CRDS

Cavity ring-down spectroscopy

Cavity ring-down spectroscopy is an absorption technique that is applied in atmospheric chemistry. The absorption of trace gases such as nitrogen dioxide (NO₂) and the nitrogen radical (NO₃) is measured within a cavity, which consists of two high reflective mirrors that are 0.5 to 1.0 m spaced. Pulsed laser light is coupled into the cavity through one of the cavity mirrors. The light that is transmitted through the cavity is observed behind the second mirror by a photomultiplier tube. An exponential decay of the light intensity is observed after the laser light has been distinguished because of its loss on the mirrors, scattering and absorption by trace gases. The time constant with which the intensity decays can be converted to the absorption and therefore the concentration of the trace gas. Because of the high mirror reflectivity effective length of the light path can be as long as 10 to 100 km. Therefore, a high sensitivity of the technique can be achieved allowing the measurement of e.g. of NO₃ concentrations of less than one pptv (parts per trillion per volume).