The ICAD (Iterative Cavity Enhanced DOAS) Method

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.

Download Full-text

The ICAD (iterative cavity-enhanced DOAS) method

Atmospheric Measurement Techniques ◽

10.5194/amt-12-3365-2019 ◽

2019 ◽

Vol 12 (6) ◽

pp. 3365-3381 ◽

Cited By ~ 3

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.

Download Full-text

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

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-1-481-2008 ◽

2008 ◽

Vol 1 (1) ◽

pp. 481-507 ◽

Cited By ~ 1

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.

Download Full-text

Caution with Spectroscopic NO2 Reference Cells (Cuvettes)

10.5194/amt-2019-130 ◽

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.

Download Full-text

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

Atmospheric Measurement Techniques ◽

10.5194/amt-2-713-2009 ◽

2009 ◽

Vol 2 (2) ◽

pp. 713-723 ◽

Cited By ~ 53

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.

Download Full-text

Caution with spectroscopic NO2 reference cells (cuvettes)

Atmospheric Measurement Techniques ◽

10.5194/amt-12-6259-2019 ◽

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.

Download Full-text

Capturing Spatial and Temporal Patterns of NO2 in Cities using mobile and stationary DOAS measurements

10.5194/dach2022-153 ◽

2021 ◽

Author(s):

Mark Wenig ◽

Sheng Ye ◽

Ying Zhu ◽

Hanlin Zhang

Keyword(s):

Air Quality ◽

Hot Spots ◽

Temporal Patterns ◽

Optical Absorption Spectroscopy ◽

Spatial And Temporal Patterns ◽

The Public ◽

Measurement Height ◽

Weekly Cycles

The problem of elevated NO2 levels in cities has gained some attention in the public in recent years and has given rise to questions about the plausibility of banning diesel engines in cities, the meaning of exceedances of air quality limits and the effects of corona lock-downs on air quality to name a few. Urban air quality is typically monitored using a relatively small number of monitoring stations. Those in-situ measurements follow certain guidelines in terms of inlet height and location relative to streets, but the question remains how a limited number of point measurements can capture the spatial variability in cities. In this talk we present two measurement campaigns in Hong Kong and Munich where we utilized a combination of mobile in-situ and stationary remote sensing differential optical absorption spectroscopy (DOAS) instruments. We developed an algorithm to separate spatial and temporal patterns in order to generate pollution maps that represent average NO2 exposure.&#160; We use those maps to identify pollution hot spots and capture the weekly cycles of on-road NO2 levels and spatial dependency of long-term changes and we analyze how on-road measurements compare to monitoring station data and how the measurement height and distance to traffic emissions have to be considered when interpreting observed concentration patterns.

Download Full-text

Spectral studies of ocean water using DOAS

Ocean Science Discussions ◽

10.5194/osd-4-459-2007 ◽

2007 ◽

Vol 4 (3) ◽

pp. 459-489

Author(s):

M. Vountas ◽

T. Dinter ◽

A. Bracher ◽

J. P. Burrows ◽

B. Sierk

Keyword(s):

Spectral Studies ◽

Optical Absorption Spectroscopy ◽

Retrieval Technique ◽

Phytoplankton Absorption ◽

Visible Wavelengths ◽

Scanning Imaging ◽

Atmospheric Trace Gas ◽

First Time ◽

Chlorophyll Concentrations

Abstract. Methods enabling the retrieval of oceanic parameter from the space borne instrumentation Scanning Imaging Absorption Spectrometer for Atmospheric ChartographY (SCIAMACHY) using Differential Optical Absorption Spectroscopy (DOAS) are presented. SCIAMACHY onboard ENVISAT measures back scattered solar radiation at a spectral resolution (0.2 to 1.5 nm). The DOAS method was used for the first time to fit modelled Vibrational Raman Scattering (VRS) in liquid water and in situ measured phytoplankton absorption reference spectra to optical depths measured by SCIAMACHY. Spectral structures of VRS and phytoplankton absorption were clearly found in these optical depths. Both fitting approaches lead to consistent results. DOAS fits correlate with estimates of chlorophyll concentrations: low fit factors for VRS retrievals correspond to large chlorophyll concentrations and vice versa; large fit factors for phytoplankton absorption correspond with high chlorophyll concentrations and vice versa. From these results a simple retrieval technique taking advantage of both measurements is shown. First maps of global chlorophyll concentrations were compared to the corresponding MODIS measurements with very promising results. In addition, results from this study will be used to improve atmospheric trace gas DOAS-retrievals from visible wavelengths by including these oceanographic signatures.

Download Full-text

NO2 seasonal evolution in the North Subtropical free troposphere

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-14473-2015 ◽

2015 ◽

Vol 15 (10) ◽

pp. 14473-14504

Author(s):

M. Gil-Ojeda ◽

M. Navarro-Comas ◽

L. Gómez-Martín ◽

J. A. Adame ◽

A. Saiz-Lopez ◽

...

