In situ measurements of H2O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 mu m

A distributed-feedback InGaAs laser diode emitting near 1.393 mu m is used in conjunction with an optical multipass cell that is open to the atmospher e to yield ambient water-vapor measurements by infrared absorption spectros copy. To obtain the high dynamic range for the measurements that is require d for continuous water-vapor monitoring in the upper troposphere and the lo wer stratosphere, we used a simple circuit that combined differential and d irect detection. Furthermore, the laser emission wavelength was tuned to ba lance the steep decrease in H2O concentration with altitude by sweeping mol ecular transitions of stronger line strengths. The technique was implemente d by use of the Spectrometre B Diodes Laser Accordables (SDLA), a tunable d iode laser spectrometer operated from a stratospheric balloon. Absorption s pectra of H2O in the 5-30-km altitude range obtained at 1-s intervals durin g recent balloon flights are reported. Water-vapor mixing ratios were retri eved from the absorption spectra by a fit to the full molecular Line shape in conjunction with in situ pressure and temperature measurements, with a p recision error ranging from 5% to 10%. (C) 2000 Optical Society of America. OCIS codes: 300.6340, 300.6380, 300.6260, 280.3420, 010.1280, 300.1030.