N₂O detection with the CEAS method

Abstract

Nitrous oxide (N 2 O) plays a significant role in many different fields therefore its monitoring is an important task. In this paper the opportunity of application of cavity enhanced absorption spectroscopy for N 2 O detection is presented. This method is a modification of cavity ring down spectroscopy. The laser radiation tuned to absorption line of N 2 O is injected into an optical cavity under a very small angle in respect to its axis. In the case of lack of the absorption, the result is determined by the mirrors reflectivity coefficient, diffraction losses and length of the cavity. When the absorber is present in the cavity, the result additionally depends on absorption and scattering of light in cavity. The method provides the determination of a very weak absorption coefficient as well as the concentration of the absorbing gas.Our N 2 O sensor consisted of a pulsed radiation source, optical cavity, detection module and a digital oscilloscope. As the light source anoptical parametric generator was applied. It enabled the wavelength tuning in a broad spectral range with resolution of 1 nm. The optical cavity was composed of two high-reflective spherical mirrors. Optical signal registration was done with detection module equipped with HgCdTe photodetector.The spectral range of 4.52-4.53 µm is the best for N 2 O detection. Operation at these wavelengths provides opportunity avoiding of interferences with other atmosphere gases, like CO 2 and H 2 O. Assuming 2% uncertainty of measurements and the effective value of the absorption cross section of about 6·10 -19 cm 2 the detection limit of 10 ppb was achieved.