Development of Mobile Open-path Cavity Ring-down Spectrometer for Measurement of Trace Atmospheric Methane Gas

摘要

Use in recent decades of methane as a 'clean' alternative to coal and gasoline has seen a rapid increase in natural gas extraction in the United States. Although combustion of methane produces less CO2 than traditional fuels, it is a powerful greenhouse gas with a 20 year Global Warming Potential (GWP20) that is 84x that reported for CO2 in the latest IPCC report; therefore, the promise of natural gas as a clean fuel can only by realized if emissions of uncombusted gas are sufficiently low. To address this problem, there is a need for both regional (basin wide) measurements of methane emissions to determine global levels, as well as localized measurements to allow identification and reduction of emissions ("leaks") from specific equipment. The goal of this research is to develop a mobile open-path cavity ring-down spectroscopy (CRDS) sensor for localized measurements of atmospheric methane. While designed with the oil and gas industry in mind, the technology also has application to study emissions from agricultural operations and those from other sectors.;This thesis presents development from proof-of-concept open-path sensor through two mobile iterations. CRDS can provide fast, non-intrusive, sensitive measurements; but in contrast to available instruments, the focus is on open-path operation (no flow-cell and pump) to provide opportunities for significant weight, size and power reductions to increase the mobility of the technique (<4 kg, <25 W). Challenges of open-path operation, such as fitting broadened spectral peaks, preserving mirror cleanliness and techniques for removing signal noise due to aerosol particles are addressed. The sensor is based on widely available and mature engineering near-infrared (NIR) opto-electronic components that have been developed for the telecom industry. Sensor validation with known methane concentrations show that the open-path sensor is capable of measuring atmospheric concentrations in the range of ∼1.8-20+ ppmv at a rate of 1-3 Hz. Sensitivity studies using Allan variance techniques show sensitivity of < 20 ppbv in 1 -- adequate for practical leak detection of small plumes <1 ppmv. Comparisons against a commercially available closed path sensor in mobile deployments are presented, along with mobile measurements from natural gas facilities in Platteville, CO and Washington County, PA. Finally, integration of the sensor onto a UAS platform for airborne measurements of methane and ammonia from agricultural applications is discussed.