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

Citation

Shen, Linhan (2018) Cavity Ring-Down and Multi-Pass Spectroscopies for Methane Source Attribution and Chemical Kinetics Studies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WE40-5W54. https://resolver.caltech.edu/CaltechTHESIS:04122018-141241438

Abstract

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.

Thesis Files