives specific values, i.e. only in top of the detector. To make a chart, one thus needs a very great number of these detectors, which is expensive. A way of curing this disadvantage, is to put a detector on a satellite . The principle is the same one except that the signal arriving on the detector is the radiation retrodiffused by the Atmosphère.

The measurement of vertical distribution of ozone is derived from the method Umkehr. This method employs reflected radiation UV, rather than the direct radiation. The distribution of ozone is derived from the change of proportion of two frequencies UV with time during a sunset. A Umkehr measurement lasts approximately three hours, and provides data until an altitude of 48 km; the most precise measurements are obtained for the area above 30 km.

Nowadays, the Dobson method is especially used to gauge data obtained by other means, of which satellite . Modernized versions of the Dobson spectrophotometer exist and provide always data, in particular the Spectrophotomètre Brewer.

Dobson unit

The Dobson unit (DU) is the standard way to express the concentration of ozone column in the atmosphere, specifically the Stratosphère. A Dobson unit corresponds to a layer of ozone which would have 10 Micromètres thickness with 1 atm. For example, 300 Dobson units of ozone brought back to the surface of the Earth would form a 3 mm thickness layer. The normal value for the ozone concentration is of 350 OF.

Sources

• Prof Hubert van den Bergh (2005), Atmosphere and climate: introduction to atmospheric pollution (LPAS)

See too

• Layer of ozone

• Chlorofluorocarbon
• Ozone
• Suspended particles
• Protocol of Ultraviolet Montreal