High precision determinations of NH3 concentration by means of diode laser spectrometry at 2.005 μm

This paper reports on the development of a compact analyzer for ammonia monitoring in air, based on a distributed feedback diode laser at 2.005 mu m. A dual-beam long-path technique was combined with wavelength modulation detection of absorption in order to reach a detection limit of about 25 ppb, with a 0.2-Hz equivalent noise bandwidth. Retrieval of NH3 concentration was accomplished through a careful spectra analysis procedure based on the formalism of Fourier expansion of the 2nd harmonic signals, also taking into account residual-amplitude-modulation effects. The system was tested on certified gas-mixtures and revealed a good performance in terms of accuracy and reproducibility. Particularly, the short-term precision was found to be about 1 parts per thousand, for NH3 mixing ratios of 10 ppm. Finally, the possibility to make use of the analyzer for measurements of ammonia fluxes from soils is discussed.