Compact diode-laser spectrometer ISOWAT for highly sensitive airborne measurements of water-isotope ratios
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- Dyroff, C., Fütterer, D. & Zahn, A. Appl. Phys. B (2010) 98: 537. doi:10.1007/s00340-009-3775-6
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The tunable diode-laser absorption spectrometer ISOWAT for airborne measurements of the water-isotope ratios 18O/16O and D/H is described. The spectrometer uses a distributed feedback (DFB) diode laser to probe fundamental rovibrational water-absorption lines at around 2.66 μm. Very-low-noise system components along with signal averaging allow for a detection limit of 1.2 and 4.5‰ for measurements of 18O/16O and D/H, respectively, for a water-vapour mixing ratio of 100 ppmv and an averaging time of 60 s. This corresponds to a minimum detectable absorbance of ∼5×10−6 or ∼6.6×10−10 cm−1 when normalized to pathlength. In addition to its high sensitivity, the spectrometer is highly compact (19-inch rack at a height of 35 cm, excluding pump and calibration unit) and light weight (<40 kg total). The total power consumption is around 350 W, and the instrument is fully automated. ISOWAT will be calibrated during flight with known water-isotope ratios using a compact calibration-gas source.