Mid-infrared Laser Absorption Spectroscopy Research Articles

Real-time monitoring of CH4 and N2O emissions from livestock using mid-infrared external cavity quantum cascade laser absorption spectroscopy

The largest share of greenhouse gas (GHG) emissions related to livestock originates from methane (CH4) and nitrous oxide (N2O) which have a far higher influence on global warming, it is therefore necessary to accurately monitor CH4 and N2O emissions to provide theoretical and practical basis for further estimating and regulating GHG emissions from livestock and improving livestock production performance. For the purpose of sensing CH4 and N2O emissions during livestock living process in real time, an optical sensor based on continuous-wave (CW) external cavity quantum cascade laser (EC-QCL) operating at room temperature was developed. CH4 and N2O absorption lines, located around 8 μm, of the ν4 and ν1 fundamental vibrational bands, respectively, were chosen for direct absorption spectroscopy, which allows for sensitive, selective and simultaneous measurement of CH4 and N2O concentrations. Use of a Herriot multi-pass cell with an effective path-length of 100 m, 1σ (SNR = 1) limits of detection of 26.8 ppbv, 20.3 ppbv and 0.01 % for CH4, N2O and H2O vapor were achieved, respectively. Field measurement of CH4 and N2O emissions from horses has been carried out in a stable over two weeks at the Vernaelde farm in Couderkerque Branche city, France. Concentrations of CH4 and N2O up to 10 times and 1.5 times higher than their levels in the local ambient air (∼ 2.12 ppmv and ∼ 427 ppbv) were observed, respectively.

Simultaneous measurement of NH3 and NO by mid-infrared tunable diode laser absorption spectroscopy based on machine-learning algorithms

Accurate measurements of NH3 and NO are essential for controlling NOx emissions from coal-fired power plants, reducing ammonia slip, and improving the efficiency of selective catalytic reduction (SCR) operation. We propose a method for measuring NH3 and NO concentrations based on machine learning algorithms to address the challenges posed by overlapping interference of CO2 and H2O spectral lines in flue gas. Our approach introduces a novel method for acquiring mixture spectra for the model. Initially, we measure the spectra of individual components under different concentrations and temperatures. Subsequently, mixed spectral samples are generated by combining the measured spectra of the individual components. This approach simplifies the spectral measurement process while preserving accuracy. The particle swarm optimization support vector machine (PSO-SVM) algorithm is leveraged, providing a reliable foundation for the continuous and synchronous measurement of NH3 and NO. Upon testing, the PSO-SVM demonstrates average relative errors of 4.0 % and 0.8 % for NH3 and NO concentrations, respectively. The corresponding measurement precision is 0.05 ppm for NH3 and 0.42 ppm for NO, better than the conventional integral absorbance method. The minimum detection limit (MDL) for NH3 is 16.1 ppb at an average time of 50 s, while for NO, it is 38.4 ppb at an average time of 71 s. The methodology of this paper is expected to play an important role in reducing the influence of interfering components and improving the accuracy of field measurements.

Determination of Air–Fuel Ratio at 1 kHz via Mid-Infrared Laser Absorption and Fast Flame Ionization Detector Measurements in Engine-Out Vehicle Exhaust

