Environmental Monitoring of Methane with Quartz-Enhanced Photoacoustic Spectroscopy Exploiting an Electronic Hygrometer to Compensate the H2O Influence on the Sensor Signal

Abstract

A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband cascade laser operating at 3.34 μm capable of targeting a CH4 absorption line at 2988.8 cm−1 and a water line at 2988.6 cm−1. Water vapor was measured with both the electronic hygrometer and the QEPAS sensor for comparison. The measurement accuracy provided by the hygrometer enabled the adjustment of methane QEPAS signal with respect to the water vapor concentration to retrieve the actual CH4 concentration. The sensor was tested by performing prolonged measurements of CH4 and H2O over 60 h to demonstrate the effectiveness of this approach for environmental monitoring applications.

Intrinsic Spectral Resolution Limitations of QEPAS Sensors for Fast and Broad Wavelength Tuning

“…As an example, a variation in water concentration causes a variation in the methane PA signal, which is not related to a change in the CH concentration. Thus, in order to guarantee reliable CH concentration measurements, the water-vapor concentration must be monitored simultaneously [ 43 ].…”

High-concentration methane and ethane QEPAS detection employing partial least squares regression to filter out energy relaxation dependence on gas matrix composition

“…This effect is strongly evident for slow-relaxing molecules, such as methane. In fact, C1 QEPAS signal proved to be dependent on the water vapor concentration in the 3 µm wavelength range [40] , therefore the H 2 O concentration was fixed employing a Nafion humidifier in the experimental setup ( Fig. 1 ).…”

Calibration of Quartz-Enhanced Photoacoustic Sensors for Real-Life Adaptation

“…This strongly underlines that PA-based concentration measurements in real-life monitoring conditions necessarily involve a highly sensitive measurement of the absolute humidity level. A sensitive humidity measurement can either be done using the PA effect [ 25 ], or, perhaps more conveniently, by embedding small state-of-the-art humidity sensors (see e.g., [ 45 , 46 ]) into the PA gas chamber.…”

“…Considering the PA measurement of CO concentration, the PA voltage signal, , may therefore be described by a relation of the form wherein P is the optical power, is related to the optical absorption of CO at the employed wavelength, and is a constant-excluding polynomial with respect to . To test and compare our calibration method with related published work, the PA signal is hypothesized to be perturbed linearly by water content [ 24 , 25 ], i.e., . This reforms Equation ( 1 ) to …”

“…Multiple strategies and methods have so far been implemented for humidity correction of PA trace-gas sensors [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Here, we develop a calibration framework based on a simple learning-based method for quantifying the influence of humidity on photoacoustic carbon-dioxide concentration measurements.…”