CO Absorption Lines Research Articles

Calibration-free infrared absorption spectroscopy using cantilever-enhanced photoacoustic detection of the optical power

We report on sensitive tunable laser absorption spectroscopy using a multipass gas cell and a solid-state photoacoustic optical power detector. Unlike photoacoustic spectroscopy (PAS), this method readily allows a low gas pressure for high spectral selectivity and a free gas flow for continuous measurements. Our photoacoustic optical power detector has a large linear dynamic range and can be used at almost any optical wavelength, including the middle infrared and THz regions that are challenging to cover with traditional optical detectors. Furthermore, our approach allows for compensation of laser power drifts with a single detector. As a proof of concept, we have measured very weak CO2 absorption lines at 9.2 µm wavelength and achieved a normalized noise equivalent absorption (NNEA) of 2.35·10−9 Wcm−1Hz−1/2 with a low-power quantum cascade laser. The absolute value of the gas absorption coefficient is obtained directly from the Beer-Lambert law, making the technique calibration-free.

Tunable optical parametric oscillator based on ZnGeP2 crystal for greenhouse gas remote sensing systems

This work is devoted to the development of a compact source of coherent radiation with frequency-energy characteristics and a spectral generation range that allows remote determination of background concentrations of greenhouse gases in the atmosphere. The aim of this work was to create a frequency parametric converter based on ZGP, pumped by Ho:YAG laser radiation. For use as a source in a mobile lidar for remote determination of the concentration of greenhouse gases in the atmosphere. In the course of the research, a layout of an Optical parametric oscillator OPO based on a ZGP crystal with Ho:YAG laser radiation pumping was developed. The system’s continuous failure-free operation time was 1.5 h at a pulse repetition rate of 10 kHz and a pulse energy of the generated radiation of 0.08 mJ. The tuning range of the OPO was from 3.3 to 5 μm when using a Lyot filter. The losses from the average generation power when the Lyot filter was introduced into the resonator were 30%. At the same time, it was possible to achieve a linewidth of the generated radiation of 0.7 nm. The divergence of the generated radiation did not exceed 1.5 mrad.The absorption spectrum of gases CO2, CH4, N2O, CO in a gas cell was simulated for the entire generation range of the ZnGeP2-based OPO. As a result of the simulation, the most intense absorption lines of gases CO2, CH4, N2O, CO in the OPO tuning range were revealed, the central wavelengths of the absorption lines and their spectral width were determined.

Ppb‐level CO2 sensor based on a miniature multipass cell with eight‐petaled spot pattern

AbstractThis paper presents an innovative carbon dioxide sensor leveraging a dense‐pattern multipass absorption cell. This novel design dramatically compacts an extensive optical path of approximately 11.4 m into a 70 mm inter‐mirror distance, effectively enhancing the system's sensitivity and reducing its size. Utilizing a 2004 nm diode laser, the sensor accurately targets a potent CO2 absorption line at 4989.9 cm−1. Experimental validation reveals remarkable linearity (R² = .999) and a detection limit of 59 ppb with 1 s integration time. The incorporation of Kalman filtering techniques further refines the sensor's performance, reducing the detection limit to 30 ppb—a substantial improvement of 49%. This advancement in sensor technology marks a significant step forward in efficient and precise CO2 monitoring.

Cold molecules in H I 21 cm absorbers across redshifts ∼0.1–4

Absorption lines at high-redshift in front of quasars are quite rare in the millimeter (mm) domain. Only five associated and five intervening systems have been reported in the literature. Nevertheless, these discoveries provide very useful information that is complementary to emission lines, allowing, for instance, to distinguish between inflows and outflows. These lines are also good candidates for studying the variations of the fundamental constants of physics. Here we report the findings of our search for CO and other molecules in emission and absorption in front of a sample of 30 targets, comprising 16 associated and 14 intervening H I 21 cm absorbers. The observations were made with the IRAM-30 m telescope simultaneously at 3 mm and 2 mm, exploring several lines of the CO ladder and HCO+, depending on the redshift. We detected eight targets in emission, of which five are new. The derived molecular gas masses range from 109 to 7 × 1011 M⊙ and the highest redshift detection (z = 3.387) corresponds to a relatively average-metallicity damped Lyman-α absorber for this redshift. We also report four new detections in absorption. Two of the associated CO absorption line detections at high-redshift (z = 1.211 and 1.275) result from high-spatial-resolution follow-up observations with NOEMA. The disparity between the mm molecular and H I 21 cm absorption lines for these and another intervening system detected in HNC at z = 1.275 is attributable to radio and mm sight lines tracing different media. We compare the atomic and molecular column densities of 14 known high-redshift (z > 0.1) molecular absorption line systems. The associated H I absorption lines are broad and exhibit multiple components, and the molecular absorption generally corresponds to the broader and weaker 21 cm absorption component. This indicates two distinct phases: one near galaxy centers with a larger CO-to-H I abundance ratio, and another with lower molecular abundance in the outer regions of the galaxy. In comparison, intervening absorption profiles correspond primarily to H I-dominated gas structure in galaxy outskirts, except for gas at low impact parameters in gravitationally lensed systems. The comparison of interferometric and single-dish observations presented here shows that the detection of absorption requires sufficient spatial resolution to overcome the dilution by emission and will be an important criterion for mm follow-up of 21 cm absorbers from ongoing large-scale surveys.

