The Differential Optical Absorption Spectroscopy technique (DOAS) is used to determine the
concentrations of several atmospheric gaseous components with one single system.
The technique, which was developed mainly in West Germany and Sweden, makes use of a broadband
light source, e.g. a high pressure xenon lamp or a halogene lamp.19 This light is collimated
by a parabolic mirror to a narrow beam, which passes through the atmosphere over a path of
several hundred metres to several kilometres. At the end of this absorption path the light is captured
again and focused onto the end of an optical fibre. The light is then led through the fibre
(the length of which can be up to several tens of metres) to the main part of this measuring systern
- the opto-analysis unit.
This unit consists of a spectrometer together with electronics for acquiring and processing measurement
data. It is also equipped with a PC with the required hard- and software. The necessary
reference spectra for both the measured and the interfering components are pre-calibrated
and stored in the computer.
A number of atmospheric trace gases with absorption lines in the visible and UV are possible to
monitor with this system, among them are nitrogen dioxide, sulphur dioxide, ozone, toluene, benzene
In this paper the principle of the DOAS technique is described. We also discuss the design of the
optical system and the evaluation technique. Finally a few results from different applications are