Keyword(s):

Optical Absorption Spectroscopy ◽

High Mountain ◽

Horizontal Line ◽

Free Troposphere ◽

Seasonal Evolution ◽

The North ◽

Measuring Period ◽

The Impact ◽

Pollution Events

Abstract. Three years of Multi-Axis Differential Optical Absorption Spectroscopy (MAXDOAS) measurements (2011–2013) have been used for estimating the NO2 mixing ratio along a horizontal line of sight from the high mountain Subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O2–O2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO2, and is applicable under low aerosols loading conditions. The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO2 contamination resulting from the arrival of MBL airmasses from thermally forced upwelling breeze during central hours of the day. Comparisons with in-situ observations show that during most of measuring period the MAXDOAS is insensitive or very little sensitive to the upwelling breeze. Exceptions are found during pollution events under southern wind conditions. On these occasions, evidence of fast efficient and irreversible transport from the surface to the free troposphere is found. Background NO2 vmr, representative of the remote free troposphere, are in the range of 20–45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry-climate CAM-Chem model are in good agreement with the NO2 measurements, and are used to further investigate the possible drivers of the NO2 seasonality observed at Izaña.

Download Full-text

Intercomparison of four different in-situ techniques for ambient formaldehyde measurements in urban air

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-2897-2005 ◽

2005 ◽

Vol 5 (3) ◽

pp. 2897-2945 ◽

Cited By ~ 6

Author(s):

C. Hak ◽

I. Pundt ◽

C. Kern ◽

U. Platt ◽

J. Dommen ◽

...

Keyword(s):

Regression Line ◽

Ambient Air ◽

Urban Site ◽

Optical Absorption Spectroscopy ◽

Measuring Instruments ◽

Chromatographic Technique ◽

In Situ Techniques ◽

Time Period ◽

The Individual

Abstract. Results from an intercomparison of several currently used in-situ techniques for the measurement of atmospheric formaldehyde (CH2O) are presented. The measurements were carried out at Bresso, an urban site in the periphery of Milan (Italy) as part of the FORMAT-I field campaign. Eight instruments were employed by six independent research groups using four different techniques: Differential Optical Absorption Spectroscopy (DOAS), Fourier Transform Infra Red (FTIR) interferometry, the fluorimetric Hantzsch reaction technique (five instruments) and a chromatographic technique employing C18-DNPH-cartridges (2,4-dinitrophenylhydrazine). White type multi-reflection systems were employed for the optical techniques in order to avoid spatial CH2O gradients and ensure the sampling of nearly the same air mass by all instruments. Between 23 and 31 July 2002, up to 13 ppbv of CH2O were observed. The concentrations lay well above the detection limits of all instruments. The formaldehyde concentrations determined with DOAS, FTIR and the Hantzsch instruments were found to agree within ±11%, with the exception of one Hantzsch instrument, which gave systematically higher values. The two hour integrated samples by DNPH yielded up to 25% lower concentrations than the data of the continuously measuring instruments averaged over the same time period. The consistency between the DOAS and the Hantzsch method was better than during previous intercomparisons in ambient air with slopes of the regression line not significantly differing from one. The differences between the individual Hantzsch instruments could be attributed in part to the calibration standards used. Possible systematic errors of the methods are discussed.

Download Full-text

In vitro studies on the induction of sporogenous tissue on leaves of cinnamon fern. I. Environmental factors

Canadian Journal of Botany ◽

10.1139/b72-341 ◽

1972 ◽

Vol 50 (12) ◽

pp. 2673-2682 ◽

Cited By ~ 7

Author(s):

William H. Harvey ◽

James D. Caponetti

Keyword(s):

Environmental Factors ◽

Light Intensity ◽

Environmental Conditions ◽

Light Exposure ◽

Progressive Increase ◽

Light Phase ◽

Sporogenous Tissue ◽

Light Darkness

Intact, set III, cinnamon fern cataphyll and frond primordia, which were shown to have no predisposition to fertility in situ, produced sporangia when excised and cultured under sterile conditions in Knudson's medium supplemented with various levels of sucrose and maintained on 11 different regimens of light, darkness, and temperature for 10 weeks. Increasing levels of sucrose resulted in increased fertility under all environmental conditions, but the highest percentage of fertility was obtained under conditions of continuous dark at 26 °C. As the length of the light phase of the photoperiods decreased, a progressive increase in induction of fertile leaves was observed, suggesting that periods of long light exposure are inhibitory to the initiation of sporangia. Conversely, as the light intensity was increased, an inhibition of sporophyll differentiation occurred. Sporangia excised from dark-induced sporophylls and cultured in the light produced viable spores which germinated yielding haploid gametophytes that ultimately produced sporophytes.

Download Full-text

The ICAD (Iterative Cavity Enhanced DOAS) Method