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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-263
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-263
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 12 Aug 2019

Submitted as: research article | 12 Aug 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Measurement Techniques (AMT).

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

Cheng-Hsien Lin1, Richard H. Grant1, Albert J. Heber2, and Cliff T. Johnston1,3 Cheng-Hsien Lin et al.
  • 1Department of Agronomy, Purdue University, West Lafayette, IN 47907, United States
  • 2Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907, United States
  • 3Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907

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 the uncertainty of nitrous oxide (N2O) and carbon dioxide (CO2) concentrations derived from OP-FTIR spectra. The presence of water vapour resulted in underestimating N2O in both lab (−3 %) and field (−12 %) experiments at 30 °C using a classical least squares (CLS) model. Differences in temperature between the sample and reference spectra also underestimated N2O concentrations 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 concentrations 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 than CLS to the influence of water vapour, temperature, and path length, and provided more accurate estimations. Uncertainties in the path-averaged concentrations increased in low wind conditions (< 2 m s-1). This study identified the most common interferences that affect OP-FTIR measurements of N2O and CO2. The quantified errors can be used as references for current or future OP-FTIR users.

Cheng-Hsien Lin et al.
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Cheng-Hsien Lin et al.
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Short summary
Gas quantification using the open-path Fourier transform infrared spectrometer (OP-FTIR) is subject to interferences of environmental variables, leading to errors in gas concentration calculations. This study investigated the effects of ambient water vapour content, temperature, path lengths, and wind speed on the quantification of N2O and CO2 concentrations, which can help the OP-FTIR users to avoid these errors and improve the precision and accuracy of the atmospheric gas quantification.
Gas quantification using the open-path Fourier transform infrared spectrometer (OP-FTIR) is...
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