Vertical Structure of Gas and Dust in Four Debris Disks

We present high-spectral-resolution M-band spectra from iSHELL on NASA’s Infrared Telescope Facility along the line of sight to the debris disk host star HD 32297. We also present a Gemini Planet Imager H-band polarimetric image of the HD 131488 debris disk. We search for fundamental CO absorption lines in the iSHELL spectra of HD 32297, but do not detect any. We place an upper limit on the CO column density of ∼6 × 1015 cm−2. By combining the column density upper limit, the CO mass measured with the Atacama Large Millimeter/submillimeter Array (ALMA), and the geometrical properties of the disk, we estimate the scale height of the CO to be ≲2 au across the radial extent of the disk (∼80–120 au). We use the same method to estimate the CO scale height of three other edge-on, CO-rich debris disks that all have CO observed in absorption with the Hubble Space Telescope as well as in emission with ALMA: β Pictoris, HD 110058, and HD 131488. We compare our estimated CO scale heights of these four systems to the millimeter dust scale heights and find that, under the assumption of hydrostatic equilibrium, there is a potential correlation between the CO and millimeter dust scale heights. There are multiple factors that affect the gas vertical structure such as turbulence, photodissociation with weak vertical mixing, as well as where the gas originates. One possible explanation for the potential correlation could be that the gas and dust are of a similar secondary origin in these four systems.

The Evaluation of FY-3E Hyperspectral Infrared Atmospheric Sounder-II Long-Wave Temperature Sounding Channels

Prior to assimilating hyperspectral infrared data on the FengYun (FY) satellite in the numerical weather prediction (NWP) system, it is necessary to identify the quality and bias characteristics of these data, especially as China’s first early-morning-orbit satellite data. Using the numerical model CMA-GFS (China Meteorological Administration Global Forecast System) and the observation of FY-3E HIRAS-II (Hyperspectral Infrared Atmospheric Sounder-II), the differences between observed and simulated brightness temperatures (O-Bs) are comprehensively analyzed, with a focus on evaluating the long-wave (LW) temperature sounding channels of HIRAS-II observation in the clear sky. The results show that the O-Bs in the LW channels are between ±1.0 K, except for the CO2 absorption line peak at 667 cm−1. Only a tiny variation in O-Bs, with relative consistency, could be observed during the day, the line of dawn and dusk, and night. The difference in the standard deviations of O-Bs in the three cases is less than 0.1 K. The O-Bs of two typical channels (channels 14 and 47) in the stratosphere have disturbances at individual times, whereas the O-Bs are much more stable in time series in the tropospheric channels. The O-Bs in different channels show the characteristics of changing with the latitude, but the bias and standard deviations of O-Bs during the ascending and descending stages are not much different, except for the bias of channel 47 in low latitude. The optimal ranking of Fields of View (FOVs) in assimilation is derived from a priori analysis of O-Bs. The results demonstrate that FOV4 and FOV5 are the best in a Field of Regard (FOR) compared to all LW channels of HIRAS-II in constructions of their O-Bs and magnitude of O-B standard deviations, and they can be used as the preferred FOVs for assimilation. While the O-Bs in FOV1 and FOV2 are slightly larger, the O-Bs’ characteristics also meet the assimilation requirements and can be used as assimilation FOVs in HIRAS-II LW channels after FOV4 and FOV5 lose their efficacy.