<div>Measurements of air–fuel ratio (AFR) and <i>λ</i> (AFR<sub>actual</sub>/AFR<sub>stoich</sub>) are crucial for understanding internal combustion engine (ICE) performance. However, current <i>λ</i> sensors suffer from long light-off times (on the order of seconds following a cold start) and limited time resolution. In this study, a four-color mid-infrared laser absorption spectroscopy (LAS) sensor was developed to provide 5 kHz measurements of temperature, CO, CO<sub>2</sub>, and NO in engine-out exhaust. This LAS sensor was then combined with 1 kHz hydrocarbon (HC) measurements from a flame ionization detector (FID), and the Spindt exhaust gas analysis method to provide 1 kHz measurements of <i>λ</i>. To the authors’ knowledge, this is the first time-resolved measurement of <i>λ</i> during engine cold starts using the full Spindt method. Three tests with various engine AFR calibrations were conducted and analyzed: (1) 10% lean, (2) stoichiometric, and (3) 10% rich. The measurements were acquired in the exhaust of a light-duty truck with an 8-cylinder gasoline engine. The LAS-FID-based <i>λ</i> sensor results were compared with those obtained from a universal exhaust gas oxygen (UEGO) sensor. The LAS-FID method provided robust <i>λ</i> measurements from the first combustion exhaust event (avoiding the light-off time associated with traditional <i>λ</i> sensors) in addition to enhanced temporal resolution (on the order of 100× increase compared to traditional diffusion-based <i>λ</i> sensors). The insight gained from this novel method could be used to benefit crank, cold start, and open- or closed-loop air–fuel ratio control strategies in gasoline engines for reduced emissions.</div>

Mid-infrared absorption spectroscopy measurements and model improvements during the oxidation of ammonia/ethanol and ammonia/diethyl ether blends in a shock tube

Blending ammonia with highly active fuels (hydrogen, hydrocarbons, oxygenated hydrocarbons, etc.) has been identified as an efficient way to improve the mixture's reactivity. In this study, the temperature, CO, CO2, H2O, NH3, and NO time-histories were measured simultaneously during the oxidation of ammonia/ethanol and ammonia/diethyl ether (DEE) blends (equivalence ratio ϕ ∼ 1.2) using mid-infrared laser absorption spectroscopy behind reflected shock waves. Various models (ten for ammonia/ethanol and three for ammonia/DEE) were selected to perform the simulation and compared with the measurements. The predictions are very different for both mixtures, among which the CRECK model was selected to perform discussion and kinetics analyses for the ammonia/ethanol mixture and the Shrestha 2022 model was selected for the ammonia/DEE mixture. The CRECK model performs well in predicting the temperature rise/plateau, H2O formation rate/plateau, CO consumption rate/plateau, and CO2 formation rate. However, it underpredicts the CO formation rate and slightly overpredicts the CO2 plateau and NH3 consumption rate; for the NO time-history, the CRECK model well captures the rapid NO formation but underpredicts the near-equilibrium NO concentration and gives an unnormal initial NO production. The Shrestha 2022 model accurately predicts the plateaus of temperature and H2O, but significantly underpredicts the reactivity of the ammonia/DEE mixture and overpredicts the near-equilibrium NO concentration. According to the kinetics analyses, key reactions affecting the species' time-histories were selected. Fourteen reactions in the CRECK model and eight reactions in the Shrestha 2022 model were updated by the reported data or estimations to improve the predictions. However, discrepancies still exist, which are mainly located in the CO formation and NH3 consumption rates. Further effort should mainly focus on these two aspects, as well as the interactions between oxygenated hydrocarbons and ammonia at high temperatures.

Open-path sensor for atmospheric NO detection based on mid-infrared laser absorption spectroscopy

Nitric oxide (NO) is one of the primary air pollutants, and open optical path detection of NO holds immense significance in safeguarding the ecological environment and human health. In this paper, a high precision and high sensitivity NO open optical path sensor based on mid-infrared laser absorption spectroscopy technology is developed. The average gas concentration inversion of the NO path is achieved by collecting the backscattered light from the cooperative target. The gas measurement deploys a wavelength modulation method to remove the effects from light intensity variations. Furthermore, a Kalman filter and a temperature–pressure sensor optimization software algorithm were utilized to reduce the effects of temperature–pressure variations during the measurement process, leading to a 33% improvement in detection accuracy. The performance of the sensor was evaluated in the laboratory environment in terms of accuracy, sensitivity, and detection limit. The atmospheric NO concentration was also measured under varying time periods and weather conditions in the field environment, thus laying a strong foundation for studying the spatial and temporal evolution of NO in the atmosphere.

