scholarly journals Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections

2009 ◽  
Vol 2 (2) ◽  
pp. 713-723 ◽  
Author(s):  
U. Platt ◽  
J. Meinen ◽  
D. Pöhler ◽  
T. Leisner
Keyword(s):  
Absorption Spectroscopy ◽  
Gas Absorption ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Light Sources ◽  
Light Path ◽  
Gas Concentration ◽  
Mirror Reflectivity ◽  
Interesting Approach ◽  
Gas Measurements

Abstract. Atmospheric trace gas measurements by cavity assisted long-path absorption spectroscopy are an emerging technology. An interesting approach is the combination of CEAS with broadband light sources, the broadband CEAS (BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to analyse the derived absorption spectra. While the DOAS approach has enormous advantages in terms of sensitivity and specificity of the measurement, an important implication is the reduction of the light path by the trace gas absorption, since cavity losses due to absorption by gases reduce the quality (Q) of the cavity. In fact, at wavelength, where the quality of the BB-CEAS cavity is dominated by the trace gas absorption (especially at very high mirror reflectivity), the average light path will vary nearly inversely with the trace gas concentration and the strength of the band will become only weakly dependent on the trace gas concentration c in the cavity, (the differential optical density being proportional to the logarithm of the trace gas concentration). Only in the limiting case where the mirror reflectivity determines Q at all wavelength, the strength of the band as seen by the CE-DOAS instrument becomes directly proportional to the concentration c. We investigate these relationships in detail and present methods to correct for the cases between the two above extremes, which are of course the important ones in practice.

scholarly journals Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections

2008 ◽  
Vol 1 (1) ◽  
pp. 481-507 ◽  
Author(s):  
U. Platt ◽  
J. Meinen ◽  
D. Pöhler ◽  
T. Leisner
Keyword(s):  
Absorption Spectroscopy ◽  
Broad Band ◽  
Gas Absorption ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Light Sources ◽  
Light Path ◽  
Gas Concentration ◽  
Mirror Reflectivity ◽  
Gas Measurements

Abstract. Atmospheric trace gas measurements by cavity assisted long-path absorption spectroscopy are an emerging technology. An interesting approach is the combination of CEAS with broad band light sources, the broad-band CEAS (BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to analyse the derived absorption spectra. While the DOAS approach has enormous advantages in terms of sensitivity and specificity of the measurement, an important implication is the reduction of the light path by the trace gas absorption, since cavity losses due to absorption by gases reduce the quality (Q) of the cavity. In fact, at wavelength, where the quality of the BB-CEAS cavity is dominated by the trace gas absorption (esp. at very high mirror reflectivity), the light path will vary inversely with the trace gas concentration and the strength of the band will become nearly independent of the trace gas concentration c in the cavity, rendering the CEAS Method useless for trace gas measurements. Only in the limiting case where the mirror reflectivity determines Q at all wavelength, the strength of the band as seen by the BB-CEAS instrument becomes proportional to the concentration c. We investigate these relationships in detail and present methods to correct for the cases between the two above extremes, which are of course the important ones in practice.

Retrieval of vertical concentration profiles from OSIRIS UV–visible limb spectra

2002 ◽  
Vol 80 (4) ◽  
pp. 409-434 ◽  
Author(s):  
K Strong ◽  
B M Joseph ◽  
R Dosanjh ◽  
I C McDade ◽  
C A McLinden ◽  
...  
Keyword(s):  
Optical Absorption ◽  
Absorption Spectroscopy ◽  
Optimal Estimation ◽  
Gas Absorption ◽  
Forward Model ◽  
Differential Optical Absorption Spectroscopy ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Vertical Profiles ◽  
Uv Visible

The OSIRIS instrument, launched on the Odin satellite in February 2001, includes an optical spectrograph that will record UV–visible spectra of sunlight scattered from the limb over a range of tangent heights. These spectra will be used to retrieve vertical profiles of ozone, NO2, OClO, BrO, NO3, O2, and aerosols, for the investigation of both stratospheric and mesospheric processes, particularly those related to ozone chemistry. In this work, the retrieval of vertical profiles of trace-gas concentrations from OSIRIS limb-radiance spectra is described. A forward model has been developed to simulate these spectra, and it consists of a single-scattering radiative-transfer model with partial spherical geometry, trace-gas absorption, Mie scattering by stratospheric aerosols, a Lambertian surface contribution, and OSIRIS instrument response and noise. Number-density profiles have been retrieved by using optimal estimation (OE) to combine an a priori profile with the information from sets of synthetic ``measurements''. For ozone, OE has been applied both to limb radiances at one or more discrete wavelengths and to effective-column abundances retrieved over a broad spectral range using differential optical absorption spectroscopy (DOAS). The results suggest that, between 15 and 35 km, ozone number densities can be retrieved to 10% accuracy or better on 1 and 2 km grids and to 5% on a 5 km grid. The combined DOAS-OE approach has also been used to retrieve NO2 number densities, yielding 13% accuracy or better for altitudes from 18 to 36 km on a 2 km grid. Differential optical absorption spectroscopy – optimal estimation retrievals of BrO and OClO reproduce the true profiles above 15 km in the noise-free case, but the quality of the retrievals is highly sensitive to noise on the simulated OSIRIS spectra because of the weak absorption of these two gases. The development of inversion methods for the retrieval of trace-gas concentrations from OSIRIS spectra is continuing, and a number of future improvements to the forward model and refinements of the retrieval algorithms are identified. PACS Nos.: 42.68Mj, 94.10Dy

scholarly journals Caution with Spectroscopic NO<sub>2</sub> Reference Cells (Cuvettes)

