Atmospheric trace gas analysis with cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive laser absorption method. It can be used for quantitative analysis of molecular species at the sub-ppb level. The absorption cell (cavity) is sealed by two high-reflective mirrors on each side, which results in an effective absorption path-length of some kilometers. Our experiments for atmospheric gas analysis have been carried out so far with an Excimer pumped dye laser in the UV-VIS and a CO overtone sideband laser in the wavelength region around 3 mum. Experiments with an all solid-state difference frequency laser system will follow. In the UV-VIS region, we measured trace gas molecules like SO2, NO2, and CH2O. In the mid-infrared, around 3 mum, we measured hydrocarbons like CH4, C2H6, and C2H4 with a detection limit of less than 1 ppb. The noise equivalent absorption coefficients in the MIR are in the order of 1.7.10(-9) cm(-1). Due to the high data acquisition rate and the high sensitivity, CRDS enables real-time detection of trace gases in ambient air.