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

Abstract. 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 investigated the accuracy of such retrievals for CO₂, CH₄ 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 most sensitive to the ILS (a function of the assumed instrument field-of-view), which is due to the narrow nature of CO absorption lines and their consequent sensitivity to convolution with the ILS. Conversely, CO₂ retrievals were most sensitive to assumed atmospheric parameters, particularly gas temperature. Our findings provide confidence that FTIR-derived trace gas retrievals of CO₂, CH₄ and CO based on modeling can yield results with high accuracies, even over very large (many order of magnitude) concentration ranges that can prove difficult to retrieve via standard classical least squares (CLS) techniques. With the methods employed here, we suggest that errors in the retrieved trace gas concentrations should remain well below 10%, even with the uncertainties in atmospheric pressure and temperature that might arise when studying plumes in more difficult field situations (e.g. at uncertain altitudes or temperatures).