High-Resolution Photoacoustic Spectroscopy of the Oxygen A-Band

Citation

Lunny, Elizabeth Margaret (2020) High-Resolution Photoacoustic Spectroscopy of the Oxygen A-Band. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qxcm-4909. https://resolver.caltech.edu/CaltechTHESIS:06082020-132244698

Abstract

There have been many advances in recent years in remote sensing and ground-based measurement technologies utilizing optical detection to identify and quantify species in the atmosphere. Many of these instruments record high signal-to-noise spectra requiring sophisticated spectral modeling beyond the Voigt profile. In order to properly quantify the higher order spectral effects, high-resolution laboratory data measuring samples of known composition under carefully controlled conditions are required.

The oxygen A-band is used in a number of atmospheric composition measurements due to the uniform, well-known concentration of oxygen throughout the atmosphere. Previous laboratory A-band measurements using cavity ring-down spectroscopy and Fourier transform spectroscopy have greatly improved the understanding of spectral parameters. However, current spectral models are insufficient to fit some high quality remote sensing data, such as the OCO missions. The largest spectroscopic uncertainties in modeling result from characterization of line mixing and collision-induced absorption. These collisional effects, resulting in small absorption changes in the baseline and wings, which become more prominent at elevated pressures can be accurately measured with photoacoustic spectroscopy, a background free measurement with a large dynamic range producing high signal-to-noise spectra.

A novel high-resolution photoacoustic spectrometer was designed and constructed to improve the understanding of A-band spectral parameters to meet the OCO mission goals. The spectrometer is capable of measuring both the P and R-branches of the A-band up to J'=28 with a signal-to-noise ratio of 30,000 for pressures of 50-4,000 Torr. A temperature control system was also implemented to allow for measurements over the range of atmospherically relevant temperatures.

Results from spectral fitting of data from the newly developed spectrometer provide the most accurate A-band pressure shift coefficients for both oxygen and air measured to date. The data also indicates the importance of lineshape profile choice for resonant absorption in order to accurately characterize line mixing and collision-induced absorption; the speed-dependent Nelkin-Ghatak profile is required for the current data set. Finally, preliminary fitting of line mixing and collision-induced absorption suggests the photoacoustic data achieves the required sensitivity to provide improved understanding of line mixing and collision-induced absorption based on fundamental physical principles.

Thesis Files