The 3-channel WISP instrument design is a hybrid of the 4-channel ALIAS
(ER-2) and 2-channel ALIAS-II (balloon and UAV) spectrometers. Like
ALIAS-II, WISP uses an open-cradle Herriott cell configuration, although WISP
has added fairings for the aircraft flights. Laser light is injected into an open-path Herriott cell suspended
beneath the aircraft wing, for a total pathlength of 94 meters, similar to the
80-meter ALIAS instrument; mixing ratios are
measured using second harmonic (2f) spectroscopy. Like ALIAS-II, the open-path arrangement
of WISP minimizes
problems of contamination by outgassing or adsorption of water on instrument surfaces.
The instrument was specifically designed for open-path measurements on NASA's WB57F aircraft, which carries atmospheric science payloads to altitudes up to 65,000 feet. WISP is made a single test flight during the September 1999 ACCENT (Atmospheric Chemistry of Combustion Emissions Near the Tropopause) campaign, from Ellington Field in Houston, TX, to test subsystems, but has yet to record any data.
Measurements of the isotopic composition of atmospheric water vapor in the upper troposphere and lower stratosphere are a critical tool for diagnosing sources of upper tropospheric humidity and processes of troposphere-stratosphere exchange. Water vapor isotopic composition can be measured by remote sensing above this region, and by collection and laboratory mass spectrometry in the wetter air below it, but to date few in situ measurements have been made in the region of maximum interest for transport questions. WISP (Water Isotope SPectrometer) is a three-channel tunable diode laser spectrometer designed for in-situ measurement of the isotopic composition of water vapor in the upper troposphere and lower stratosphere.