IN-SITU SENSING OF TROPOSPHERIC WATER-VAPOR USING AN AIRBORNE NEAR-IRDIODE-LASER HYGROMETER

We describe a near-IR diode laser hygrometer developed for in-situ mea surement of water vapor from airborne platforms throughout the troposp here. The sensor is based on the absorption of a 1.39-mu m diode laser beam and employs an open, folded 50-cm absorption path placed in the freestream air using an external probe. Water vapor number density is continuously recorded at 10 Hz. Extensive laboratory testing showed a sensitivity of 10(12) molecules cm(-3) Hz(-1/2) (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midla titude tropopause conditions or a mixing ratio of 0.6 ppmv under bound ary-layer conditions. The corresponding minimum measurable absorbance is 10-5 Hz(-1/2). Th, laser hygrometer was field-demonstrated aboard t he NASA P3B research aircraft, during a series of flights spanning sev eral weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled minor hygromete rs. In general, the laser hygrometer performed well; however, under so me conditions, it reported water vapor number densities 20% greater th an the cryogenic frost-point hygrometer. This difference is currently under study.