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NRMRL-RTP-P- 568 Childers, J.W., Phillips, W.J., Thompson*, E.L., Harris*, D.B., Kirchgessner*, D.A., Natschke, D.F., and Clayton, M.J. Comparison of an Innovative Nonlinear Algorithm to Classical Least Squares for Analyzing Open-Path Fourier Transform Infrared Spectra Collected at a Concentrated Swine Production Facility. Applied Spectroscopy 56 (3):325- 336 (2002). 12/28/2001 Open-path Fourier transform infrared (OP/FTIR) spectrometry was used to measure the concentrations of ammonia, methane, and other atmospheric gases at an integrated swine production facility. The cononcentration-pathlength products of the target gases at this site often exceeded the linear dynamic range of the OP/FTIR spectrometer. A piecewise, linear classical least squares (CLS) analysis method with a calibration set containing multiple reference spectra for each target gas at concentration-pathlength products that encompassed those found in the field spectra and an innovative nonlinear algorithm (NLA) were used to determine the path-averaged concentrations of the target gases in the OP/FTIR field spectra. The results from these two methods are compared to those produced by conventional linear CLS methods. The conventional CLS methods with a calibration set consisting of only one reference spectrum per target gas under- reported the target gas concentrations when the field spectra exhibited concentration-pathlength products larger than those of the reference spectra. The path-averaged concentrations reported by the NLA were generally higher than those determined by the piecewise, linear CLS method. The mean absolute percent differences between these two methods were 3.9% for methane, 6.8% for nitrous oxide, 7.7% for ammonia, and 14. 1 % for carbon dioxide.

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