Differential optical absorption spectroscopy

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The differential (also differential ) optical absorption spectroscopy (DOAS short) is used in environmental monitoring physicochemical remote sensing method , traces can be detected chemical compounds qualitatively and quantitatively with the. The DOAS is used to analyze atmospheric trace substances. It can be used, for example, to measure the ozone concentration in the earth's atmosphere . Other trace gases are u. a. Nitrogen dioxide (NO 2 ), nitrogen monoxide (NO), sulfur dioxide (SO 2 ), methane , formaldehyde , bromine monoxide (BrO), iodine monoxide (IO), chlorine monoxide (ClO) and chlorine peroxide (OClO) , water , nitrous acid and glyoxal .

overview

The DOAS is based on the frequency or wavelength dependent absorption of light in gaseous matter. The light can come from an artificial source such as a high-pressure discharge lamp , a light-emitting diode or a natural, extraterrestrial source such as the sun . Since there is little light absorption in the atmosphere, a sufficiently long distance in the atmosphere between the light source and the spectrometer is required to measure trace gases (approx. 100 m to several km ). If the scattered sunlight is used as a light source, the absorption is recorded in the entire atmosphere. Depending on the gas, a sensitivity in the ppb to ppt (10 −12 ) range can be achieved.

In contrast to other methods of absorption spectroscopy , differential optical absorption spectroscopy does not examine the entire spectrum, but only wavelengths close to the absorption structures of a gas to be examined and the gas concentration there is calculated from the characteristic difference in absorption directly at the absorption structures, often simultaneously for several absorbers. This is necessary in order to distinguish the broadband absorption cross- sections of Mie and Rayleigh scattering from the higher-frequency cross- sections of the trace substances. The evaluation wavelength range is limited due to various factors. In the case of passive atmospheric measurements, this can also be, for example, the different radiation transport through different air layers due to the wavelength dependence of Rayleigh scattering. The absorption structures examined 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 UV / VIS spectroscopy commonly used in analytical chemistry .

MAX DOAS

With passive DOAS systems, changes in the measurement geometry, e.g. the elevation angle of the telescope , can provide additional information about the height distribution of trace substances. This procedure is then called i. A. MAX-DOAS (Multi AXial Differential Optical Absorption Spectroscopy) and can also be used to determine aerosol profiles when measuring different O 4 absorption bands . This makes use of the fact that one knows the distribution of O 4 in the atmosphere and can also measure its absorption in different wavelength ranges. Thus, an inverse model based on radiation transfer models such as SCIATRAN can be used to infer the original aerosol and trace substance distribution.

Since DOAS is a calibration-free measuring method, MAX-DOAS measurements, for example, can be used to calibrate other measuring methods. For cameras that observe the spread of SO 2 on volcanoes, this is an obvious procedure. The fact that no calibration is required for measurements results from the fact that common DOAS measuring devices measure the optical thickness using two spectra, one with and one without absorption, and the column density and then the concentration directly from this optical density and the , time-constant, effective cross-section of the absorber.

Satellite measurements

Satellite-based measuring instruments such as SCIAMACHY can be used to create global trace substance maps with the help of the DOAS process. There are cards and a. of nitrogen dioxide (NO 2 ), nitrogen monoxide (NO), sulfur dioxide (SO 2 ), methane , formaldehyde , bromine monoxide (BrO), chlorine monoxide (ClO) and chlorine peroxide (OClO) , water and various aerosol parameters. GOME and GOME-2 were built to determine ozone (O 3 ); however, various other trace substances (NO 2 , SO 2 , BrO and OClO) are also measured.

Active DOAS variants

Measurements with their own light source are summarized under the term 'active'. These include long-path DOAS, multi-reflection cells and cavity-enhanced DOAS . In the first case a long light path is achieved with mirrors and a telescope, in the last two methods this is achieved by folding the light path in a limited space. The path lengths achieved vary between a few hundred meters for multi-reflection cells and several kilometers for long path and cavity systems.

Long path DOAS system at Cape Verde Atmospheric Observatory CVAO on São Vicente , Cape Verde

Web links

Individual evidence

  1. ↑ A. Nawahda: Ozone monitoring using differential optical absorption spectroscopy (DOAS) and UV photometry instruments in Sohar, Oman. In: Environ Monit Assess. 187 (8), Aug 2015, p. 485. PMID 26138853
  2. ↑ R. Sinreich, U. Frieß, T. Wagner, S. Yilmaz, U. Platt: Retrieval of Aerosol Distributions by Multi-Axis Differential Absorption Spectroscopy (MAX-DOAS). In: Nucleation and Atmospheric Aerosols. 17th International Conference, Galway, Ireland, 2007. 2008, pp. 1145–1149 doi : 10.1007 / 978-1-4020-6475-3_227
  3. ↑ SCIAMACHY homepage at the Institute for Environmental Physics (IUP), University of Bremen.
  4. ↑ Instruments - GOME-2. at ESA .
  5. ↑ GOME-2 Results and GOME-2 / SCIAMACHY DOAS nadir data browser at the IUP, University of Bremen.