Application of Scanning Open Path Fourier Transform Infrared Spectroscopy (OP/FT-IR) to Measure Greenhouse Gas (GHG) Concentrations Emitted from Agricultural Soils.

Open-path Fourier Transform Infrared spectroscopy (OP/FT-IR) is a remote sensing approach to measuring greenhouse gases (GHGs) emitted from area agricultural sources. Since scanning OP/FT-IR is capable of measuring the concentrations of multiple gases simultaneously in real time (1-minute sampling intervals) over long paths (order of 100 m) and extended period (days or months), we can provide temporally and spatially representative gas concentrations over multiple area sources. However, this technique is also subject to the challenges as follows. First of all is that the lack of being able to collect a “GHG-free” reference/background OP/FT-IR spectrum (e.g. evacuation of the field). Secondly, the strong contribution of water vapor to the mid-IR spectrum affects the gas quantification from the spectral analysis. Third, the variations in air temperature change the absorbance characteristics of gases and affect the accuracy of gas quantification. In this study, we used OP/FT-IR to determine the concentrations of CO₂, N₂O, and CH₄ emitted from agricultural soils and developed the quantitative methods that can deal with the challenges mentioned above and increase the accuracy of predicted GHG concentrations. Results showed that the biases in the FTIR-predicted GHG concentrations increased with the increasing the interference of water vapor in the spectral windows. Based on the optimal quantitative methods, the biases in CO₂ and N₂O predictions reduced from 8.8±4.8% to 0.3±2.5% and from 13.7±4.6% to -0.4±3.1%, respectively. Furthermore, the biases in N₂O predictions were also associated with the varying air temperature and the increasing temperature led higher biases.