Absorption Spectrum Of Methane Research Articles

Optoelectronic methane sensor based on two-color led module

The article proposes an optoelectronic two-wave method for monitoring the methane content in the atmosphere. Spectral characteristics are given two-color LED module LED39, LED32, photodiode PD36 and methane absorption spectrum. An optoelectronic sensor for monitoring the methane content in the atmosphere with high measurement accuracy has been developed, and its block diagram is shown. In an optoelectronic sensor for monitoring methane content in the atmosphere A two-color LED module with emission spectra of 3.2 microns (reference) and LEDs with emission spectra of 3.4 microns (working) were used . To increase the optical power of LEDs (3.2 and 3.4 microns), a design with more efficient heat removal and a parabolic reflector design for LEDs, which focuses IR radiation at an angle of 8-10 degrees, have been proposed. LED crystals with emission spectra of 3.2 microns and 3.4 microns are mounted in one housing to ensure high accuracy and sensitivity of the device.

Development of methane leakage telemetry system based on tunable diode laser absorption spectroscopy

A methane telemetry system based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) is proposed for the long-range, high-precision, and real-time measurement of methane concentration in coal mine safety production. The output wavelength of the tunable semiconductor laser is controlled to scan at 1654nm (6045.94cm−1) of the methane absorption spectrum. The harmonic signal is obtained by digital phase locking technique, and the effect of light intensity variation on the system measurement results is eliminated by 2f/1f signal processing technique. Through calibration experiment and measurement error analysis, the measurement error of the system is less than 10%, which proves that the device has the advantages of high accuracy.

Photoacoustic phase-controlled Fourier-transform infrared spectroscopy

We demonstrate a 13-fold speed improvement in broadband cantilever-enhanced photoacoustic spectroscopy (CEPAS) by combining it with phase-controlled Fourier-transform spectroscopy (PC-FTS) instead of traditional Fourier-transform infrared spectroscopy (FTIR). PC-FTS is a modification of FTIR and capable of fundamentally faster interferogram acquisitions. The speed-improvement is beneficial for CEPAS, which is an especially sensitive version of the background-free photoacoustic spectroscopy technique. We used the PC-FTS-CEPAS technique to measure the absorption spectrum of methane in the mid-infrared region (3.3–3.5 µm) with an optical frequency comb as the light source.

The high resolution absorption spectrum of methane in the 10 800-14 000 cm-1 region: literature review, new results and perspectives.

The recent development of high resolution spectrographs for exoplanetary research in the visible range makes suitable an improvement of our knowledge of the high resolution spectrum of methane. In this contribution, the weak and highly congested absorption spectrum of methane in the 10 800-14 000 cm-1 region (0.71-0.93 μm) is considered on the basis of (i) an exhaustive review of the literature over the lasts decades, (ii) the analysis of a spectrum recorded at Kitt Peak by Fourier transform spectroscopy at room temperature, (iii) a very high sensitivity spectrum recorded by cavity ring down spectroscopy near 760 nm. The line list retrieved from the Kitt Peak spectrum includes 12 800 lines between 10 802 and 13 922 cm-1. Together with the CRDS line list in the 13 060-13 300 cm-1 interval (about 2650 lines), the reported FTS dataset represents the first high resolution extensive intensity measurements of methane for wavenumbers above 11 502 cm-1. A very good agreement between our Kitt Peak line list and HITRAN list is found in the 10 800-11 502 cm-1 interval. The "quasi-continuum" absorption background underlying the congested spectrum around 11 200 cm-1 is quantitatively evaluated to about 42% of the absorption by CH4 lines. Previous laser-based investigations are critically reviewed by comparison to the FTS and CRDS experimental data retrieved in the present work. The review of the studies of the minor isotopologues (13CH4, CH3D, CH2D2, and CHD3) is also presented. Intensity comparison with band models used for planetary applications is discussed and confirms the importance of the "quasi-continuum" absorption in the methane spectrum at room temperature. The comparison to the TheoReTs line list obtained by ab initio calculations gives valuable hints for future assignments but the TheoReTS line positions are not sufficiently accurate for application to high resolution exoplanetary spectra in the region. From the various comparisons and results obtained in this work, we conclude that the high frequency absorption spectrum of methane deserves to be revisited by modern cavity-enhanced absorption techniques to fulfil needs both for future analysis of high resolution exoplanetary spectra and for theoretical analysis.

