Water vapor resulting from particle evaporation is measured by absorption in the infrared, using a custom instrument originally built by Dr. Randy May of SpectraSensors, Inc. This sensor is virtually identical, except for its internal absorption cell, to open-path instruments flown by the JPL group (May and Herman) on the NASA ER-2, DC-8 and WB-57 during past missions. These water vapor spectrometers use near-infrared (1.37 micron) tunable diode laser (TDL) sources that provide about 5 mW of single-mode (i.e., single absorption feature) output. In addition to their small size and dependability, these lasers are attractive because they operate near room temperature, eliminating the need for cryogens. Instead, a thermoelectric element is used to stabilize the laser temperature to about 15 oC. The infrared detector -- a standard InGaAs sensor -- also does not require cryogens, but is mounted to a thermostated baseplate to maintain a stable operating temperature.
The absorption path is defined by the length of the internal sample cell, a 6-mm I.D. stainless-steel tube, which is capped at both ends with anti-reflection-coated quartz windows. The path length is 27.62 cm, with a single pass between the laser and detector, which is sufficient for tropospheric water (the original intent), but is limited to mixing ratios above about 10 ppmv in the upper troposphere and lower stratosphere for 1-second integrations.
On aircraft, the CLH measures behind a forward-facing, heated inlet. The mass flow rate within the instrument is fixed at less than 5 m/s using a mass flow controller, considerably slowing the flow compared to the free-stream velocity. This subisokinetic condition enhances sampling of particulates relative to the gas-phase. The particles are evaporated primarily by ram-heating, and the resulting water vapor is measured by absorption. The inlet has been designed such that the instrument is sensitive to particles larger than about 5 micron in diameter.Home page for this instrument.