In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer
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molecules cm-3 Hz-1/2 (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midlatitude tropopause conditions or a mixing ratio of 0.6 ppmv under boundary-layer conditions. The corresponding minimum measurable absorbance is 10-5 Hz-1/2. The laser hygrometer was field-demonstrated aboard the NASA P3B research aircraft, during a series of flights spanning several weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled mirror hygrometers. In general, the laser hygrometer performed well; however, under some conditions, it reported water vapor number densities 20% greater than the cryogenic frost-point hygrometer. This difference is currently under study.
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