Sources of error in open-path FTIR measurements of N2 O and CO2 emitted from agricultural fields

Abstract

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 (N2 O) and carbon dioxide (CO2 ) derived from OP-FTIR spectra. The presence of water vapour in spectra underestimated N2 O 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 C). Differences in temperature between sample and reference spectra also underestimated N2 O 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. N2 O and CO2 mole fractions were underestimated by 30% and 7.5 %, respectively, at the optical path of 100m using CLS models). For N2 O 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 (< 2ms-1). This study identified the most common interferences that affect OP-FTIR measurements of N2 O and CO2 , which can serve as a quality assurance/control guide for current or future OPFTIR users.