Mid-Ir Cavity Ringdown Spectroscopy Measurements of Atmospheric Ethane to Methane Ratio for Methane Source Attribution

Atmospheric methane is a very important greenhouse gas. Source of methane plumes could be attributed using isotopic composition measurements and methane coemitting gas (ethane, propane, and butane) detections. Since isotopic source attribution process requires sensitive detection of more than three methane isotopologues (12CH4, 13CH4, and 12CH3D), we choose to measure the highest abundant coemitting gas, ethane. Ethane to methane ratio from biogenic and thermogenic sources have known to vary from 0.1% to 30%. In this work, we demonstrated a mid-IR (3.3 µm) cw cavity ringdown spectrometer capable of measuring atmospheric ethane abundance and ethane to methane ratio. This technique utilizes the long effective pathlengh of cavity ringdown spectroscopy to increase sensitivity of atmospheric ethane detection. Our spectrometer can measure atmospheric ethane concentration as low as 70 ppb. We have demonstrated the capability of this instrument by measuring the atmospheric ethane composition and ethane to methane ration in ambient air in Pasadena, California and soil gas samples collected from the Porter Ranch SoCal Gas Storage Facility.