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 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.

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.

Technological advances to improve the quantification of volcanic emissions

2021 ◽  
Author(s):  
Christopher Fuchs ◽  
Jonas Kuhn ◽  
Nicole Bobrowski ◽  
Ulrich Platt
Keyword(s):  
Remote Sensing ◽  
Temporal Resolution ◽  
Volcanic Activity ◽  
Imaging Techniques ◽  
Correlation Spectroscopy ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Integration Time ◽  
Volcanic Plume ◽  
Flux Measurements

&lt;p&gt;Variations in volcanic trace gas composition and fluxes are a valuable indicator for changes in magmatic systems and therefore allow monitoring of the volcanic activity. An established method to measure trace gas emissions is to use remote sensing techniques like, for example, Differential Optical Absorption Spectroscopy (DOAS) and more recently SO&lt;sub&gt;2&lt;/sub&gt;-cameras, that can quantify volcanic sulphur dioxide (SO&lt;sub&gt;2&lt;/sub&gt;) emissions during quiescent degassing and eruptive phases, making it possible to correlate fluxes with volcanic activity.&amp;#160;&lt;/p&gt;&lt;p&gt;We present flux measurements of volcanic SO&lt;sub&gt;2&lt;/sub&gt; emissions based on the novel remote sensing technique of Imaging Fabry-P&amp;#233;rot Interferometer Correlation Spectroscopy (IFPICS) in the UV spectral range. The basic principle of IFPICS lies in the application of an Fabry-P&amp;#233;rot Interferometer (FPI) as wavelength selective element. The FPIs periodic transmission profile is matched to the periodic spectral absorption features of SO&lt;sub&gt;2&lt;/sub&gt;, resulting in high spectral information for its detection. This technique yields a higher trace gas selectivity and sensitivity than imaging approaches based on interference filters, e.g. SO&lt;sub&gt;2&lt;/sub&gt;-cameras and an increased spatio-temporal resolution over spectroscopic imaging techniques, e.g. imaging DOAS. Hence, IFPICS shows reduced cross sensitivities to broadband absorption (e.g. to ozone, aerosols), which allows the application to weaker volcanic SO&lt;sub&gt;2&lt;/sub&gt; emitters and increases the range of possible atmospheric conditions. It further raises the possibility to apply IFPICS to other trace gas species like, for example, bromine monoxide, that still can be characterized with a high spatial and temporal resolution (&lt; 1 HZ).&lt;/p&gt;&lt;p&gt;In October 2020, we acquired SO&lt;sub&gt;2&lt;/sub&gt; column density distribution images of Mt Etna volcanic plume with a detection limit of 2x10&lt;sup&gt;17&lt;/sup&gt; molec cm&lt;sup&gt;-2&lt;/sup&gt;, 1 s integration time, 400x400 pixel spatial, and 0.3 Hz temporal resolution.&amp;#160; We compare the SO&lt;sub&gt;2&lt;/sub&gt; fluxes retrieved by IFPICS with simultaneous flux measurements using the mutli-axis DOAS technique.&lt;/p&gt;

The Determination of HF, HCl, SiF4, and HCF3 in WF6 Gas

1974 ◽  
Vol 28 (2) ◽  
pp. 139-142 ◽  
Author(s):  
C. L. Chaney ◽  
J. Chin
Keyword(s):  
Infrared Absorption ◽  
Gas Absorption ◽  
Absorption Method ◽  
Absorption Cell ◽  
Cell Sensitivity ◽  
Gas Sampling ◽  
Calibration Curves

An infrared absorption method in the 1- to 15-µ range has been developed for determining HF, HCl, HCF3, and SiF4 in WF6, gas. Because of the reactivity and corrosiveness of WF6, a special gas absorption cell and gas-sampling apparatus were designed and built. Calibration curves were generated for a 10-cm cell. Sensitivity values were 100 wppm for HF and HCl, and 10 wppm for HCF3 and SiF4.

