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)

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-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)

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

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Direct measurements of the seasonality of emission factors from savanna fires in northern Australia

Current good practice guidelines for national greenhouse gas inventories requires that seasonal variation in emission factors from savanna fires be considered when compiling national accounts. African studies concluded that the emission factor for methane decreases during the dry season principally due to curing of the fuels. However, available data from Australian tropical savannas shows no effect of seasonality on emission factors, consistent with observations that the fine fuels appear to cure fully soon after the start of the fire season. To test whether the seasonality in greenhouse gas emission factors reported for Africa also occurs in Australia, methane and nitrous oxide emission factors were measured in early and in late dry season fires in Western Arnhem Land, a region typical of much of the northern Australia savanna zone. We found no significant seasonality in methane emission factors, but there was substantial variation in emission factors associated with inter-fire differences in vegetation and fuel. This variation could be explained almost completely by combustion efficiency. Nitrous oxide emission factors were not related to combustion efficiency but showed some variation across vegetation and fuel size class. Both methane and nitrous oxide emission factors were consistent with previous work in northern Australia and with some published values from Africa. The absence of a significant seasonal trend in emission factors indicates that savanna fire emissions in northern Australia can be managed by strategic prescribed burning