Investigation of Methane Emissions and Sources

Methane (CH4) is an important trace gas and the most potent greenhouse gas in the global atmosphere. This study addressed CH4 related issues using a multi-pronged approach to monitor, characterize, and quantify CH4 emissions and sources through three observational platforms including Remote Methane Leak Detector mounted on an Unmanned Aerial Vehicle (RMLD-UAV), the UH mobile laboratory, and the Moody tower stationary site. The RMLD-UAV system provided an advanced capability in detecting CH4 leaks autonomously in the natural-gas industry. A quantification algorithm was derived based on a mass-balance approach, which was accurate to approximately 50% under preferred wind conditions with higher wind speeds and lower wind variation. The influence of systematic error was investigated by introducing simulated noises, of which GPS noise presented the greatest impact on leak rate errors. Zero-leak cases can be ascertained via a skewness indicator which is unique and promising. Key information concerning regional anthropogenic CH4 sources was obtained through the stable carbon isotope δ13CCH4 during two field campaigns in Texas using a state-of-the-art mobile laboratory. The background δ13CCH4 signatures and overall CH4 distributions depicted the different dominate CH4 sources in the studied areas. δ13CCH4 signatures of thirty-three sources were identified varying from -76‰ to -23‰. Several repeated measurements revealed the variability of typical source signatures. The detection of an unexpected CH4 leaking demonstrated the capability of the system that can capture and identify unknown CH4 leaks. One-year continuous measuring of CH4 mixing ratios and related meteorological factors conducted at the Moody Tower site indicated the relative long-term seasonal and diurnal dynamics of regional CH4. The diurnal variation of CH4 in different seasons showed the same trend with the highest values happened in the early morning and lower values during the daytime. The calculated δ13CCH4 signature was -58.9‰ yearly and was diverse in different seasons. The prevailing wind was southerly; whereas, most of the high CH4 spikes came from the northeast of the site where the polluted ship-channel area is located. The results of this study supplied valuable references for regulatory programs, regional emission inventories, and atmospheric model inputs.