Simultaneous Monitoring of Atmospheric CH 4 , N 2 O, and H 2 O Using a Single Gas Sensor Based on Mid-IR Quartz-Enhanced Photoacoustic Spectroscopy.

An optical sensor based on external-cavity quantum cascade laser (EC-QCL) was developed for simultaneous triple-species monitoring of CH 4 , N 2 O, and H 2 O vapor using off-beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS). The EC-QCL wavelength was scanned over three neighboring absorption lines of CH 4 (1260.81 cm -1 ), N 2 O (1261.06 cm -1 ), and H 2 O vapor (1261.58 cm -1 ) by tuning the grating of the EC-QCL with a piezoelectric actuator. Molecular relaxation effects impacting the generation of the QEPAS signals resulting from light absorption by CH 4 and N 2 O molecules were investigated in the mid-infrared region near 8 μm. A theoretical model was introduced for the mid-infrared region, including the beneficial influence of water vapor. An enhancement of the QEPAS signals by a factor of 3 for CH 4 in air and of 20% for N 2 O in air was observed in humidified samples compared to that in dry samples. The QEPAS measurement was scaled by the calibrated reference spectrometers; detection limits of 98 ppbv for CH 4 , 12 ppbv for N 2 O, and 750 ppmv for H 2 O vapor were obtained with a 1σ signal-to-noise ratio (SNR = 1) in humidified gas mixtures. Real-time Kalman filtering was applied to improve the measurement precision by a factor of approximately 4 while keeping the same temporal resolution, leading to measurement precisions of 60 ppbv for CH 4 , 10 ppbv for N 2 O, and 0.07% for H 2 O in the measurements of 1.99 ppmv CH 4 and 312 ppbv N 2 O humidified with 2.8% H 2 O vapor, with a 1 s lock-in amplifier time constant and an equivalent bandwidth of 0.1 Hz.