Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes"
When compared to established point-samplingmethods, Open-Path Fourier Transform Infrared (OP-FTIR)spectroscopy can provide path-integrated concentrations ofmultiple gases simultaneously, in situ and near-continuously.The trace gas pathlength amounts can be retrieved from themeasured IR spectra using a forward model coupled to anon-linear least squares fitting procedure, without requiring¿background¿ spectral measurements unaffected by the gasesof interest. However, few studies have investigated the accuracyof such retrievals for CO2, CH4 and CO, particularlyacross broad concentration ranges covering those characteristicof ambient to highly polluted air (e.g. from biomassburning or industrial plumes). Here we perform such an assessmentusing data collected by a field-portable FTIR spectrometer.The FTIR was positioned to view a fixed IR sourceplaced at the other end of an IR-transparent cell filled withthe gases of interest, whose target concentrations were variedby more than two orders of magnitude. Retrievals madeusing the model are complicated by absorption line pressurebroadening, the effects of temperature on absorption bandshape, and by convolution of the gas absorption lines and theinstrument line shape (ILS). Despite this, with careful modelparameterisation (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 retrievedto within 5%. Sensitivity to the aforementioned model inputswas also investigated. CO retrievals were shown to be most sensitive to the ILS (a function of the assumed instrumentfield-of-view), which is due to the narrow nature of COabsorption lines and their consequent sensitivity to convolutionwith the ILS. Conversely, CO2 retrievals were most sensitiveto assumed atmospheric parameters, particularly gastemperature. Our findings provide confidence that FTIRderivedtrace gas retrievals of CO2, CH4 and CO based onmodeling can yield results with high accuracies, even oververy large (many order of magnitude) concentration rangesthat can prove difficult to retrieve via standard classical leastsquares (CLS) techniques. With the methods employed here,we suggest that errors in the retrieved trace gas concentrationsshould remain well below 10%, even with the uncertaintiesin atmospheric pressure and temperature that mightarise when studying plumes in more difficult field situations(e.g. at uncertain altitudes or temperatures).
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