Nitromethane as a nitric oxide precursor for studying high-temperature interactions between ammonia and nitric oxide in a shock tube

Ammonia (NH3) is a promising carbon-free fuel for carbon neutrality that has recently received increasing attention. However, NOx emissions are one of the main problems for NH3 or NH3-blended combustion. This study used nitromethane (CH3NO2) as a nitric oxide precursor to study NH3/NO interactions at high temperatures. The temperature, CO, CO2, NO, and NH3 time-histories were measured simultaneously during the pyrolysis and oxidation (equivalence ratios ϕ = 2.0 and 1.0) of CH3NO2 and CH3NO2/NH3 mixtures behind reflected shock waves using mid-infrared laser absorption spectroscopy to analyze the NH3/NO interactions. The measurements were used to validate and evaluate seven kinetic models which gave very different predictions for the different mixtures, among which the Glarborg et al. (2018) and Shang et al. (2020) models agreed best with the temperature, NO, and NH3 measurements. The measured CO and CO2 time-histories are shown in the Supplementary Material. Both mechanisms accurately capture the temperature plateau while the main difference is the temperature increase during oxidation. The NO formation rate and plateau concentrations during CH3NO2 pyrolysis are better predicted by the Shang model than by the Glarborg model. Both mechanisms capture the NO formation rate for CH3NO2 oxidation with ϕ = 2.0 but underestimate it for ϕ = 1.0. With NH3, the Glarborg model better predicts the NO removal during CH3NO2/NH3 pyrolysis. However, both mechanisms underestimate the NO formation rate and maximum but overestimate the NO removal rate and NH3 consumption for CH3NO2/NH3 oxidation. Rate-of-production and sensitivity analyses show that: for the NO formation, CH3 + NO2 = CH3O + NO and NO2 + H = NO + OH are the primary pathways and CH3NO2 (+ M) = CH3 + NO2 (+ M) is the most sensitive reaction. NH3 + OH = NH2 + H2O contributes the most to the NH3 consumption during CH3NO2/NH3 pyrolysis, while NH3 + H = NH2 + H2 and NH3 + O = NH2 + OH are increasingly important at oxidation condition. NH2 + NO = N2 + H2O and NH2 + NO = NNH + OH are the most important reactions for the NO reduction with NH3 existing. The rate constants of some key reactions were revised to improve the predictions.

In situ probing of atmospheric-pressure warm air glow discharge for nitrogen fixation by multiple laser spectroscopies

The fixation of atmospheric nitrogen into valuable compounds through reactive plasma processes has attracted intense interests due to its easy operation and compatibility with distributed renewable energy sources. However, practical implementation of plasma-assisted nitrogen fixation is hampered because of its relatively low throughput, which is dominantly limited by the unclear underlying mechanisms. In this study, effort was focused on the in situ production of key species in a DC-driven warm air glow discharge at atmospheric pressure with the help of advanced laser spectroscopic diagnostics. Laser Rayleigh scattering was applied to determine the gas temperature distribution in the discharge column. And mid-infrared quantum cascade laser absorption spectroscopy and one/two-photon absorption laser-induced fluorescence were performed on molecular nitric oxide (NO), atomic oxygen and nitrogen (O, N) for their absolute densities in the discharge. It is found that the spatial distributions of gas temperature, O and N atoms show peaks in the hot discharge center. In contrast, a hollow ‘doughnut’ shape characterized by the NO molecule was observed, particularly under conditions of high discharge current but low airflow rate. The steady-state simulation shows that the hollow pattern of NO is dominantly induced by the radial diffusion of species due to the steep spatial gradient of gas temperature in the discharge cross-section. Moreover, the reverse conversion by atomic N leads to a negative effect on the NO synthesis, especially at the discharge center where the N density and gas temperature are high. From the steady-state modeling, a similar hollow distribution of NO2 was depicted in the air glow discharge. These results demonstrate the strong dependence on atomic O for the major formation process of NO, and the importance of suppressing the reverse paths dominated by atomic N for higher NO production in the studied warm air plasma.

