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Linking Remote Sensing Estimates of XCO2 and XCH4 to Emissions
by Dr. David Crisp


Space-based remote sensing observations are now providing the data needed to estimate the atmospheric column-averaged dry air mole fractions of carbon dioxide (XCO2) and methane (XCH4) with unprecedented precision, accuracy, resolution and coverage. These products are being analyzed with atmospheric inverse models to estimate fluxes (emissions and removals) on spatial scales ranging from individual large fossil fuel-fired powerplants and large urban areas to nations and the globe. In spite of their potential utility, these remote sensing products are not yet widely used by the greenhouse gas (GHG) inventory community. Often-cited obstacles include (i) lingering concerns about the accuracy of remotely-sensed concentrations and fluxes; (ii) atmospheric measurement do not distinguish policy-relevant anthropogenic and natural processes; and (iii) bottom-up inventory compilers do not yet understand the information content and limitations of these data. Fortunately, there is progress in all three areas. Rigorous calibration and validation efforts are improving the accuracy of space-based XCO2 and XCH4 estimates. These estimates are now being routinely compared to ground-based results from the Total Carbon Column Observing Network (TCCON) network, which serves as a transfer standard between the space-based products and WMO atmospheric in situ CO2 and CH4 standards. Once corrected for known biases, space-based soundings have random errors near 0.2% and accuracies near 1 ppm for XCO2 and 10 ppb for XCH4. These estimates are being combined with ground-based and airborne in situ CO2 and CH4 data and analyzed with ensembles of atmospheric inverse models to estimate fluxes. These products are more challenging to validate on scales larger than individual smokestacks. Regional-scale fluxes are validated by comparing adjusted (posterior) CO2 and CH4 fields to withheld ground-based and airborne data or through comparisons with estimates from land and ocean biogeochemical models. Neither technique is completely satisfactory. In the future, vertical profiles of CO2 and CH4 from in situ sensors deployed on commercial aircraft (e.g., CONTRAIL, IAGOS) could be used to validate fluxes from large urban areas with busy airports. To resolve emission sources, space-based CO2 estimates are being combined with coincident observations of carbon monoxide (CO) and nitrogen dioxide (NO2) to distinguish low-temperature (biomass burning) and high-temperature (fossil fuel) combustion sources. For CH4, estimates from high-spatial-resolution hyperspectral imagers are being used to identify large emission sources. Finally, to build understanding and trust of these space-based remote sensing products by national inventory developers and their stakeholders, several organizations (NASA ARSET, IG3IS, CEOS) have begun developing capacity building activities.

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Topic : Theme 2: Strengthening the linkage of remote sensing GHG concentration measurements to emission fluxes.
Reference : T2-D6

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