System for evaluation of trace gas concentration in the atmosphere based on the differential optical absorption spectroscopy technique

Abstract

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 and formaldehyde. 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 presented.