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National annual total CO2 emissions from combustion of fossil fuelsare likely known to within 5–10 % for most developed countries. However,uncertainties are inevitably larger (by unknown amounts) for emissionestimates at regional and monthly scales, or for developing countries. Givenrecent international efforts to establish emission reduction targets,independent determination and verification of regional and national scalefossil fuel CO2 emissions are likely to become increasinglyimportant. Here, we take advantage of the fact that precise measurements of14C in CO2 provide a largely unbiased tracer for recentlyadded fossil-fuel-derived CO2 in the atmosphere and present anatmospheric inversion technique to jointly assimilate observations ofCO2 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 totalfossil fuel emission to better than 1 % for the year and to within5 % for most months. Increasing the number of 14CO2observations to  ∼ 5000 per year over North America, as recentlyrecommended 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 awhole and also for smaller, highly emissive regions over which the specifieddata coverage is relatively dense, such as for the New England states or theNY-NJ-PA tri-state area. This result suggests that, given continuedimprovement in state-of-the art transport models, a measurement programsimilar in scale to that recommended by the NAS can provide for independentverification of bottom-up inventories of fossil fuel CO2 at theregional and national scale. In addition, we show that the dual tracerinversion framework can detect and minimize biases in estimates of thebiospheric flux that would otherwise arise in a traditional CO2-onlyinversion when prescribing fixed but inaccurate fossil fuel fluxes

atmospheric chemistry and physics

Separation of biospheric and fossil fuel fluxes of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; by atmospheric inversion of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; measurements: Observation System Simulations