Development and field deployment of a mid-infrared methane sensor without pressure control using interband cascade laser absorption spectroscopy

A mid-infrared methane (CH4) sensor without pressure control was developed using a continuous-wave (CW) interband cascade laser (ICL) for targeting a CH4 absorption line located at 3038.5 cm−1. A multi-pass gas cell with an absorption path length of 54.6 m was utilized for enhancing gas absorption. The pressure inside the MPGC was measured using direct Lorentzian absorption fitting for the compensation of CH4 concentration changes resulting from pressure variations. Laboratory pressure calibration was conducted in the range of 25-800 Torr using 1.3-, 1.5-, 1.7- and 2.1-ppmv CH4 samples. A pressure precision of ∼1.65 Torr with a ∼2.5-s averaging time was achieved based on the measurement of a 2.1-ppmv CH4 sample at 700-Torr. Concentration level measurements of a 2.1-ppmv CH4 sample at a 700-Torr pressure yielded an Allan deviation of 2.25 ppbv for an averaging time of 2.5 s. The sensor functioned normally with CH4 samples at 1.0, 1.2, 1.4, 1.6 and 2.1 ppmv concentration levels as the pressure changes from 25 to 800 Torr. Indoor/outdoor CH4 concentration measurements on the Rice University campus and a field campaign in the Greater Houston Area (GHA) were conducted to evaluate the performance of the sensor system.