scholarly journals The ICAD (Iterative Cavity Enhanced DOAS) Method

2019 ◽  
Author(s):  
Martin Horbanski ◽  
Denis Pöhler ◽  
Johannes Lampel ◽  
Ulrich Platt
Keyword(s):  
Light Intensity ◽  
Gas Absorption ◽  
Optical Absorption Spectroscopy ◽  
Light Path ◽  
Calibration Source ◽  
Optical Cavities ◽  
Total Light ◽  
Averaging Time ◽  
Chemiluminescence Detector

Abstract. Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS or BB-CEAS DOAS) allows to make in-situ measurements while maintaining the km-long light paths required by DOAS. These technique have been successfully used for several years to measure in-situ atmospheric trace gases. A property of optical cavities is that in presence of strong absorbers or scatterers the light path is reduced, opposite to classical Long Path DOAS measurements. Typical CE-DOAS or BB-CEAS evaluation schemes correct this effect using the measured total light intensity attenuation. This makes them sensitive to any variations of the light intensity not arising from the trace gas absorption. That means an important DOAS advantage, to be independent of total light intensity, is actually lost. In order to cope with this problem, the instrument setup would require a thorough stabilisation of the light source and a very rigid mechanical setup, which would make instrumentation more complex and error prone. We present a new approach to Cavity Enhanced (CE-) DOAS based on an iterative algorithm (ICAD) which actually models the light path reduction from the derived absorbers in the optical resonator. It allows a sensitive and robust data analysis that does not depend on the total light intensity allowing a simpler and more compact instrument setup. The algorithm is discussed and simulated measurements demonstrate its sensitivity and robustness. Furthermore, a new NO2 ICAD instrument is presented. It takes advantage of the advanced data evaluation to build a compact (50 cm cavity) and light weight instrument (<10 kg) with low power consumption (25 W) for sensitive measurements of NO2 with a detection limit of 0.02 ppbv at an averaging time of 7 minutes. The instrument is characterized with a NO2 calibration source and good long term stability is demonstrated in a comparison with a commercial chemiluminescence detector. As a new application of ICAD we show measurements on an auto mobile platform to investigate the two dimensional NO2 distribution in an urban area. The instrument is so robust that even strong vibrations do not lead to any measurement problems.

scholarly journals Determination of an effective trace gas mixing height by differential optical absorption spectroscopy (DOAS)

2009 ◽  
Vol 2 (4) ◽  
pp. 1663-1692 ◽  
Author(s):  
B. Zhou ◽  
S. N. Yang ◽  
S. S. Wang ◽  
T. Wagner
Keyword(s):  
Trace Gases ◽  
Mixing Layer ◽  
Correlation Coefficients ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Gas Mixing ◽  
Mixing Height ◽  
Free Atmosphere ◽  
Layer Height

Abstract. A new method for the determination of the Mixing layer Height (MH) by the DOAS technique is proposed in this article. The MH can be retrieved by a combination of active DOAS and passive DOAS observations of atmospheric trace gases; here we focus on observations of NO2. Because our observations are sensitive to the vertical distribution of trace gases, we refer to the retrieved layer height as an ''effective trace gas mixing height'' (ETMH). By analyzing trace gas observations in Shanghai over one year (1017 hourly means in 93 days in 2007), the retrieved ETMH was found to range between 0.1 km and 2.8 km (average is 0.78 km); more than 90% of the measurements yield an ETMH between 0.2 km and 2.0 km. The seasonal and diurnal variation of the ETMH shows good agreement with mixing layer heights derived from meteorological observations. We investigated the relationship of the derived ETMH to temperature and wind speed and found correlation coefficients of 0.65 and 0.37, respectively. Also the wind direction has an impact on the measurement to some extent. Especially in cases when the air flow comes from highly polluted areas and the atmospheric lifetime of NO2 is long (e.g. in winter), the NO2 concentration at high altitudes over the measurement site can be enhanced, which leads to an overestimation of the ETMH. Enhanced NO2 concentrations in the free atmosphere and heterogeneity within the mixing layer can cause additional uncertainties. Our method could be easily extended to other species like e.g. SO2, HCHO or Glyoxal. Simultaneous studies of these molecules could yield valuable information on their respective atmospheric lifetimes.

