Near-Infrared Tunable Diode Laser Spectrometer for In Situ Stratospheric Measurements of H₂O

 :

Measurement Description: Water vapor volume mixing ratio is measured using second harmonic (2f) absorption spectroscopy at 1.37 µm wavelength (near 7300 cm-1). A single- frequency tunable diode laser (TDL) is repetitively swept in wavelength over a narrow spectral interval containing the target spectral line. A full 2f spectrum is recorded every 0.128 s, and typically ten spectra are averaged in software to produce mixing ratio data at a 1 s rate continuously during flights. On-the-fly data processing software allows preliminary mixing ratios to be obtained in flight. Laboratory calibration data based on standard gas mixtures is utilized for final calibration of the flight data.

To ensure measurements in clean air, the optical path is defined by a pair of external mirrors mounted in a multipass (Herriott) arrangement. 54 passes provides an optical absorption pathlength of 11.1 m. The multipass mirrors are located two inches below the mounting panel to avoid the boundary layer. The optical head containing the laser, detector, and collimating optics is also located external to the super-pod, and is evacuated to eliminate detectable water vapor absorption within the optical head. This arrangement also reduces the total non-atmospheric optical path to <1 cm. The instrument provided data throughout the 1997 POLARIS mission, and was also flown on the NASA WB57F aircraft during the 1998 WAM mission. A similar instrument participated in the 1998 CAMEX-3 mission on the DC-8, and a permanent WB57F version is under construction for the 1999 ACCENT mission. Both the ER-2 and DC-8 instruments will participate in the 1999 - 2000 SOLVE mission.