2019 ◽  
Author(s):  
Ulrich Platt ◽  
Jonas Kuhn
Keyword(s):  
Correlation Spectroscopy ◽  
Gas Absorption ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Central Component ◽  
Light Path ◽  
Absorption Cell ◽  
Model Studies ◽  
Partial Pressures

Abstract. Spectroscopic measurements of atmospheric trace gases, e.g. by Differential Optical Absorption Spectroscopy (DOAS) are frequently supported by recording the trace gas column density (CD) in absorption cells (cuvettes), which are temporarily inserted into the light-path. The idea is to verify the proper working of the instruments, to check the spectral registration (wavelength calibration and spectral resolution), and to perform some kind of calibration (absolute determination of trace gas CDs). In principle DOAS applications do not require absorption cell calibration, however in practice measurements with absorption cells in the spectrometer’s light path are frequently performed. In addition, trace gas absorption cells are used as a central component in gas correlation spectroscopy instruments. Here we show at the example of NO2 absorption cells that the effective CD seen by the instrument can deviate greatly from expected values (by orders of magnitude). Analytical calculations and kinetic model studies show the dominating influence of photolysis and dimerisation of NO2. In particular, this means that the partial pressure of NO2 in the cell matters. However, problems can be particular severe at high NO2 pressures (around 105 Pa) as well as low NO2 partial pressures (of the order of a few 100 Pa). Also, it can be of importance whether the cell contains pure NO2 or is topped up with air or oxygen (O2). Some suggestions to improve the situation are discussed.

scholarly journals Caution with spectroscopic NO<sub>2</sub> reference cells (cuvettes)

2019 ◽  
Vol 12 (12) ◽  
pp. 6259-6272
Author(s):  
Ulrich Platt ◽  
Jonas Kuhn
Keyword(s):  
Correlation Spectroscopy ◽  
Gas Absorption ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Central Component ◽  
Light Path ◽  
Absorption Cell ◽  
Model Studies ◽  
Partial Pressures

Abstract. Spectroscopic measurements of atmospheric trace gases, for example, by differential optical absorption spectroscopy (DOAS), are frequently supported by recording the trace-gas column density (CD) in absorption cells (cuvettes), which are temporarily inserted into the light path. The idea is to verify the proper functioning of the instruments, to check the spectral registration (wavelength calibration and spectral resolution), and to perform some kind of calibration (absolute determination of trace-gas CDs). In addition, trace-gas absorption cells are a central component in gas correlation spectroscopy instruments. In principle DOAS applications do not require absorption-cell calibration; however, in practice, measurements with absorption cells in the spectrometer's light path are frequently performed. Since NO2 is a particularly popular molecule to be studied by DOAS, and at the same time it can be unstable in cells, we chose it as an example to demonstrate that the effective CD seen by the instrument can deviate greatly (by orders of magnitude) from expected values. Analytical calculations and kinetic model studies show the dominating influence of photolysis and dimerization of NO2. In particular, this means that the partial pressure of NO2 in the cell matters. However, problems can be particularly severe at high NO2 pressures (around 105 Pa) as well as low NO2 partial pressures (of the order of a few 100 Pa). Also, it can be of importance whether the cell contains pure NO2 or is topped up with air or oxygen (O2). Some suggestions to improve the situation are discussed.

scholarly journals Satellite monitoring of different vegetation types by differential optical absorption spectroscopy (DOAS) in the red spectral range

2007 ◽  
Vol 7 (1) ◽  
pp. 69-79 ◽  
Author(s):  
T. Wagner ◽  
S. Beirle ◽  
T. Deutschmann ◽  
M. Grzegorski ◽  
U. Platt
Keyword(s):  
Optical Absorption ◽  
Absorption Spectroscopy ◽  
Spectral Range ◽  
Near Infrared ◽  
Vegetation Type ◽  
Differential Optical Absorption Spectroscopy ◽  
Trace Gas ◽  
Satellite Monitoring ◽  
Optical Absorption Spectroscopy ◽  
Vegetation Types

Abstract. A new method for the satellite remote sensing of different types of vegetation and ocean colour is presented. In contrast to existing algorithms relying on the strong change of the reflectivity in the red and near infrared spectral region, our method analyses weak narrow-band (few nm) reflectance structures (i.e. "fingerprint" structures) of vegetation in the red spectral range. It is based on differential optical absorption spectroscopy (DOAS), which is usually applied for the analysis of atmospheric trace gas absorptions. Since the spectra of atmospheric absorption and vegetation reflectance are simultaneously included in the analysis, the effects of atmospheric absorptions are automatically corrected (in contrast to other algorithms). The inclusion of the vegetation spectra also significantly improves the results of the trace gas retrieval. The global maps of the results illustrate the seasonal cycles of different vegetation types. In addition to the vegetation distribution on land, they also show patterns of biological activity in the oceans. Our results indicate that improved sets of vegetation spectra might lead to more accurate and more specific identification of vegetation type in the future.