CW DFB RT diode laser-based sensor for trace-gas detection of ethane using a novel compact multipass gas absorption cell

2013 ◽  
Vol 112 (4) ◽  
pp. 461-465 ◽  
Author(s):  
Karol Krzempek ◽  
Mohammad Jahjah ◽  
Rafał Lewicki ◽  
Przemysław Stefański ◽  
Stephen So ◽  
...  
Keyword(s):  
Diode Laser ◽  
Gas Absorption ◽  
Gas Detection ◽  
Trace Gas ◽  
Absorption Cell ◽  
Trace Gas Detection

CW DFB RT diode laser based sensor for trace-gas detection of ethane using novel compact multipass gas absorption cell

2013 ◽  
Author(s):  
Mohammad Jahjah ◽  
Rafal Lewicki ◽  
Frank K. Tittel ◽  
Karol Krzempek ◽  
Przemyslaw Stefanski ◽  
...  
Keyword(s):  
Diode Laser ◽  
Gas Absorption ◽  
Gas Detection ◽  
Trace Gas ◽  
Absorption Cell ◽  
Trace Gas Detection

scholarly journals The ICAD (iterative cavity-enhanced DOAS) method

2019 ◽  
Vol 12 (6) ◽  
pp. 3365-3381 ◽  
Author(s):  
Martin Horbanski ◽  
Denis Pöhler ◽  
Johannes Lampel ◽  
Ulrich Platt
Keyword(s):  
Light Intensity ◽  
Gas Absorption ◽  
Optical Absorption Spectroscopy ◽  
Light Path ◽  
Calibration Source ◽  
Optical Cavities ◽  
Total Light ◽  
Averaging Time ◽  
Chemiluminescence Detector

Abstract. Cavity-enhanced differential optical absorption spectroscopy (CE-DOAS or BB-CEAS DOAS) allows us to make in situ measurements while maintaining the kilometre-long light paths required by DOAS. This technique has been successfully used for several years to measure in situ atmospheric trace gases. A property of optical cavities is that in the presence of strong absorbers or scatterers the light path is reduced, in contrast to classical long-path DOAS measurements where the light path is fixed. Typical CE-DOAS or BB-CEAS evaluation schemes correct this effect using the measured total light intensity attenuation. This makes them sensitive to any variations in the light intensity not arising from the trace gas absorption. That means an important DOAS advantage, to be independent of total light intensity, is actually lost. In order to cope with this problem, the instrument setup would require a thorough stabilisation of the light source and a very rigid mechanical setup, which would make instrumentation more complex and error prone. We present a new approach to cavity-enhanced (CE) DOAS based on an iterative algorithm (ICAD) which actually models the light path reduction from the derived absorbers in the optical resonator. It allows a sensitive and robust data analysis that does not depend on the total light intensity, allowing a simpler and more compact instrument setup. The algorithm is discussed and simulated measurements demonstrate its sensitivity and robustness. Furthermore, a new ICAD NO2 instrument is presented. It takes advantage of the advanced data evaluation to build a compact (50 cm cavity) and lightweight instrument (<10 kg) with low power consumption (25 W) for sensitive measurements of NO2 with a detection limit of 0.02 ppbv at an averaging time of 7 min. The instrument is characterised with a NO2 calibration source and good long-term stability is demonstrated in a comparison with a commercial chemiluminescence detector. As a new application of ICAD we show measurements on an automobile platform to investigate the two-dimensional NO2 distribution in an urban area. The instrument is so robust that even strong vibrations do not lead to any measurement problems.

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