High-speed mid-infrared laser absorption spectroscopy of CO_2 for shock-induced thermal non-equilibrium studies of planetary entry

A high-speed laser absorption technique is employed to resolve spectral transitions of CO_2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO_2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO_2 asymmetric stretch fundamental bands (Delta v_3 = 1) near 4.19 upmu hbox {m}. The sensor probes a wide range of rotational energies as well as two vibrational states (00^00 and 01^10). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO_2 non-equilibrium with a single vibrational temperature.

V-cavity stabilized quantum cascade laser-based cavity ringdown spectroscopy for rapid detection of radiocarbon below natural abundance

Mid-infrared laser absorption spectroscopy utilizing a high-finesse optical cavity enables high precision trace analysis of gas molecules. In particular, optical detection of radiocarbon (14C) based on cavity ringdown spectroscopy using a quantum cascade laser (QCL) is gaining attention as an alternative to accelerator mass spectrometry. This paper reports a compact-packaged narrow-linewidth QCL system utilizing resonant optical feedback from an external V-shaped cavity. Based on frequency noise analysis, the derived laser linewidth is 44 kHz for 100 μs integration time with the capability to perform seamless frequency scanning around 10 GHz. We installed this laser system within a table-top cavity ringdown spectrometer for 14CO2. A single-shot detection limit of 1.2 × 10−9 cm−1 Hz−1/2 leading to a detectable abundance evaluated from a noise analysis of 0.2 in fraction modern 14C for a 10-s averaging time was achieved. This capability of rapid analysis for 14CO2 is suitable for various applications requiring trace 14C analysis.

Intercomparison of upper tropospheric and lower stratospheric water vapor measurements over the Asian Summer Monsoon during the StratoClim campaign

Abstract. In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration–dehydration of the tropical tropopause layer. Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (<5 ppmv) and a challenging low temperature–pressure environment. The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer). Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence. In clear-sky UTLS conditions (H2O<10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %. Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %. Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively. This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.

Shock Tube and Flame Speed Measurements of 2,4,4-Trimethyl-1-Pentene: A Co-Optima Biofuel

Abstract The combustion of 2,4,4-trimethyl-1-pentene (diisobutylene, C8H16), which is a biofuel and a component of surrogate fuels, is examined in this work. Carbon monoxide time–histories and ignition delay times are collected behind reflected shock waves utilizing a shock tube and mid-infrared laser absorption spectroscopy. Measurements were obtained near 10 atm pressure during stoichiometric oxidation of 0.15%C8H16/O2/Ar. Simulated results from chemical kinetic models are provided, and sensitivity analyses are used to discuss differences between models for both ignition delay times and carbon monoxide formation. In addition, laminar burning speeds are obtained at 1 atm, 428 K, and equivalence ratios, phi, between 0.91 and 1.52 inside a spherical chamber facility. Measured burning speeds are found to be less than that of ethanol over the equivalence ratio span. Burning speed measurements are compared to predictions of chemical kinetic mechanisms and are in agreement for the richest conditions; however, at lean conditions, the model predicts a far slower-burning speed. The maximum burning speed occurs at an equivalence ratio of 1.08 with a magnitude of 0.70 m/s. The current work provides the crucial experimental data needed for assessing the feasibility of this biofuel and for the development of future combustion chemical kinetics models.

