English: Illustration showing how trace gases are detected in the field using a mobile dual-frequency comb laser spectrometer. The spectrometer sits in the center of a circle which is ringed with retroreflecting mirrors. Laser light from the spectrometer (yellow line) passes through a gas cloud, strikes the retroreflector and is returned directly to its point of origin. The data collected are used to identify leaking trace gases (including methane), as well as leak locations and their emission rates.