RedEye-1: a compact SWIR hyperspectral imager for observation of atmospheric methane and carbon dioxide

We present the development of our first prototype of a compact short-wavelength infrared (SWIR) hyperspectral imager, RedEye-1, designed to observe and measure the concentrations of methane (CH4) and carbon dioxide (CO2) in the atmosphere. It operates in the spectral range of 1588 nm to 1673 nm with a nominal spectral resolution of 0.5 nm and has a total weight of approximately 1.8 kg. We outline the optical design of the instrument, including its fore-optics (telescope), and evaluate its performance using the OpticStudio ZEMAX software. In addition, we explain the spectral and radiometric calibration procedures and present the results. Our preliminary data reduction and analysis revealed a high quality SWIR image and a single path measurement of an atmospheric profile displaying the absorption lines of CH4 and CO2.

Confirmation of Subsolar Metallicity for WASP-77Ab from JWST Thermal Emission Spectroscopy

We present the dayside thermal emission spectrum of WASP-77Ab from 2.8 to 5.2 μm as observed with the NIRSpec instrument on the James Webb Space Telescope (JWST). WASP-77Ab was previously found to have a subsolar metallicity and a solar carbon-to-oxygen (C/O) ratio from H2O and CO absorption lines detected using high-resolution spectroscopy. By performing atmospheric retrievals on the JWST spectrum assuming chemical equilibrium, we find a subsolar metallicity [M/H] = and C/O ratio . We identify H2O and CO and constrain their abundances, and we find no CO2 in the spectrum. The JWST and high-resolution spectroscopy results agree within ∼1σ for the metallicity and within 1.8σ for the C/O ratio. However, our results fit less well in the picture painted by the shorter wavelength spectrum measured by Hubble Space Telescope Wide Field Camera 3. Comparing the JWST thermal emission spectra of WASP-77Ab and HD 149026b shows that both hot Jupiters have nearly identical brightness temperatures in the near-infrared but distinctly different atmospheric compositions. Our results reaffirm high-resolution spectroscopy as a powerful and reliable method to measure molecular abundances. Our results also highlight the incredible diversity of hot Jupiter atmospheric compositions.

The State of CO and CO2 Ices in the Kuiper Belt as Seen by JWST

JWST has shown that CO2 and CO are common on the surfaces of objects in the Kuiper Belt and have apparent surface coverages even higher than that of water ice, though water ice is expected to be significantly more abundant in the bulk composition. Using full Mie scattering theory, we show that the high abundance and the unusual spectral behavior around the 4.26 μm ν 1 band of CO2 can be explained by a surface covered in a few micron thick layer of ∼1–2 μm CO2 particles. CO is unstable at the temperatures in the Kuiper Belt, so the CO must be trapped in some more stable species. While hydrate clathrates or amorphous water ice are often invoked as a trapping mechanism for outer solar system ices, the expected spectral shift of the absorption line for a CO hydrate clathrates or trapping in amorphous ice is not seen, nor does the H2O abundance appear to be high enough to explain the depth of the CO absorption line. Instead, we suggest that the CO is created via irradiation of CO2 and trapped in the CO2 grains during this process. The presence of a thin surface layer of CO2 with embedded CO suggests volatile differentiation driving CO2 from the interior as a major process driving the surface appearance of these mid-sized Kuiper Belt objects, but the mechanisms that control the small grain size and depth of the surface layer remain unclear.

Baseline extraction algorithm for mixed absorption spectrum of multiple gases under different pressure

This study developed an algorithm for determining gas pressure and concentration in a mixed absorption feature using direct absorption spectroscopy. Blending spectra is observed when multiple absorption lines are approached when conducting a laser scan and/or when the pressure begins to increase. The proposed blended-feature baseline fitting method estimated the baseline by coupling measured data with simulated fractional transmission at the peaks between absorption features. In this study, the CH4 and CO absorption lines in the range 4299.70 to 4300.10 cm −1 were selected to demonstrate the method. Three experiments were conducted at different pressures of 0.5, 1, and 2 atm, and a mixture of CH4 and CO with different concentrations was measured at each pressure. Subsequently, the transmission spectra under the optimal parameters were compared with the simulated transmission spectra. Finally, the three results obtained via the proposed common baseline fitting method were compared with simulation results to verify the accuracy of the method. The method of data analysis based on direct absorption spectroscopy was confirmed to be applicable for spectra with a mixture of features, and was suitable for scanning ranges with multiple intensive spectral absorption lines and high pressures.

Portable TDLAS Sensor for Online Monitoring of CO2 and H2O Using a Miniaturized Multi-Pass Cell.

