Separation of biospheric and fossil fuel fluxes of CO₂ by atmospheric inversion of CO₂ and ¹⁴CO₂ measurements: Observation System Simulations

Abstract. National annual total CO₂ emissions from combustion of fossil fuels are likely known to within 5–10 % for most developed countries. However, uncertainties are inevitably larger (by unknown amounts) for emission estimates at regional and monthly scales, or for developing countries. Given recent international efforts to establish emission reduction targets, independent determination and verification of regional and national scale fossil fuel CO₂ emissions are likely to become increasingly important. Here, we take advantage of the fact that precise measurements of ¹⁴C in CO₂ provide a largely unbiased tracer for recently added fossil-fuel-derived CO₂ in the atmosphere and present an atmospheric inversion technique to jointly assimilate observations of CO₂ and ¹⁴CO₂ in order to simultaneously estimate fossil fuel emissions and biospheric exchange fluxes of CO₂. Using this method in a set of Observation System Simulation Experiments (OSSEs), we show that given the coverage of ¹⁴CO₂ measurements available in 2010 (969 over North America, 1063 globally), we can recover the US national total fossil fuel emission to better than 1 % for the year and to within 5 % for most months. Increasing the number of ¹⁴CO₂ observations to  ∼ 5000 per year over North America, as recently recommended by the National Academy of Science (NAS) (Pacala et al., 2010), we recover monthly emissions to within 5 % for all months for the US as a whole and also for smaller, highly emissive regions over which the specified data coverage is relatively dense, such as for the New England states or the NY-NJ-PA tri-state area. This result suggests that, given continued improvement in state-of-the art transport models, a measurement program similar in scale to that recommended by the NAS can provide for independent verification of bottom-up inventories of fossil fuel CO₂ at the regional and national scale. In addition, we show that the dual tracer inversion framework can detect and minimize biases in estimates of the biospheric flux that would otherwise arise in a traditional CO₂-only inversion when prescribing fixed but inaccurate fossil fuel fluxes.