Identification, monitoring, and reaction kinetics of reactive trace species using time-resolved mid-infrared quantum cascade laser absorption spectroscopy: development, characterisation, and initial results for the CH<sub>2</sub>OO Criegee intermediate

Abstract. The chemistry and reaction kinetics of reactive species dominate changes to the composition of complex chemical systems, including Earth's atmosphere. Laboratory experiments to identify reactive species and their reaction products, and to monitor their reaction kinetics and product yields, are key to our understanding of complex systems. In this work we describe the development and characterisation of an experiment using laser flash photolysis coupled with time-resolved mid-infrared (mid-IR) quantum cascade laser (QCL) absorption spectroscopy, with initial results reported for measurements of the infrared spectrum, kinetics, and product yields for the reaction of the CH2OO Criegee intermediate with SO2. The instrument presented has high spectral (< 0.004 cm−1) and temporal (< 5 µs) resolution and is able to monitor kinetics with a dynamic range to at least 20 000 s−1. Results obtained at 298 K and pressures between 20 and 100 Torr gave a rate coefficient for the reaction of CH2OO with SO2 of (3.83 ± 0.63) × 10−11 cm3 s−1, which compares well to the current IUPAC recommendation of 3.70-0.40+0.45 × 10−11 cm3 s−1. A limit of detection of 4.0 × 10−5, in absorbance terms, can be achieved, which equates to a limit of detection of ∼ 2 × 1011 cm−3 for CH2OO, monitored at 1285.7 cm−1, based on the detection path length of (218 ± 20) cm. Initial results, directly monitoring SO3 at 1388.7 cm−1, demonstrate that SO3 is the reaction product for CH2OO + SO2. The use of mid-IR QCL absorption spectroscopy offers significant advantages over alternative techniques commonly used to determine reaction kinetics, such as laser-induced fluorescence (LIF) or ultraviolet absorption spectroscopy, owing to the greater number of species to which IR measurements can be applied. There are also significant advantages over alternative IR techniques, such as step-scan FT-IR, owing to the coherence and increased intensity and spectral resolution of the QCL source and in terms of cost. The instrument described in this work has potential applications in atmospheric chemistry, astrochemistry, combustion chemistry, and in the monitoring of trace species in industrial processes and medical diagnostics.

Measurement of CO, HCN, and NO productions in atmospheric reaction induced by femtosecond laser filament

It is proved that the chemical reaction induced by femtosecond laser filament in the atmosphere produces CO, HCN, and NO, and the production CO and HCN are observed for the first time. The concentrations of the products are measured by mid-infrared tunable laser absorption spectroscopy. In the reduced pressure air, the decomposition of CO2 is enhanced by vibration excitation induced by laser filament, resulting in the enhanced production of CO and HCN. At the same time, the CO and HCN generated from the atmosphere suffer rotation excitation induced by laser filament, enhancing their absorption spectra. It is found that NO, CO, and HCN accumulate to 134 ppm, 80 ppm, and 1.6 ppm in sealed air after sufficient reaction time. The atmospheric chemical reaction induced by laser filament opens the way to changing the air composition while maintaining environmental benefits.

In situ quantification of NO synthesis in a warm air glow discharge by WMS-based Mid-IR QCL absorption spectroscopy

Nitric oxide (NO) is one of the most crucial products in the plasma-based nitrogen fixation process. In this work, in situ measurements were performed for quantifying the NO synthesis spatially in a warm air glow discharge, through the method of Mid-infrared quantum cascade laser absorption spectroscopy (QCL-AS). Two ro-vibrational transitions at 1900.076 cm−1 and 1900.517 cm−1 of the ground-state NO(X) were probed sensitively by the help of the wavelength modulation spectroscopy (WMS) approach to increase the signal/noise (S/N) level. The results show a decline trend of NO synthesis rate along the discharge channel from the cathode to the anode. However, from the point of energy efficiency, the cathode region is of significantly low energy efficiency of NO production. Severe disproportionality was found for the high energy consumption but low NO production in the region of cathode area, compared to that in the positive column zone. Further analysis demonstrates the high energy cost of NO production in the cathode region, is ascribed to the extremely high reduced electric field therein not selectively preferable for the processes of vibrational excitation or dissociation of N2 and O2 molecules. This drags down the overall energy efficiency of NO synthesis by this typical warm air glow discharge, particularly for the ones with short electrode gaps. Limitations of further improving the energy cost of NO synthesis by variations of the discharge operation conditions, such as discharge current or airflow rate, imply other effective manners able to tune the energy delivery selectively to the NO formation process, are sorely needed.

