Rapid formaldehyde monitoring in ambient air by means of mid-infrared cavity leak-out spectroscopy

Abstract.We report the spectroscopic detection of formaldehyde in ambient air using cavity leak-out spectroscopy, a cw variant of cavity ring-down spectroscopy. This technique proved to be suitable for a real-time quantitative analysis of polluted air without any preprocessing of the air sample. Using a tunable CO-overtone sideband laser for the λ=3 μm spectral region and a ring-down cell with R=99.95% mirrors, we achieved a detection limit of 2 parts per billion formaldehyde in ambient air, corresponding to a minimum detectable absorption coefficient of 7×10-9/cm (sampling time: 2 s). Calibration problems arising from the polarity of the molecule and due to HITRAN database uncertainties are discussed.