In situ Sensing of Atmospheric Trace Gases Using Airborne Near-IR Diode Laser Sensors

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.