Characterizing combustion of a hybrid rocket using laser absorption spectroscopy

The combustion of an oxygen/paraffin hybrid rocket motor was experimentally characterized. Firing tests were conducted for different oxidizer mass fluxes ranging from 2.47 to 3.40 g/ (cm2·s). Variations in temperature and H2O partial pressure at the nozzle exit were diagnosed using mid-infrared tunable diode laser absorption spectroscopy (TDLAS) based on H2O absorption near 2.5 μm. Three H2O absorption lines were simultaneously covered by only one distributed feedback (DFB) laser using scanned-wavelength direct absorption (DA) mode with 2.0 kHz repetition rate. Measurement uncertainty was analyzed in detail considering line-strength uncertainty and Voigt fitting residuals. A two-dimensional (2D) model of the nozzle was constructed using the ANSYS FLUENT CFD software package. The combustion efficiency of the hybrid rocket motor was evaluated from the perspectives of chemical reaction and heat release, respectively, based on TDLAS results and CFD simulations. The effectiveness of the evaluation was validated by comparing its results with characteristic velocity (C*)-based combustion efficiency. Finally, comparisons of combustion efficiencies among different cases show that increasing the oxidizer mass flux or oxidizer-to-fuel ratio improves the combustion efficiency of the hybrid rocket motor under our experimental conditions.

Impact of breath sampling on exhaled carbon monoxide

The influence of breath sampling on exhaled carbon monoxide (eCO) and related pulmonary gas exchange parameters is investigated in a study with 32 healthy non-smokers. Mid-infrared tunable diode laser absorption spectroscopy and well-controlled online sampling is used to precisely measure mouth- and nose-exhaled CO expirograms at exhalation flow rates (EFRs) of 250, 120 and 60 ml s−1, and for 10 s of breath-holding followed by exhalation at 120 ml s−1. A trumpet model with axial diffusion is employed to fit simulated exhalation profiles to the experimental expirograms, which provides equilibrium airway and alveolar CO concentrations and the average lung diffusing capacity in addition to end-tidal concentrations. For all breathing maneuvers, excellent agreement is found between mouth- and nose-exhaled end-tidal CO (ETCO), and the individual values for ETCO and alveolar diffusing capacity are consistent across maneuvers. The eCO parameters clearly show a dependence on EFR, where the lung diffusing capacity increases with EFR, while ETCO slightly decreases. End-tidal CO is largely independent of ambient air CO and alveolar diffusing capacity. While airway CO is slightly higher than, and correlates strongly with, ambient air CO, and there is a weak correlation with ETCO, the results point to negligible endogenous airway CO production in healthy subjects. An EFR of around 120 ml s−1 can be recommended for clinical eCO measurements. The employed method provides means to measure variations in endogenous CO, which can improve the interpretation of exhaled CO concentrations and the diagnostic value of eCO tests in clinical studies. Clinical trial registration number: 2017/306-31

Shock tube measurement of NO time-histories in nitromethane pyrolysis using a quantum cascade laser at 5.26 µm

