Sources of error in open-path FTIR measurements of N 2O and CO 2 emitted from agricultural fields

Lin, Cheng-Hsien; Grant, Richard H.; Heber, Albert J.; Johnston, Cliff T.

Open-path Fourier transform infrared spectroscopy (OP-FTIR) is susceptible to environmental variables which can become sources of errors for gas quantification. In this study, we assessed the effects of water vapour, temperature, path length, and wind speed on quantitative uncertainties of nitrous oxide (inline-formulaN2O) and carbon dioxide (inline-formulaCO2) derived from OP-FTIR spectra. The presence of water vapour in spectra underestimated inline-formulaN2O mole fractions by 3 % and 12 %, respectively, from both lab and field experiments using a classical least squares (CLS) model when the reference and sample spectra were collected at the same temperature (i.e. 30 inline-formulaC). Differences in temperature between sample and reference spectra also underestimated inline-formulaN2O mole fractions due to temperature broadening and the increased interferences of water vapour in spectra of wet samples. Changes in path length resulted in a non-linear response of spectra and bias (e.g. inline-formulaN2O and inline-formulaCO2 mole fractions were underestimated by 30 % and 7.5 %, respectively, at the optical path of 100 m using CLS models). For inline-formulaN2O quantification, partial least squares (PLS) models were less sensitive to water vapour, temperature, and path length and provided more accurate estimations than CLS. Uncertainties in the path-averaged mole fractions increased in low-wind conditions (inline-formula<2 m sinline-formula−1). This study identified the most common interferences that affect OP-FTIR measurements of inline-formulaN2O and inline-formulaCO2, which can serve as a quality assurance/control guide for current or future OP-FTIR users.

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Lin, Cheng-Hsien / Grant, Richard H. / Heber, Albert J. / et al: Sources of error in open-path FTIR measurements of N2O and CO2 emitted from agricultural fields. 2020. Copernicus Publications.

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