Optimization of a compact photoacoustic quantum cascade laser spectrometer for atmospheric flux measurements: application to the detection of methane and nitrous oxide

Room temperature (RT) quantum cascade lasers (QCL) are now available even in continuous wave (cw) mode, which is very promising for in situ gas detectors. Ambient air monitoring requires high sensitivity with robust and simple apparatus. For that purpose, a compact photoacoustic setup was combined with two cw QCLs to measure ambient methane and nitrous oxide in the 8 μm range. The first laser had already been used to calibrate the sensitivity of the photoacoustic cell and a detection limit of 3 ppb of CH4 with a 1s integration time per point was demonstrated. In situ monitoring with this laser was difficult because of liquid nitrogen cooling. The second laser is a new RT cw QCL with lower power, which enabled one to reach a detection limit of 34 ppb of methane in flow. The loss in sensitivity is mainly due to the weaker power as photoacoustic signal is proportional to light power. The calibration for methane detection leads to an estimated detection limit of 14 ppb for N2O flux measurements. Various ways of modulation have been tested. The possibility to monitor ambient air CH4 and N2O at ground level with this PA spectrometer was demonstrated in flux with these QCLs.