AUTOMATIC METHANE PLUMES DETECTION IN TIME SERIES OF SENTINEL-5P L1B IMAGES

Abstract. Reducing methane emissions is essential to tackle climate change. Here, we address the problem of detecting automatically large methane leaks using hyperspectral data from the Level 1B product of the Sentinel-5P satellite. To do this, two features of TROPOMI (TROPOspheric Monitoring Instrument), the Sentinel-5P satellite sensor, are exploited. The first one is the fine spectral sampling of the data which allows to isolate features of the methane absorption spectrum in the shortwave infrared wavelength range (SWIR). The second one is the daily coverage of the whole Earth which allows to perform time series analysis. Our method involves three main steps: i) a pixel reconstruction, ii) an angle correction and iii) a plume detection with a time series. In the first step, a simplified absorption model is inverted to recover, for each pixel, a coefficient representative of the presence of methane which we call the methane coefficient. In the second step, a correction is made to the methane coefficient to take into account the viewing angle of the satellite. In the third step, the obtained coefficient is normalized spatially and then the detection is carried out pixel by pixel, by looking for anomalies in a time series. We validate our method by comparing the detected plumes against a recently published dataset of plumes manually detected in the Sentinel-5P L2 methane product. We then show how our method can complement the Sentinel-5P L2 methane product for the detection of methane plumes.

DETECTION OF METHANE PLUMES IN HYPERSPECTRAL IMAGES FROM SENTINEL-5P BY COUPLING ANOMALY DETECTION AND PATTERN RECOGNITION

Abstract. Reducing methane emissions is essential to tackle climate change. Here, we address the problem of detecting large methane leaks using hyperspectral data from the Sentinel-5P satellite. For that we exploit the fine spectral sampling of Sentinel-5P data to detect methane absorption features visible in the shortwave infrared wavelength range (SWIR). Our method involves three separate steps: i) background subtraction, ii) detection of local maxima in the negative logarithmic spectrum of each pixel and iii) anomaly detection in the background-free image. In the first step, we remove the impact of the albedo using albedo maps and the impact of the atmosphere by using a principal component analysis (PCA) over a time series of past observations. In the second step, we count for each pixel the number of local maxima that correspond to a subset of local maxima in the methane absorption spectrum. This counting method allows us to set up a statistical a contrario test that controls the false alarm rate of our detections. In the last step we use an anomaly detector to isolate potential methane plumes and we intersect those potential plumes with what have been detected in the second step. This process strongly reduces the number of false alarms. We validate our method by comparing the detected plumes against a dataset of plumes manually annotated on the Sentinel-5P L2 methane product.

Fiber-based optical frequency comb at 3.3 µm for broadband spectroscopy of hydrocarbons [Invited

A 125 MHz fiber-based frequency comb source in the mid-infrared wavelength region is presented. The source is based on difference frequency generation from a polarization-maintaining Er-doped fiber pump laser and covers a spectrum between 2900 cm−1 and 3400 cm−1 with a simultaneous bandwidth of 170 cm−1 and an average output power up to 70 mW. The source is equipped with actuators and active feedback loops, ensuring long-term stability of the repetition rate, output power, and spectral envelope. An absorption spectrum of ethane and methane was measured using a Fourier transform spectrometer to verify the applicability of the mid-infrared comb to multispecies detection. The robustness and good long- and short-term stability of the source make it suitable for optical frequency comb spectroscopy of hydrocarbons.

Application of Derivative Transform Spectroscopy in Gas Detection

Digital filtering technique is of great significance in real-time signal processing and analysis, but the stability, efficiency and flexibility of filter algorithm are important indicators to reflect its application value. In this paper, an adaptive derivative transformation based on Savitzky-Golay filter algorithm was proposed for laser absorption spectroscopy analysis. To demonstrate this analysis algorithm, first-order and second-order derivative spectroscopy are evaluated for the analysis of infrared methane absorption spectra, and compared with the original direct absorption spectral signals. The results indicated that the proposed signal processing algorithm has good performance on noise suppression and spectral resolution improvement, and the 2nd derivative spectroscopy shows better de-noising efficiency.

260 kHz mode-spacing optical frequency combs for scan-free high-resolution direct-comb spectroscopy

For scan-free high-resolution direct-comb spectroscopy, mode spacing of an optical frequency comb is reduced down to 260 kHz by phase modulation. It turns out that time-domain signal is degraded by averaging because of slow optical path length fluctuations and fast optical pulse timing jitter. In this study, compensation of these effects is introduced, and signal degradation by averaging is avoided. With demonstrations of direct-comb spectroscopy with the narrow-mode-spacing optical frequency comb, Doppler-limited absorption spectrum of methane and reflection spectrum from an optical ring cavity are observed. As a result, detailed resonance spectral line profile of 8 MHz linewidth for the optical ring cavity is obtained in 50 ms measurement time.

Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer

Abstract. We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L−1 to mmol L−1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % (2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L−1 at 25 ∘C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L−1. Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

High-resolution and gapless dual comb spectroscopy with current-tuned quantum cascade lasers.