The pyrolysis of nitromethane, CH3NO2, was studied at ∼3.5 atm and 1013 K–1418 K in a heated shock tube by measuring the key product nitric oxide (NO) using mid-infrared laser absorption spectroscopy. We used a quantum cascade laser (QCL) at 5.26 µm to exploit the strong NO absorption at 1900.08 cm−1. With the NO absorption cross-section data characterized at 1006 K–1789 K and 2.7 atm–3.5 atm behind reflected shock waves, we measured the NO concentration time-histories during the pyrolysis of nitromethane at two different concentrations (1.05% and 0.6%). The absorption interference from other major products such as CO and H2O was analyzed to be negligible, leading to an interference-free NO diagnostic in nitromethane pyrolysis. A recent kinetic model of Shang et al. (2019) was adopted to interpret the shock tube data. All the NO time-histories measured over the entire temperature range 1013 K–1418 K were well-predicted by this model in terms of the initial NO formation rate and the final plateau level. The rate-of-production, sensitivity, and reaction flux analyses were performed to identify four important reactions (CH3NO2 = CH3 + NO2, CH3NO2 ↔ CH3ONO = CH3O + NO, CH3 + NO2 = CH3O + NO, and NO2 + H = NO + OH) that determine the NO formation during CH3NO2 pyrolysis. The satisfactory agreement between the simulation and shock tube/laser absorption measurement further validated the kinetic mechanism of nitromethane decomposition. The developed mid-infrared NO absorption sensor provides a promising diagnostic tool for studying fuel-nitrogen chemical kinetics in the shock tube experiments.

Structural characterization of the NO(X2Π)-N2O complex with mid-infrared laser absorption spectroscopy and quantum chemical calculations.

Both positive and negative ions of N3O2 have been observed in various experiments. The neutral N3O2 was predicted to exist either as a weakly bound NO·N2O complex or a covalent molecule. The rovibrational spectrum of the NO(X2Π)-N2O complex has been measured for the first time in the 5.3 µm region using distributed quantum cascade lasers to probe the direct absorption in a slit-jet supersonic expansion. The observed spectrum is analyzed with a semi-rigid asymmetric rotor Hamiltonian for a planar open-shell complex, giving a bent geometry with an a-axis-NO angle of about 21.9°. The vibrationally averaged 2A'-2A″ energy separation is determined to be ε = 144.56(95) cm-1 for the ground state, indicating that the electronic orbital angular momentum is partially quenched upon complexation. Geometry optimizations of the complex restricted to a planar configuration at the RCCSD(T)/aug-cc-pVTZ level of theory show that the 2A″ state is more stable than the 2A' state by about 110 cm-1 and the N atom of NO points to the central N atom of N2O at the minimum of the 2A″ state.

Structural optimization and algorithm design for temperature control system of mid-infrared diode laser

Mid-infrared tunable diode laser absorption spectroscopy (TDLAS) has been extensively applied in high-sensitivity gas detection. Diode laser temperature control systems exert considerable influence on the comprehensive performance of gas detection systems. The overall design of temperature control systems for mid-infrared diode lasers is established in this research on the basis of the system’s structural optimization, thermoelectric cooler (TEC) selection, transfer function identification and control algorithm optimization, and the first-stage fuzzy PID combined with a second-stage PID control algorithm is applied to the actual system finally. The system can be quickly stabilized at any setting temperature between 10 °C–50 °C with a precision of ±10 m °C, and the energy consumption efficiency is high. The results lay the foundation for the optimization of the TDLAS systems’ overall performance.

Time-resolved characterization of non-thermal plasma-assisted photocatalytic removal of nitric oxide

We performed time-resolved characterization of plasma-photocatalytic removal of nitric oxide (NO) in a ceramic foam packing double dielectric barrier discharge reactor using mid-infrared laser absorption spectroscopy. Non-thermal plasma (NTP) was generated in the porous ceramic foam coated with mesoporous TiO2 thin film. Real-time NO monitoring was performed using a quantum cascade laser at 5.26 µm combined with a custom-designed multipass gas cell, leading to a sub-ppm detection limit at 0.1 s measurement time. By analyzing the feed gas of 90 ppm NO/1% O2 balanced in N2, the plasma treatment without TiO2 photocatalyst had no effect on NO removal when the discharge power was below 45 W. In comparison, the NTP-TiO2 system achieved an efficiency of 49% for NO removal even at the relatively low discharge power of 18 W. The synergy between NTP and TiO2 photocatalyst effectively promotes the NO removal at low plasma discharge powers. However, we also observed a negative effect of TiO2 photocatalyst on NO removal at a relatively high discharge power of 135 W.