Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes"

T. E. L. Smith, M. J. Wooster, M. Tattaris, D. W. T. Griffith
2011 Atmospheric Measurement Techniques  
When compared to established point-sampling methods, Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy can provide path-integrated concentrations of multiple gases simultaneously, in situ and near-continuously. The trace gas pathlength amounts can be retrieved from the measured IR spectra using a forward model coupled to a non-linear least squares fitting procedure, without requiring "background" spectral measurements unaffected by the gases of interest. However, few studies have
more » ... w studies have investigated the accuracy of such retrievals for CO 2 , CH 4 and CO, particularly across broad concentration ranges covering those characteristic of ambient to highly polluted air (e.g. from biomass burning or industrial plumes). Here we perform such an assessment using data collected by a field-portable FTIR spectrometer. The FTIR was positioned to view a fixed IR source placed at the other end of an IR-transparent cell filled with the gases of interest, whose target concentrations were varied by more than two orders of magnitude. Retrievals made using the model are complicated by absorption line pressure broadening, the effects of temperature on absorption band shape, and by convolution of the gas absorption lines and the instrument line shape (ILS). Despite this, with careful model parameterisation (i.e. the optimum wavenumber range, ILS, and assumed gas temperature and pressure for the retrieval), concentrations for all target gases were able to be retrieved to within 5%. Sensitivity to the aforementioned model inputs was also investigated. CO retrievals were shown to be Introduction In many applications, remote sensing of gas species presence and concentrations offer advantages over established point-sampling and/or laboratory analysis methods. Openpath (OP) Fourier Transform Infrared (FTIR) spectroscopy can be used to detect and quantify a wide range of gases simultaneously; can operate in situ, eliminating contamination from tubing or sample handling; can operate continuously, providing real-time data at a relatively high temporal resolution (seconds); and can be used over long path lengths, providing path-integrated gas concentrations less prone to Published by Copernicus Publications on behalf of the European Geosciences Union.
doi:10.5194/amt-4-97-2011 fatcat:wklh3bzdrfho7kg5d5poatxmuu