We present gapless, high-resolution absorption and dispersion spectra obtained with quantum cascade laser frequency combs covering 55 cm-1. Using phase-sensitive dual comb design, the comb lines are gradually swept over 10 GHz, corresponding to the free spectral range of the laser devices, by applying a current modulation. We show that with interleaving the spectral point spacing is reduced by more than four orders of magnitude over the full spectral span of the frequency comb. The potential of this technique for high-precision gas sensing is illustrated by measuring the low pressure (107 hPa) absorption and dispersion spectra of methane spanning the range of 1170 cm-1 - 1225 cm-1 with a resolution of 0.001 cm-1.

Spectroscopic Measurements of Methane Solid-Gas Equilibrium Clapeyron Curve between 40 and 77 K.

The infrared gas-phase absorption spectrum of methane was used to determine its Clapeyron solid-gas equilibrium curve in the 40-77 K temperature range. For comparative purposes and to obtain more reliable results, two different optical experimental setups were used. At higher temperatures (53-77 K), a single pass cryogenically cooled cell was coupled to a standard low-resolution Fourier transform infrared spectrometer. The second system was a state-of-the-art vertical-external-cavity surface-emitting laser tunable source operating at around 2.3 μm, combined with a 7 m path Herriott cell, to record methane absorption features down to 40 K. From the measurements, the vapor pressure curve ln( p/Pa) = -(1191.92 ± 8.92)/( T/K) + (22.49 ± 0.16) was derived in the range 40-77 K. This corresponds to a value of 9910 ± 75 J mol-1 for the sublimation enthalpy. The relation was validated down to 40 K, increasing our knowledge of the saturation pressure by 2 orders of magnitude. Data were compared with available pressure measurements from the literature, obtained by manometric or mass spectrometry techniques, and the sublimation enthalpy was compared with a thermodynamic approach based on heat capacity measurements in the solid and gas phases.

Correlation of absorption spectrum properties of methane with coupling variables between temperature and concentration

When using tunable diode laser absorption spectroscopy to detect methane on-line, the line intensity of methane absorption spectrum is affected by the change of ambient temperature, which has inevitable impacts on measurement results. Based on the direct absorption spectroscopy method, the absorption spectrum properties of methane with different concentrations (1%–15%) and temperatures (290–350 K) were analyzed, and the correlation between temperature-concentration coupling variables and spectral absorbance of methane was obtained by polynomial regression and global linear fitting of shared parameters. The results show that the spectral absorbance A of methane increases with the rise of its concentration at the same temperature. At the same concentration, the ratio of total partition function rQ plays a dominant role in the process of methane absorption spectrum line intensity changing with temperature, and the linear decrease of rQ with the increase of temperature, which results in the methane line intensity decreasing when the temperature increases. When the methane concentration is in the range of 1%–15% and the ambient temperature is 290–350 K, the relative error between the inversion result of correlation and the simulation data of HITEMP spectrum platform is less than 1%.

Large regional shortwave forcing by anthropogenic methane informed by Jovian observations.

Recently, it was recognized that widely used calculations of methane radiative forcing systematically underestimated its global value by 15% by omitting its shortwave effects. We show that shortwave forcing by methane can be accurately calculated despite considerable uncertainty and large gaps in its shortwave spectroscopy. We demonstrate that the forcing is insensitive, even when confronted with much more complete methane absorption spectra extending to violet light wavelengths derived from observations of methane-rich Jovian planets. We undertake the first spatially resolved global calculations of this forcing and find that it is dependent on bright surface features and clouds. Localized annual mean forcing from preindustrial to present-day methane increases approaches +0.25 W/m2, 10 times the global annualized shortwave forcing and 43% of the total direct CH4 forcing. Shortwave forcing by anthropogenic methane is sufficiently large and accurate to warrant its inclusion in historical analyses, projections, and mitigation strategies for climate change.

Quasi-distributed multipoint laser methane detection system and its application in cable trench safety monitoring

A low power consumption, quasi-distributed, multi-point laser methane detection system incorporating a Vertical Cavity Surface Emitting Laser (VCSEL) has been designed and its performance discussed. Being based on the features of the methane absorption spectrum and adaptive tracking technology, the system can realize on-line methane monitoring across 14 channels of sensors, with the maximum transmission distance for each sensor being 10km. Working across a temperature range of 0°C-40°C, the measurement error was determined to be less than ±0.03% (of the gas concentration) when the methane volume concentration lies in the range up to 10% methane. The system has been tested in-the-field in a cable tunnel belonging to a power supply company in Jinan in China and the test results achieved show that the system has the high accuracy and good stability needed for use in industrial safety monitoring.

