Sources of error in open-path FTIR measurements of N2O and CO2 emitted from agricultural fields

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 o...

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Bibliographic Details
Published in:Atmospheric measurement techniques 2020-04, Vol.13 (4), p.2001-2013
Main Authors: Cheng-Hsien, Lin, Grant, Richard H, Heber, Albert J, Johnston, Cliff T
Format: Article
Language:English
Subjects:
Accuracy
Agricultural land
Analytical methods
Carbon dioxide
Error analysis
Field tests
Fourier transforms
Gases
Humidity
Influence
Infrared spectroscopy
Least squares
Length
Methods
Nitrous oxide
Nonlinear response
Quality assurance
Spectra
Spectrum analysis
Temperature
Temperature effects
Uncertainty
Water vapor
Water vapour
Wind
Wind speed
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Summary: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 (N2O) and carbon dioxide (CO2) derived from OP-FTIR spectra. The presence of water vapour in spectra underestimated N2O 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 N2O 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. N2O and CO2 mole fractions were underestimated by 30 % and 7.5 %, respectively, at the optical path of 100 m using CLS models). For N2O 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 (
ISSN:1867-1381
1867-8548
DOI:10.5194/amt-13-2001-2020