Abstract
Photoacoustic spectroscopy is a highly sensitive technique, well suited for and used in applications targeting the accurate measurement of water vapor in a wide range of concentrations. This work demonstrates the nonlinear photoacoustic response obtained for water vapor in air at typical atmospheric concentration levels, which is a result of the resonant vibrational coupling of water and oxygen. Relevant processes in the relaxation path of water in a mixture with air, excited with near-infrared radiation, are identified and a physical model for the acoustic signal measured with a resonant photoacoustic cell is presented. The model is valid for modulation frequencies typical for conventional and quartz-enhanced photoacoustic spectroscopy and provides a simplified means of calibration for photoacoustic water vapor sensors. Estimated values for comprised model coefficients are evaluated from photoacoustic measurements of water vapor in synthetic air. Furthermore, it is shown experimentally that the process of vibrational excitation of nitrogen is of negligible importance in the relaxation path of water vapor and thus insignificant in the photoacoustic heat production in atmospheric measurement environments.
Highlights
The amount of published research on photoacoustic spectroscopy and the number of commercially available sensor systems based on this method are rising steadily, as technical advances allowed substantial progress in the limits of detection (LOD) and reduction of size and cost (e.g., [1, 2])
We demonstrated the significant and unfavorable effects of relaxation processes involving molecular oxygen, on the vibrational photoacoustic measurement of water vapor in air
The strong resonant coupling of the first vibrationally excited state of water vapor, H2O (0,1,0), with the long-living, first vibrationally excited state of molecular oxygen, O2(1), leads to a relaxation time that is large in comparison to typical modulation periods in photoacoustic spectroscopy
Summary
The amount of published research on photoacoustic spectroscopy and the number of commercially available sensor systems based on this method are rising steadily, as technical advances allowed substantial progress in the limits of detection (LOD) and reduction of size and cost (e.g., [1, 2]). While the technical advances lead to increased sensitivities and allow for detection at trace levels, the upper limits remain more or less unaltered, yielding increased dynamic ranges of the methods. Water vapor in atmospheric measurement environments can vary over a wide range of concentrations, which makes photoacoustic (PA) spectroscopy an ideal detection and measurement technique. In the case of an incorrect linear assumption, extrapolation inevitably leads to large errors in the predicted concentrations and in the predicted theoretical LOD
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