Laser absorption spectroscopy of methane at 1000 K near 1.7 µm: A validation test of the spectroscopic databases

A tubular furnace is combined with a diode laser spectrometer to record the high temperature near infrared absorption spectrum of methane at a pressure of 80 Torr. The investigated spectral range covers the 5693–6257 cm−1 (1.75–1.60 µm) range of the tetradecad dominated by the 2ν3 band near 6000 cm−1. Various determinations (thermocouples, Doppler line broadening, rotational temperature) provide a value of about 964 K for the gas temperature. Lines with intensity larger than 5 × 10−24 cm/molecule at 1000 K are detected. As a result of the high temperature, high-J rovibrational transitions up to J ≈ 20 and hot bands are identified in the spectrum. The recorded spectra provide tests of various methane line lists recently released in the literature: HITRAN2016, experimental list derived from emission spectra, the MeCaSDa empirical line list and the TheoReTs and ExoMol theoretical line lists. The agreement with the TheoReTs line list is found to be very good in the studied region.

The methane absorption spectrum near 1.73 µm (5695–5850 cm−1): Empirical line lists at 80 K and 296 K and rovibrational assignments

The methane absorption spectrum is studied at 297 K and 80 K in the center of the Tetradecad between 5695 and 5850 cm−1. The spectra are recorded by differential absorption spectroscopy (DAS) with a noise equivalent absorption of about αmin ≈ 1.5 × 10−7 cm−1. Two empirical line lists are constructed including about 4000 and 2300 lines at 297 K and 80 K, respectively. Lines due to 13CH4 present in natural abundance were identified by comparison with a spectrum of pure 13CH4 recorded in the same temperature conditions. About 1700 empirical values of the lower state energy level, Eemp, were derived from the ratios of the line intensities at 80 K and 296 K. They provide accurate temperature dependence for most of the absorption in the region (93% and 82% at 80 K and 296 K, respectively). The quality of the derived empirical values is illustrated by the clear propensity of the corresponding lower state rotational quantum number, Jemp, to be close to integer values. Using an effective Hamiltonian model derived from a previously published ab initio potential energy surface, about 2060 lines are rovibrationnally assigned, adding about 1660 new assignments to those provided in the HITRAN database for 12CH4 in the region.

Broadening and Shift of the Methane Absorption Lines in the 11000–11400 cm–1 Region

Absorption spectra of methane in the 11000–11400 cm–1 spectral region were recorded with an IFS-125M Fourier spectrometer at pressures from 11 to 100 mbar, a room temperature, and a spectral resolution of 0.03 cm–1. A multipass cell 60 cm long with 44 passes provided a total path length of 2640 cm and threshold sensitivity to absorption of about 10–8 cm–1. Line centers, intensities, self-broadening, and selfshift coefficients of methane lines were determined by fitting Voigt profile parameters.

Feed-forward coherent link from a comb to a diode laser: Application to widely tunable cavity ring-down spectroscopy.

We apply a feed-forward frequency control scheme to establish a phase-coherent link from an optical frequency comb to a distributed feedback (DFB) diode laser: This allows us to exploit the full laser tuning range (up to 1 THz) with the linewidth and frequency accuracy of the comb modes. The approach relies on the combination of an RF single-sideband modulator (SSM) and of an electro-optical SSM, providing a correction bandwidth in excess of 10 MHz and a comb-referenced RF-driven agile tuning over several GHz. As a demonstration, we obtain a 0.3 THz cavity ring-down scan of the low-pressure methane absorption spectrum. The spectral resolution is 100 kHz, limited by the self-referenced comb, starting from a DFB diode linewidth of 3 MHz. To illustrate the spectral resolution, we obtain saturation dips for the 2ν3 R(6) methane multiplet at μbar pressure. Repeated measurements of the Lamb-dip positions provide a statistical uncertainty in the kHz range.

High performance direct absorption spectroscopy of pure and binary mixture hydrocarbon gases in the 6\u201311 upmum range

The availability of accurate and fast hydrocarbon analyzers, capable of real-time operation while enabling feedback-loops, would lead to a paradigm change in the petro-chemical industry. Primarily gas chromatographs measure the composition of hydrocarbon process streams. Due to sophisticated gas sampling, these analyzers are limited in response time. As hydrocarbons absorb in the mid-infrared spectral range, the employment of fast spectroscopic systems is highly attractive due to significantly reduced maintenance costs and the capability to setup real-time process control. New developments in mid-infrared laser systems pave the way for the development of high-performance analyzers provided that accurate spectral models are available for multi-species detection. In order to overcome current deficiencies in the availability of spectroscopic data, we developed a laser-based setup covering the 6–11 upmum wavelength range. The presented system is designated as laboratory reference system. Its spectral accuracy is at least 6.6times 10^{-3} cm^{-1} with a precision of 3times 10^{-3} cm^{-1}. With a “per point” minimum detectable absorption of 1.3times 10^{-3} cm^{-1} Hz^{{-}{1/2}} it allows us to perform systematic measurements of hydrocarbon spectra of the first 7 alkanes under conditions which are not tabulated in spectroscopic database. We exemplify the system performance with measured direct absorption spectra of methane, propane, iso-butane, and a mixture of methane and propane.