The total ozone content of the atmosphere can be measured from satellites or by ground based instruments. For ground based observation UV-spectrophotometers are most commonly used. Their measurement principle is based on the differential absorption of solar light by ozone: the intensities of the solar light arriving at the Earth's surface at two (or more) wavelengths in the UV (between 280 and 330 nm) are measured and compared. Ozone is mostly absorbed by light at short wavelength. From the knowledge of the absorption spectrum of ozone and the calibration of the instrument, the ozone content of the atmosphere is derived. There are two main types of these instruments: the Dobson and the Brewer spectrophotometers.
The Dobson spectrophotometer
The Dobson instrument nr 40 at Uccle.
The oldest type of ozone spectrophotometer is the Dobson instrument, which uses quartz prisms to select the wavelengths of the incoming light. The instrument was designed in the 1920's by Dobson. During the following decades it has undergone upgrades with respect to the electronics, but the basic optical parts are unchanged. After the International Geophysical year (1958), and especially when the impact of human activities on the ozone layer became clear (1970's), many instruments were installed to form a world wide network. Although other instruments were developed, the Dobson instrument is still considered as the reference instrument for total ozone observations. At Uccle Dobson instrument nr 40 is operational since mid 1971. To assure the quality of the measurements, the instrument participated in different intercomparison campaigns organised under supervision of WMO. More detailed information on these calibrations can be found on the Dobson calibration web-pages.
Total ozone is expressed in Dobson Units (DU). One Dobson unit corresponds with a layer of 0.01 mm of ozone if all the ozone in the column were to be brought to standard conditions (i.e. air pressure of 1013.25hPa and 0°C). Over Uccle the value of total ozone may vary between 200 and 500 DU. Its mean value is about 325 DU; this corresponds with a reduced thickness (at standard conditions) of 3.25mm.
The Brewer spectrophotometers
The Brewer instruments nr 016 and 178 on the roof of RMIB.
During the 1970's, the Brewer instrument was developed. It is based on the same measurement principle as the Dobson instrument, but uses more state-of-the-art technology. Holographic gratings ensure the wavelength selection. In addition the Brewer instruments are completely automated and data are collected by a PC.
This makes it possible to show automatically recent data on the web-site of the institute.
The Brewer instrument can also measure the UV intensities falling on a horizontal surface. From these data the actual UV index can be calculated.
The total ozone data of these spectrophotometers are used to monitor the state of the ozone layer. To that end trends are calculated.