Cavity Ring-Down and Multi-Pass Spectroscopies for Methane Source Attribution and Chemical Kinetics Studies

Methane is the most abundant hydrocarbon in the Earth atmosphere, is also an important greenhouse gas, energy source, and microbial metabolic energy source and product. With the rapid increase of atmospheric methane concentration, it has become very important to quantify methane emissions from different sources. This thesis describes the applications of cavity ring-down applications on atmospheric ethane measurements and measurements of doubly substituted methane for methane source attributions. We also present our work on chemical kinetics studies of an alkene ozonolysis intermediate, Criegee intermediate, using a multi-pass absorption technique. In Chapter 2, we demonstrated the performance of a continuous-wave (cw) interband cascade laser (ICL) based mid-infrared cavity ring-down spectroscopy (CRDS) sensor for atmospheric ethane (C2H6) detection. A 3.36 µm cw ICL with an was used to target two ethane absorption bands at 2976.788 cm-1 and 2983.383 cm-1. This technique utilizes the long effective pathlength (~ 4.5 km) of CRDS to increase sensitivity of atmospheric ethane detection. Our spectrometer can measure atmospheric ethane concentration as low as 200 pptv at standard temperature and pressure. We have used this instrument to measure the atmospheric ethane composition in ambient air collected in Pasadena, California. We have utilized this instrument to aid in the study of soil microbial response post the Porter Ranch gas leak. Results were shown in Chapter 3. In Chapter 4, we demonstrate high sensitivity measurements of both 13CH3D and 12CH2D2 isotopologues using a high precision and high resolution spectroscopy technique, frequency stabilized cavity ring-down spectroscopy (FS-CRDS). Measurements of the abundances of doubly-substituted methane isotopologues (13CH3D and 12CH2D2) are important in methane source attributions. Currently, methods developed for 13CH3D and 12CH2D2 measurements have been mostly focused on the use of isotope ratio mass spectrometry (IRMS), which faces the challenges of mass resolutions. In this work, we focus on measuring these low abundant methane isotopologues optically, taking advantage of the distinct absorption features of them. This technique can be used as a potential complement to IRMS measurements for its ability to measure abundances of rare methane isotopologues with a short time average (~1 hour average per isotope ratio measurement). In Chapter 5, we utilized our IR kinetic spectroscopy (IRKS) apparatus to study the formation of HCO radicals from the smallest Criegee Intermediate (CH2OO), which is an important intermediate from oznolysis of the smallest alkene.</p

Measurements doubly-substituted methane isotopologue (13CH3D and 12CH2D2) abudance using frequency stabilized mid-ir cavity ringdown spectroscopy

In this work, we demonstrated a spectroscopic method of measuring abundances of doubly-substituted methane isotopologues ($^{13}$CH$_{3}$D, $^{12}$CH$_2$D${2}$). In this method, we use a frequency stabilized cavity ringdown spectroscopy (FS-CRDS) technique to measure $\Delta$$^{12}$CH$_2$D${2}$ in naturally abundant methane to sub 0.1$\%$ level within one hour of average. Compare to traditional isotope-ratio mass spectrometer, which requires more than 24 hours of average to achieve comparable precision, this method provides a fast way of measuring clumped isotopologue abundance optically without destroying samples

Mid-ir cavity ringdown spectroscopy measurements of atmospheric ethane to methane ratio

In this work, we demonstrated a mid-IR (3.3 $\mu$m) cw cavity ringdown spectrometer capable of measuring atmospheric ethane abundance and ethane to methane ratio. This technique can measure atmospheric ethane concentration as low as 70 ppb. Since ethane is a tracer for thermogenic methane emissions, this technique could be used to identify sources of atmospheric methane. We have demonstrated the capability of this instrument by measuring the atmospheric ethane composition and ethane to methane ratio in ambient air in Pasadena, California

MID-IR CAVITY RINGDOWN SPECTROSCOPY FOR ATMOSPHERIC ETHANE ABUNDANCE MEASUREMENTS

We demonstrate a mid-IR (3.3 $mu$m) cw cavity ringdown spectrometer capable of measuring atmospheric ethane abundances. This technique can measure atmospheric ethane concentration as low as 100 ppb. The atmospheric ethane to methane ratio could also be observed by measuring methane concentration using a high precision near-IR (1.65 $mu$m) cavity ringdown spectrometer. We will also discuss the daily variation of ethane abundance and ethane to methane ratio in Pasadena observed using this tecnhique

Vacuum ultraviolet photoionization cross section of the hydroxyl radical

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(^1D) + H_2O in a flow reactor in He at 8 Torr. The initial O(^1D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(^3P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(^3P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(^3P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra

