Application of Open Path Fourier Transform Infrared Spectroscopy
(OP-FTIR) to Measure Greenhouse Gas Concentrations from
Agricultural Soils
Abstract. Open-path Fourier transform infrared spectroscopy (OP-FTIR) has often been used to measure hazardous or trace gases from the "hot" point sources (e.g., volcano, industrial or agricultural facilities) but seldom used in the field-scale source areas, such as soil emissions. OP-FTIR, the close-path mid-IR laser-based N2O, and the nondispersive-IR CO2 analyzers were used to measure the concentrations of greenhouse gases (e.g., N2O and CO2) emitted from agricultural soils over a period of 9−19 June in 2014. We developed a quantitative method of N2O/CO2 analysis that minimized the interferences from diurnal changes of humidity and temperature in order to measure N2O/CO2 concentrations accurately. Two chemometric multivariate models were developed, a classical least squares (CLS) and a partial least squares (PLS), respectively. This study evaluated different methods to generate the single beam background spectra, and different spectral regions to determine N2O/CO2 concentrations from OP-FTIR spectra. A standard extractive method was used to measure the "actual" path-averaged concentrations along an OP-FTIR optical path in situ, as a benchmark to assess the feasibilities of these quantitative methods. Within the absolute humidity of 5000−20 000 ppmv and the temperature of 10−35 °C, we found that the CLS model underestimated N2O concentrations (Bias = −4.9 ± 3.1 %) calculated from OP-FTIR spectra, and the PLS model improved the accuracy of the calculated N2O (Bias = 1.4 ± 2.3 %). The bias of the calculated CO2 was −1.0 ± 2.8 % using the CLS model. These methods suggested that the changed ambient factors potentially led to biases in N2O/CO2 estimations from OP-FTIR spectra, and may help the OP-FTIR user to escape from the dependency of extractive methods used to calibrate the concentration determined by OP-FTIR.