In situ measurements of H_(2)O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 μm

摘要

A distributed-feedback InGaAs laser diode emitting near 1.393 μm is used in conjunction with an optical multipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infrared absorption spectroscopy. To obtain the high dynamic range for the measurements that is required for continuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used a simple circuit that combined differential and direct detection. Furthermore, the laser emission wave-length was tuned to balance the steep decrease in H_(2)O concentration with altitude by sweeping molecular transitions of stronger line strengths. The technique was implemented by use of the Spectrometre a Diodes Laser Accordables (SDLA), a tunable diode laser spectrometer operated from a stratospheric balloon. Absorption spectra of H_(2)O in the 5-30-km altitude range obtained at 1-s intervals during recent balloon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra by a fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements, with a precision error ranging from 5% to 10%.