We designed a tunable diode laser absorption spectroscopy (TDLAS) sensor for the online monitoring of CO2 and H2O concentrations. It comprised a small self-design multi-pass cell, home-made laser drive circuits, and a data acquisition circuit. The optical and electrical parts and the gas circuit were integrated into a portable carrying case (height = 134 mm, length = 388 mm, and width = 290 mm). A TDLAS drive module (size: 90 mm × 45 mm) was designed to realize the function of laser current and temperature control with a temperature control accuracy of ±1.4 mK and a current control accuracy of ±0.5 μA, and signal acquisition and demodulation. The weight and power consumption of the TDLAS system were only 5 kg and 10 W, respectively. Distributed feedback lasers (2004 nm and 1392 nm) were employed to target CO2 and H2O absorption lines, respectively. According to Allan analysis, the detection limits of CO2 and H2O were 0.13 ppm and 3.7 ppm at an average time of 18 s and 35 s, respectively. The system response time was approximately 10 s. Sensor performance was verified by measuring atmospheric CO2 and H2O concentrations for 240 h. Experimental results were compared with those obtained using a commercial instrument LI-7500, which uses non-dispersive infrared technology. Measurements of the developed gas analyzer were in good agreement with those of the commercial instrument, and its accuracy was comparable. Therefore, the TDLAS sensor has strong application prospects in atmospheric CO2 and H2O concentration detection and ecological soil flux monitoring.

Optimized Wavelength Sampling for Thermal Radiative Transfer in Numerical Weather Prediction Models

In the thermal spectral range, there are millions of individual absorption lines of water vapor, CO2, and other trace gases. Radiative transfer calculations of wavelength-integrated quantities, such as irradiance and heating rate, are computationally expensive, requiring a high spectral resolution for accurate numerical weather prediction and climate modeling. This paper introduces a method that could highly reduce the cost of integration in the thermal spectrum by employing an optimized wavelength sampling method. Absorption optical thicknesses for various trace gases were calculated from the HITRAN 2012 spectroscopic dataset using the ARTS line-by-line model as input to a fast Schwarzschild radiative transfer model. Using a simulated annealing algorithm, different optimized sets of wavelengths and corresponding weights were identified, which allowed for accurate integrated quantities to be computed as a weighted sum, reducing the computational time by several orders of magnitude. For each set of wavelengths, a lookup table, including the corresponding weights and absorption cross-sections, is created and can be applied to any atmospheric setups for which it was trained. We applied the lookup table to calculate irradiances and heating rates for a large set of atmospheric profiles from the ECMWF 91-level short-range forecast. Ten wavelength nodes are sufficient to obtain irradiances within an average root mean square error (RMSE) of upward and downward radiation at any height below 1 Wm−2 while 100 wavelengths allowed for an RSME of below 0.05 Wm−2. The applicability of this method was confirmed for irradiances and heating rates in clear conditions and for an exemplary cloud at 3.2 km height. Representative spectral gridpoints for integrated quantities in the thermal spectrum (REPINT) is available as absorption parameterization in the libRadtran radiative transfer package, where it can be used as an efficient molecular absorption parameterization for a variety of radiative transfer solvers.

Smoldering peat fire detection by time-resolved measurements of transient CO2 and CH4 emissions using a novel dual-gas optical sensor

A compact and sensitive dual-gas laser absorption sensor was developed for smoldering peat fire detection by real-time monitoring of transient CO2 and CH4 emissions from peat combustion exhaust. The sensor combines two infrared lasers to exploit CH4 and CO2 absorption lines centered at 6057.09 cm−1 and 6359.96 cm−1, respectively. Scanned-wavelength modulation spectroscopy with the second-harmonic detection (WMS-2f) and a custom-designed Herriot multipass gas cell (10.3 m path length in a 40.5 mL volume) were employed to improve detection sensitivity. The simultaneous real-time transient emissions (CH4 and CO2) were measured at an interval of 0.1 s under various smoldering peat combustion conditions in a top open cylindrical reactor. An increasing trend of CH4/CO2 ratio from 0.053 to 0.075 and the greenhouse gas (GHG) flux from 1.5 g/m2 s to 3.4 g/m2 s was observed with increasing oxygen supplies from 10 mm/s to 24 mm/s. However, a decreasing trend of CH4/CO2 ratio from 0.06 to 0.043 and GHG flux from 2.6 g/m2 s to 0.4 g/m2 s was noticed with increasing peat moisture content from 8% (dry peat) to 100%. The measurement results agree well with commercial non-dispersive infrared CO2 and CH4 analyzers which are not fast enough to capture transient emissions due to their low time response. The developed dual-gas sensor has the potential to be applied for detecting underground peatland fires and evaluating the overall GHGs emissions from smoldering wildfires due to its excellent temporal resolution, hardware simplicity, and compactness.

