Dual Comb Spectroscopy for Emissions Measurements*

Trace gas measurements are critical for understanding city contributions to greenhouse gas emissions and for identifying and quantifying natural gas leaks from oil and gas wells. Here we present a near-infrared dual frequency comb spectroscopy system that we use for trace gas measurements. Our system is comprised of a dual comb spectroscopy instrument, a telescope and co-located detector, and a retroreflector located at the far end of our measurement path. This enables us to measure over path lengths of several hundred meters to several kilometers. We have compared this system against a near-identical system over the open atmosphere with excellent agreement of retrieved trace gas concentrations. We then deployed this system at NIST to measure the carbon dioxide enhancement over the city of Boulder, Colorado which we primarily attribute to vehicle exhaust. This is coupled with Gaussian plume modeling to estimate the emissions from the city and we have good agreement with the city estimate of vehicle emissions. Finally, we deploy the system in the field in a van to measure emissions from a simulated natural gas leak using a mobile UAS-mounted retroreflector. This work shows the promise of using horizontal column-integrated open-path measurements for emissions quantification.