Development of open-path interband cascade laser-based ethane sensor

Abstract:

We present development of an open-path, fast response, laser spectrometer for sensing atmospheric ethane on ground-based and mobile laboratory platforms. A 3336.8 nm interband cascade laser probes a fundamental infrared absorption band for sensitivity to ethane under ambient conditions, as enabled by employing wavelength modulation spectroscopy. Simultaneous measurement of methane on an adjacent line corrects for cross-sensitivity with ethane in the air-broadened second harmonic spectrum. The sensor has an average power consumption of 20 W with an optical pathlength of 60 m and has been mounted alongside CO₂ and CH₄ analyzers on our mobile laboratory. With a noise equivalent absorbance of 2 x 10^-5, precision and sensitivity are expected to be better than 1 ppbv, comparable to background levels away from localized ethane sources. Results are presented using the sensor for identification of fugitive methane leakage from natural gas production in the Marcellus Shale, helping to exclude other potentially collocated sources including wetlands and small-scale animal feeding operations. Ethane is applied as a highly effective tracer for distinguishing biologically produced methane from the thermogenic methane of interest. For medium- to high- emitting wells, we also obtain information about natural gas composition. Finally, performance of the instrument for measuring ethane and methane under varying regimes (enhancement over ambient atmospheric levels, methane/ethane ratios) is discussed along with future applications enabled by the reduced power and weight specifications and 10 Hz time response.