A CO2-independent cloud mask from Infrared Atmospheric Sounding Interferometer (IASI) radiances for climate applications

Abstract. With more than 15 years of continuous and consistent measurements, the Infrared Atmospheric Sounding Interferometer (IASI) radiance dataset is becoming a reference climate data record. To be exploited to its full potential, it requires a cloud filter that is accurate, unbiased over the full IASI life span and strict enough to be used in satellite data retrieval schemes. Here, we present a new cloud detection algorithm which combines (1) a high sensitivity, (2) a good consistency over the whole IASI time series and between the different copies of the instrument flying on board the suite of Metop satellites, and (3) simplicity in its parametrization. The method is based on a supervised neural network (NN) and relies, as input parameters, on the IASI radiance measurements only. The robustness of the cloud mask over time is ensured in particular by avoiding the IASI channels that are influenced by CO2, N2O, CH4, CFC-11 and CFC-12 absorption lines and those corresponding to the ν2 H2O absorption band. As a reference dataset for the training, version 6.5 of the operational IASI Level 2 (L2) cloud product is used. We provide different illustrations of the NN cloud product, including comparisons with other existing products. We find very good agreement overall with version 6.5 of the operational IASI L2 with an identical mean annual cloud amount and a pixel-by-pixel correspondence of about 87 %. The comparison with the other cloud products shows a good correspondence in the main cloud regimes but with sometimes large differences in the mean cloud amount (up to 10 %) due to the specificities of each of the different products. We also show the good capability of the NN product to differentiate clouds from dust plumes.

Open-path anti-pollution multi-pass cell-based TDLAS sensor for the online measurement of atmospheric H2O and CO2 fluxes.

We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H2O and CO2 fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H2O and CO2 absorption lines, respectively. Allan analysis of variance indicated that detection limits of H2O and CO2 were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO2 and H2O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.

Shock tube study of the interaction between ammonia and nitric oxide at high temperatures using laser absorption spectroscopy

The interaction between ammonia (NH3) and nitric oxide (NO) at high temperatures is studied in this work using a shock tube combined with laser absorption diagnostics. The system simultaneously measured the NH3 and NO time-histories during the reaction processes of the shock-heated NH3/NO/CO/Ar mixtures (NH3:NO ≈ 0.9:1.0 and 1.4:1.0). The absorption cross-sections of NH3 near 1122.10 cm–1 and NO at 1900.52 cm–1 (characterized in this study) were used for measuring NH3 and NO time-histories with the temperature measured by two CO absorption lines. The measured NH3 and NO time-histories at 1614–1968 K and 2.4–2.8 atm were compared with predictions of seven recent kinetics models. The predictions that based on different mechanisms are very different and the measured profiles are within the range of the predictions. The Glarborg, NUI Galway Syngas-NOx, and Mathieu mechanisms give the closest predictions to the measurements. Kinetics analyses indicate that the NH3 and NO consumption rates are extremely sensitive to the rate constants and branching ratio of NH2 + NO = N2 + H2O and NH2 + NO = NNH + OH, which are more reliably represented in the Glarborg and NUI Galway Syngas-NOx mechanisms. The performances of Glarborg mechanisms at lower initial temperatures can be apparently improved by revising the rate constants and branching ratio of NH2 + NO = N2 + H2O and NH2 + NO = NNH + OH. These two reactions are also the primary pathways for NO reduction and NH3 is mainly consumed via NH3 + OH = NH2 + H2O and NH3 + H = NH2 + H2. Trace amounts of NO2 and N2O impurities decompose to form O radical followed by the generation of OH radical via H-abstraction reactions, which significantly affects the predictions of NH3 and NO according to kinetics analyses.

Simultaneous detection of atmospheric CO2 and H2O using a DFB diode laser based absorption spectrometer

In this paper, a high-resolution absorption spectrometer based on a continuous wave (CW) distributed feedback (DFB) diode laser near 2682.8 nm spectral region is developed, which is suitable for high sensitive detection of trace CO2 and H2O simultaneously. Spectral parameters (line strengths, air- and self-broadening coefficients) for CO2 absorption line at 3727.08279 cm−1 and H2O absorption line at 3727.73765 cm−1 are experimentally investigated, and a good agreement was obtained by comparing with the HITRAN database. H2O induced CO2 broadening coefficient is especially measured to upgrade the spectral signal processing algorithm model for field application. Finally, the developed diode laser absorption spectrometer was continuously deployed at AECORSQ Observatory onMt. Qomolangma, China, for approximate two days, which demonstrated the high sensitivity and stability of the sensor system for continuously atmospheric CO2 and H2O background observation.

