Differential optical absorption spectroscopy
The differential optical absorption spectroscopy ( DOAS short ) is a physico-chemical remote sensing method used in environmental monitoring, which can be detected with traces of chemical compounds qualitatively and quantitatively. The DOAS is used for the analysis of atmospheric trace gases. It can be used for example to measure the concentration of ozone in the atmosphere.
Overview
The DOAS based on the frequency or wavelength-dependent absorption of light in the gaseous matter. The light can in this case derived from an artificial source such as a high-pressure discharge lamp, a light emitting diode or a natural source like the sun extraterrestrial. Since the light absorption in the atmosphere is low, a sufficiently large distance in the atmosphere between the light source and spectrometer required (approximately 100 m to several km ) for the measurement of trace gases. Is the scattered sunlight as a light source, the absorption in the whole atmosphere is detected. It can be to ppt (10-12 ) range depending on the gas reaches a sensitivity in the ppb.
In contrast to other methods, the absorption spectroscopy is not examined the whole range in the differential optical Absorptionsspektroskospie, but only wavelengths detected near absorption structures one to the gas under study, and it is calculated the gas concentration therein of the characteristic difference of the absorption directly in the absorbent structures, often simultaneously for several absorber. This is necessary to distinguish the broad-band absorption cross sections of Mie and Rayleigh scattering of the higher-frequency cross sections of trace substances. Due to various factors of Auswertewellenlängenbereich is limited. In passive atmospheric measurements, for example, this can also be the different radiative transfer through different layers of the atmosphere due to the wavelength dependence of Rayleigh scattering. The investigated absorption structures are mostly in the range of ultraviolet or visible light, but also in the near infrared. The DOA spectroscopy is therefore closely related to the commonly used in analytical chemistry, UV / VIS spectroscopy.
MAX - DOAS
In passive DOAS systems of the elevation angle of the telescope, more information on the height distribution of the trace gases can be obtained by changes in the measurement geometry, for example. This method is then called A., MAX DOAS ( differential optical absorption spectroscopy multi axial ) and can be also used in measurement of various O4 absorption bands in order to determine profiles aerosol. For this purpose, use is made knowing the distribution of O4 in the atmosphere and can also measure the absorption in different wavelength ranges. Thus one can conclude on an inverse model based on radiative transfer models such as SCIATRAN to the original aerosol and trace gas distribution.
As DOAS is a calibration- measuring method, for example, MAX DOAS measurements can be used to calibrate another measurement method. For cameras observing the spread of SO2 in volcanoes, this is obvious approach. The fact that you need for measurements no calibration results from the fact that current DOAS instruments, the optical thickness, measured by two spectra, one with and one without absorption and the column density and then the concentration directly optical from this density and the, is constant over time, the cross section of the absorber arise.
Satellite measurements
Satellite-based instruments such as SCIAMACHY can be used to create global trace gas cards using the DOAS method. Here exist, inter alia cards of nitrogen dioxide ( NO2), nitric oxide (NO ), sulfur dioxide (SO2 ), methane, formaldehyde, bromine oxide ( BrO ), chlorine monoxide (ClO ) and chlorine peroxide ( OClO ), water and various Aeorosolparametern. GOME and GOME -2 were constructed for the determination of ozone (O3 ); but also various other trace gases (NO2, SO2, BrO and OClO ) are measured.
Active DOAS variants
Measurements with its own light source can be 'active' grouped under the term. These include long- path DOAS, multi reflection cells and Cavity Enhanced DOAS. In the first case is achieved by a telescope mirrors and a long optical path, this is achieved by a folding of the light path to a more limited area at the last two procedures. The achieved path lengths vary between a few hundred meters for multi- reflection cells and several kilometers for long path and cavity systems.