Robust, spatially scanning, open-path TDLAS hygrometer using retro-reflective foils for fast tomographic 2-D water vapor concentration field measurements

摘要

We have developed a fast, spatially scanning direct tunable diode laserabsorption spectrometer (dTDLAS) that combines four polygon-mirror basedscanning units with low-cost retro-reflective foils. With this instrument,tomographic measurements of absolute 2-D water vapor concentration profilesare possible without any calibration using a reference gas.A spatial area of 0.8 m × 0.8 m was covered, which allows for application insoil physics, where greenhouse gas emission from certain soil structuresshall be monitored. The whole concentration field was measured with up to2.5 Hz. In this paper, we present the setup and spectroscopic performance ofthe instrument regarding the influence of the polygon rotation speed andmode on the absorption signal. Homogeneous HO distributions weremeasured and compared to a single channel, bi-static reference TDLASspectrometer for validation of the instrument. Good accuracy and precisionwith errors of less than 6% of the absolute concentration and length andbandwidth normalized detection limits of up to 1.1 ppmv · m (Hz) were achieved.The spectrometer is a robust and easy to set up instrument for tomographicreconstructions of 2-D-concentration fields that can be considered as a goodbasis for future field measurements in environmental research.