Probing Dynamics and Thermal Properties Inside Molecular Tori with CO Rovibrational Absorption Lines

A recent hydrodynamic model, the radiation-driven fountain model (Wada et al. 2016), presented a dynamical picture that active galactic nuclei (AGNs) tori sustain their geometrical thickness by gas circulation around AGNs, and previous papers have confirmed that this picture is consistent with multiwavelength observations of nearby Seyfert galaxies. Recent near-infrared observations implied that CO rovibrational absorption lines (ΔJ = ± 1, v = 0 − 1, λ ∼ 4.7 μm) could probe the physical properties of the inside tori. However, the origin of the CO absorption lines has been under debate. In this paper, we investigate the origin of the absorption lines and conditions for detecting them by performing line radiative transfer calculations based on the radiation-driven fountain model. We find that CO rovibrational absorption lines are detected at inclination angles θ obs = 50°–80°. At the inclination angle θ obs = 77°, we observe multi-velocity components: inflow (v LOS = 30 km s−1), systemic (v LOS = 0 km s−1), and outflows (v LOS = −75, − 95, and −105 km s−1). The inflow and outflow components (v LOS = 30 and −95 km s−1) are collisionally excited at the excitation temperatures of 186 and 380 K up to J = 12 and 4, respectively. The inflow and outflow components originate from the accreting gas on the equatorial plane at 1.5 pc from the AGN center and the outflowing gas driven by AGN radiation pressure at 1.0 pc, respectively. These results suggest that CO rovibrational absorption lines can provide us with the velocities and kinetic temperatures of the inflow and outflow in the inner few parsec region of AGN tori, and the observations can probe the gas circulation inside the tori.

Frequency-Domain Detection for Frequency-Division Multiplexing QEPAS

To achieve multi-gas measurements of quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors under a frequency-division multiplexing mode with a narrow modulation frequency interval, we report a frequency-domain detection method. A CH4 absorption line at 1653.72 nm and a CO2 absorption line at 2004.02 nm were investigated in this experiment. A modulation frequency interval of as narrow as 0.6 Hz for CH4 and CO2 detection was achieved. Frequency-domain 2f signals were obtained with a resolution of 0.125 Hz using a real-time frequency analyzer. With the multiple linear regressions of the frequency-domain 2f signals of various gas mixtures, small deviations within 2.5% and good linear relationships for gas detection were observed under the frequency-division multiplexing mode. Detection limits of 0.6 ppm for CH4 and 2.9 ppm for CO2 were simultaneously obtained. With the 0.6-Hz interval, the amplitudes of QEPAS signals will increase substantially since the modulation frequencies are closer to the resonant frequency of a QTF. Furthermore, the frequency-domain detection method with a narrow interval can realize precise gas measurements of more species with more lasers operating under the frequency-division multiplexing mode. Additionally, this method, with a narrow interval of modulation frequencies, can also realize frequency-division multiplexing detection for QEPAS sensors under low pressure despite the ultra-narrow bandwidth of the QTF.

Mid‐infrared multiline absorption tomography for in situ analysis of thermochemical structure in natural gas‐fired cooker flame

AbstractA mid‐infrared laser absorption tomography technique was developed to quantify the nonuniform thermochemical structures of flames stabilized on a natural gas‐fired cooker. Multispectral absorption spectroscopy coupled with Tikhonov‐regularized three‐point Abel inversion was used to resolve the spatially resolved temperature and species concentration. A tunable interband cascade laser (ICL) near 4.2 μm was used to access multiple CO2 absorption lines in the fundamental vibration band. Multiline Voigt fitting strategy was used to improve the fitting of the overlap absorption features. Comprehensive measurements at a different height above the cooker were performed and assembled to display the contour of temperature and CO2 concentrations, resolving the nonuniform thermochemical structure of the high‐temperature combustion field. The measurement uncertainty analysis for temperature and CO2 concentration was also discussed. The current technique is demonstrated to be capable of quantitatively distinguishing flame conditions under different fuel supply modes.