Near-Infrared Off-Axis Integrated Cavity Output Spectroscopic Gas Sensor for Real-Time, In Situ Atmospheric Methane Monitoring

Abstract

A near-infrared methane (CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) sensor system was demonstrated by using off-axis integrated cavity output spectroscopy (OA-ICOS), which was an effective gas-phase analytical technique and was adopted for fast, real-time and in-situ ambient atmospheric CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> monitoring. A 60 cm-long cavity with an effective absorption path length of 2150 m was fabricated with a fast response time of ~ 0.8 s. An Allan deviation analysis yielded a minimum detection limit of 13 parts-per-billion in volume (ppbv) using wavelength modulation spectroscopy (WMS) technique, which corresponds to a noise equivalent absorption sensitivity (NEAS) of 9.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> . By applying multiple denoising scheme of wavelet transform (WT), Kalman filtering (KF) and empirical mode decomposition (EMD) method, the precision levels of CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> measurements were improved by a factor of 4, 5 and 6, respectively. For field application, dual-gas sensing was implemented for simultaneous measurement of CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and water vapor (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) in a single sensor system. Also, a three-day continuous monitoring of atmospheric CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration levels was carried out to verify the validity and reliability of the demonstrated ppbv-level CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> sensor system.