A Fire Warning Method Using Tunable Diode Laser Absorption Spectroscopy

2021 ◽  
Vol 16 (2) ◽  
pp. 222-229
Author(s):  
Lin Feng ◽  
Jian Wang ◽  
Chao Ding
Keyword(s):  
Diode Laser ◽  
Absorption Spectroscopy ◽  
Gas Absorption ◽  
Tunable Diode Laser ◽  
Laser Absorption Spectroscopy ◽  
Gas Concentration ◽  
Laser Absorption ◽  
Output Wavelength ◽  
Light Beams ◽  
Diode Laser Absorption

Tunable diode laser absorption spectroscopy (TDLAS) technology is adopted herein to detect fire gas produced in the early stage of the fire. Based on this technology, a fire warning detection system with multiple lasers and detectors is proposed. Multiple drivers input laser’s temperature and injected current data, making its output wavelength consistent with the measured gas’ absorption peak wavelengths in absorption spectroscopy. Multiple light beams are coupled to the same optical fiber. After the light beams pass through the long optical path absorption cell filled with fire gas, the beams are separated by a converter. The signals are demodulated by different detectors and further analyzed for fire warnings. After the fire warning system’s design, the system’s various hardware modules are designed, including the light source module, TDLAS controller, gas chamber module, photoelectric detector, and data collection. When the temperature remains unchanged, the output wavelength is linearly related to the injected current. When the injected current remains unchanged, the output wavelength is linearly related to the operating temperature. With a semiconductor laser’s injected current of 40 mA, the initial temperature of 38.6 °C, and the output wavelength of 1578.16 nm, the output wavelength increases continuously as the temperature increases. The harmonic signal amplitude after gas absorption is positively correlated with the measured gas concentration, indicating that the second harmonic signals can estimate the fire gas concentration.

scholarly journals Recent improvements of long-path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations

2019 ◽  
Vol 12 (8) ◽  
pp. 4149-4169 ◽  
Author(s):  
Jan-Marcus Nasse ◽  
Philipp G. Eger ◽  
Denis Pöhler ◽  
Stefan Schmitt ◽  
Udo Frieß ◽  
...  
Keyword(s):  
Light Source ◽  
Optical Density ◽  
Measurement Accuracy ◽  
Trace Gases ◽  
Detection Limits ◽  
Trace Gas ◽  
Light Sources ◽  
Light Path ◽  
Mixing Ratios ◽  
Spectral Residual

Abstract. Over the last few decades, differential optical absorption spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace-gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer–Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km, with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace-gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light-emitting diodes (LEDs) as light sources. Here we present the application of a laser-driven light source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition, the lifetime of LDLS is about an order of magnitude higher than of typical Xe arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode-mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5-10×10-5 root mean square (rms; in units of optical density). This represents a reduction of detection limits of typical trace-gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance, automated LP-DOAS systems, as demonstrated here by more than 2 years of continuous observations in Antarctica.

scholarly journals Extending differential optical absorption spectroscopy for limb measurements in the UV

2010 ◽  
Vol 3 (3) ◽  
pp. 631-653 ◽  
Author(s):  
J. Puķīte ◽  
S. Kühl ◽  
T. Deutschmann ◽  
U. Platt ◽  
T. Wagner
Keyword(s):  
Optical Depth ◽  
Absorption Spectroscopy ◽  
Weighting Function ◽  
Scattered Light ◽  
A Priori ◽  
Optical Absorption Spectroscopy ◽  
A Priori Knowledge ◽  
Light Path ◽  
Air Mass ◽  
Mass Factor

Abstract. Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the Beer-Lambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light path only, the relation between the optical depth and the concentration of any absorber can be approximated as linear also for scattered light observations at a single wavelength if the absorption is weak. If the light path distribution is approximated not to vary with wavelength, also linearity between the optical depth and the product of the cross-section and the concentration of an absorber can be assumed. These assumptions are widely made for DOAS applications for scattered light observations. For medium and strong absorption of scattered light (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is no longer linear. In addition, for broad wavelength intervals the differences in the travelled light-paths at different wavelengths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength. However, the DOAS method can be extended to cases with medium to strong absorptions and for broader wavelength intervals by the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a priori knowledge for the air mass factor or the weighting function from radiative transfer modelling. We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself. This new approach provides a description of the SCD the exactness of which depends on the order of the Taylor expansion, and is independent from any assumptions or a priori knowledge of the considered absorbers. In case studies of simulated and measured spectra in the UV range (332–357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. The results for BrO obtained from the simulated spectra are closer to the true profiles, when applying the new method for the SCDs of ozone, than when the standard DOAS approach is used. For the measured spectra the agreement with validation measurements is also improved significantly, especially for cases with strong ozone absorption. While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied to a wide range of DOAS retrievals.

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