QCL-based open-path, single-pass measurement of ambient carbon monoxide using R1f/ΔI1 WMS

This letter outlines a new robust wavelength modulation spectroscopy technique to achieve calibration-free measurements. The technique normalizes the magnitude of the $1^{st}$ harmonic signal by the amplitude of the laser’s linear intensity modulation, and uses the fact that the ratio of the intensity to the amplitude of the linear intensity modulation remains invariant at every point within a system and over time despite significant and unpredictable variations in the individual quantities. The technique is demonstrated using a mid-infrared quantum cascade laser for open-path measurements of ambient carbon monoxide with a single-pass detection limit of 12 ppb-m. The method is simple to implement and performs robustly for long durations of outdoor measurements.