Abstract. Open-path Fourier transform infrared spectroscopy
(OP-FTIR) has often been used to measure hazardous or trace gases from hot
point sources (e.g. volcano, industrial, or agricultural facilities) but
seldom used to measure greenhouse gases (GHGs) from field-scale sources
(e.g. agricultural soils). Closed-path mid-IR laser-based N2O,
nondispersive-IR CO2 analysers, and OP-FTIR were used to measure
concentrations of N2O and CO2 at a maize cropping system during
9–19 June 2014. To measure N2O and CO2 concentrations accurately, we
developed a quantitative method of N2O∕CO2 analysis that minimized
interferences from diurnal changes of humidity and temperature. Two
chemometric multivariate models, classical least squares (CLS) and partial
least squares (PLS), were developed. This study evaluated various methods to
generate the single-beam background spectra and different spectral regions
for determining N2O and 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 an absolute
humidity range of 5000–20 000 ppmv and a temperature range 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 calculated N2O
concentrations (bias =1.4±2.3 %). The bias of calculated
CO2 concentrations was -1.0±2.8 % using the CLS model. These
methods suggested that environmental variables potentially lead to biases in
N2O and CO2 estimations from OP-FTIR spectra and may help OP-FTIR
users avoid dependency on extractive methods of calibrations.
Trace gas emissions from the production and use of domestic biofuels in Zambia measured by open-path Fourier transform infrared spectroscopy