Abstract
A rapid tunable diode laser (TDL) absorption sensor was developed for real-time measurements of ammonia concentration by using wavelength modulation spectroscopy (WMS) at 992.698cm−1 of the ν2 vibrational band. This line has patterns free from interference with other species in the atmosphere, and can be used for open-path detection. The 1f signal was used to normalize the 2f signal thereby eliminating the need for calibration and explaining the laser transmission variations. Using WMS with a large modulation depth and integrating the absolute value of the resulting spectra increased the limit of detection (LOD) of the sensor by a factor of seven, compared with the LOD achieved by using the maximum value of the WMS 2f signal. Furthermore, an increase by a factor of 25 compared with the direct absorption spectroscopy was achieved, which allowed obtaining LOD∼1ppb with a resolution time of <2s for the detection of NH3 in the atmosphere using a short-path cell (a 60-cm absorption cell with four passes).
Published Version
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