Abstract

Over the last few years, there has been considerable effort at Physical Sciences Inc. (PSI) and elsewhere to develop ultrasensitive trace gas sensors for in situ monitoring of several important atmospheric species including H₂O, CO₂ and CH₄. At PSI, high performance field sensors have been created by combining near-IR diode laser absorption with balanced ratiometric detection. One such sensor is a laser hygrometer developed for NASA which is capable of monitoring water vapor from ground level to the tropopause. The laser hygrometer monitors water vapor through absorption via a strong, isolated line at 7181.172 cm⁻¹ in the ν₁ + ν₃ combination band. Through a series of laboratory measurements, we have demonstrated a sensitivity of 2 × 10¹² molecules cm⁻³ which corresponds to ~0.3 ppmv water vapor at typical mid-latitude tropopause conditions (12 km or 40 kft) using a 50 cm absorption pathlength. We have also demonstrated a dynamic range of 4 orders of magnitude and a measurement rate of 10 Hz. The hygrometer uses a unique, compact, external air probe for true in situ sampling, thereby obviating well-known limitations in time-response and sensitivity imposed by extractive air sampling systems.

© 1998 Optical Society of America