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" Sensitive Detection of Atmospheric Methane and Nitrous Oxide Using Higher Harmonic Wavelength Modulation Spectroscopy "
Hlaing, May Hnin
Khan, Mohammad Amir
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Latin Dissertation
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English
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| Record Number
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1107209
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TLpq2436433432
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| Main Entry
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Hlaing, May Hnin
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Khan, Mohammad Amir
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| Title & Author
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Sensitive Detection of Atmospheric Methane and Nitrous Oxide Using Higher Harmonic Wavelength Modulation Spectroscopy\ Hlaing, May HninKhan, Mohammad Amir
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| College
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Delaware State University
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| Date
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2020
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| student score
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2020
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| Degree
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Ph.D.
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| Page No
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268
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| Abstract
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Atmospheric methane (CH4), and nitrous oxide (N2O) are two of the most potent greenhouse gases with relatively significant radiative forcing, contributing to global warming and climate change. In addition, due to their atmospheric residence time, both CH4 and N2O have much higher global warming potentials than the most prevalent greenhouse gas, carbon dioxide (CO2). Over the last several decades, the concentrations of CH4 and N2O have rapidly increased due to diverse natural and anthropogenic sources and sinks resulting in significant uncertainty in their emission budget. Currently, there are several commercial and academic research technologies and field instruments (earth, aerial, and geostationary satellite-based) to precisely quantify and profile atmospheric CH4 and N2O in real-time. However, there are limited arrays of technologies that can simultaneously and synchronously sample these two species on a local and global scale. Simultaneous detection of CH4 and N2O is of great interest in discriminating and correlating carbon and nitrogen emissions from biogenic and abiogenic sources, for instance, in measurements of soil flux and atmospheric monitoring near the surface and in the atmospheric boundary layer region. Semiconductor laser-based sensing techniques have been widely in use to develop precise sensor systems for detecting and monitoring trace gases in the atmosphere. Among many sensing techniques, wavelength modulation spectroscopy (WMS) is a non-intrusive and highly sensitive technique for probing atmospheric broadened rotational-vibrational molecular transitions. By employing the principles of modulation technique, WMS higher harmonic detection (WMS-HHD) not only can simultaneously measure concentrations of trace species but also can improve detection sensitivity and spectral line resolution. In addition, semiconductor mid-infrared laser sources integrated with optical multipass cells in an open-path configuration, and optomechanical components with WMS methodology provide a simple, compact, low-power, and low-cost sensing technology suitable for deployment in diverse platforms, including mobile, ground-based and airborne systems. This dissertation focuses on the design and development of a precise, non-intrusive, and ultra-sensitive sensor system to detect atmospheric CH4 and N2O in the 7.8 μm spectral region. The system is built upon a room-temperature thermo-electrically cooled mid-infrared quantum-cascade laser source, and Heriot design optical multipass cell implemented with WMS-HHD sensing technique. We show a central aspect of WMS-HHD where the structure of the higher harmonic signal and optimal WMS detection order is employed to, (i) accurately quantity laser tuning properties and frequency modulation response, and (ii) resolve overlapping rotational, vibrational molecular transition of multiple (CH4, N2O, and H2O) line-transitions of different oscillator strengths. Finally, we quantify the performance of the sensor system with detection limits of 52 parts per billion by volume (ppbv) for N2O and 162 ppbv for CH4. The laboratory precisions are 0.59%–1.12% for CH4 and 0.82%–1.85% for N2O, which translate to uncertainties of 10–22 ppbv for CH4 and 3 ppbv for N2O over the background of 1800 ppbv CH4 and 352 ppbv N2O, respectively.
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Optics
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