VUV Photoionization Cross Sections of HO_2, H_2O_2, and H_2CO

The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO_2), hydrogen peroxide (H_2O_2), and formaldehyde (H_2CO) have been measured from their first ionization thresholds to 12.008 eV. HO_2, H_2O_2, and H_2CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl_2 in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO_2, H_2O_2, and H_2CO on an absolute scale, resulting in absolute photoionization spectra

Vacuum ultraviolet photoionization cross section of the hydroxyl radical

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(^1D) + H_2O in a flow reactor in He at 8 Torr. The initial O(^1D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(^3P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(^3P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(^3P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra

Study of flow behaviour in a three products hydrocyclone screen: numerical simulation and experimental validation

A novel three products hydrocyclone screen (TPHS) has been successfully developed; it consists of a cylindrical screen embedded in a conventional hydrocyclone (CH). In the new liquid cyclone, the combination of centrifugal classification and screening was employed for particle separation based on size. The aim of this study is to investigate the flow behaviour in TPHS using numerical simulation and experimental validation. A computational fluid dynamics simulation with a 4.35 million grid scheme and linear pressure–strain RSM generated the economic and grid-independence solution, which agreed well with the experiments of particle image velocimetry and water split. The velocity vector profile reveals that TPHS represented similar flow patterns to CH, wherein in addition to the outer downward swirl flow, inner upward swirl flow, central down-flow, second circulatory flow, and mantle, a particular fluid flow named screen underflow was created in TPHS owing to the presence of a cylindrical screen. The velocity distribution demonstrates that in TPHS, relative to CH, with the increase in radius, the lower tangential and higher radial velocity first increased to a peak and subsequently decreased, while the axial velocity primarily reduced to zero, increased in the opposite direction, and finally decreased rapidly to zero again. In addition, a disadvantageous flow, namely, screen backflow, was generated in TPHS, wherein the farther away the flow is from the feed inlet, the earlier this flow behaviour occurred. However, the rational scheme of aperture size and screen length can completely remove the screen backflow in TPHS

Effect of the Metro Train on the Smoke Back-Layering Length under Different Tunnel Cross-Sections

Smoke back-layering length is an important aspect of tunnel fire research, and the influence of metro train blockage cannot be ignored. In previous studies, less attention has been paid to the phenomenon of back-layering caused by the same train blocked in tunnels with different cross-sections. This paper investigates this point through dimensionless analysis and fire dynamic simulator (FDS) numerical simulation. Several full-scale model tunnels (300 m in length), with different tunnel cross-sections, were constructed in FDS. According to the FDS simulation result, the smoke back-layering length was compared and analyzed. To investigate the effect of the tunnel cross-sections on smoke back-layering length under metro train blocking, the headroom ratio &epsilon; was proposed. Then, the influence of the tunnel cross-section on the smoke back-layering length was discussed in detail. Based on the dimensional analysis and FDS simulation results, a new model for predicting the smoke back-layering length was proposed. The prediction model obtained in this paper was compared with the FDS simulation results and the prediction model proposed by our predecessors. It is proven that the proposed model can better predict the length of smoke back-layering when metro trains are blocked in tunnels with different cross-sections. The outcomes of this work are expected to provide theoretical guidance for fire smoke control in metro tunnels with different cross-sections

Effect of the Metro Train on the Smoke Back-Layering Length under Different Tunnel Cross-Sections

Smoke back-layering length is an important aspect of tunnel fire research, and the influence of metro train blockage cannot be ignored. In previous studies, less attention has been paid to the phenomenon of back-layering caused by the same train blocked in tunnels with different cross-sections. This paper investigates this point through dimensionless analysis and fire dynamic simulator (FDS) numerical simulation. Several full-scale model tunnels (300 m in length), with different tunnel cross-sections, were constructed in FDS. According to the FDS simulation result, the smoke back-layering length was compared and analyzed. To investigate the effect of the tunnel cross-sections on smoke back-layering length under metro train blocking, the headroom ratio ε was proposed. Then, the influence of the tunnel cross-section on the smoke back-layering length was discussed in detail. Based on the dimensional analysis and FDS simulation results, a new model for predicting the smoke back-layering length was proposed. The prediction model obtained in this paper was compared with the FDS simulation results and the prediction model proposed by our predecessors. It is proven that the proposed model can better predict the length of smoke back-layering when metro trains are blocked in tunnels with different cross-sections. The outcomes of this work are expected to provide theoretical guidance for fire smoke control in metro tunnels with different cross-sections