Compact and portable open-path sensor for simultaneous measurements of atmospheric N2O and CO using a quantum cascade laser

A compact and portable open-path sensor for simultaneous detection of atmospheric N2O and CO has been developed with a 4.5 mu m quantum cascade laser (QCL). An in-line acetylene (C2H2) gas reference cell allows for continuous monitoring of the sensor drift and calibration in rapidly changing field environments and thereby allows for open-path detection at high precision and stability. Wavelength modulation spectroscopy (WMS) is used to detect simultaneously both the second and fourth harmonic absorption spectra with an optimized dual modulation amplitude scheme. Multi-harmonic spectra containing atmospheric N2O, CO, and the reference C2H2 signals are fit in real-time ( 10 Hz) by combining a software-based lock-in amplifier with a computationally fast numerical model for WMS. The sensor consumes similar to 50 W of power and has a mass of similar to 15 kg. Precision of 0.15 ppbv N2O and 0.36 ppbv CO at 10 Hz under laboratory conditions was demonstrated. The sensor has been deployed for extended periods in the field. Simultaneous N2O and CO measurements distinguished between natural and fossil fuel combustion sources of N2O, an important greenhouse gas with poorly quantified emissions in space and time. (C)2012 Optical Society of America