Independent evaluation of point source fossil fuel CO2 emissions to better than 10%

Independent estimates of fossil fuel CO₂ (CO₂ff) emissions are key to ensuring that emission reductions and regulations are effective and provide needed transparency and trust. Point source emissions are a key target because a small number of power plants represent a large portion of total global emissions. Currently, emission rates are known only from self-reported data. Atmospheric observations have the potential to meet the need for independent evaluation, but useful results from this method have been elusive, due to challenges in distinguishing CO₂ff emissions from the large and varying CO₂ background and in relating atmospheric observations to emission flux rates with high accuracy. Here we use time-integrated observations of the radiocarbon content of CO₂ (¹⁴CO₂) to quantify the recently added CO₂ff mole fraction at surface sites surrounding a point source. We demonstrate that both fast-growing plant material (grass) and CO₂ collected by absorption into sodium hydroxide solution provide excellent time-integrated records of atmospheric ¹⁴CO₂ These time-integrated samples allow us to evaluate emissions over a period of days to weeks with only a modest number of measurements. Applying the same time integration in an atmospheric transport model eliminates the need to resolve highly variable short-term turbulence. Together these techniques allow us to independently evaluate point source CO₂ff emission rates from atmospheric observations with uncertainties of better than 10%. This uncertainty represents an improvement by a factor of 2 over current bottom-up inventory estimates and previous atmospheric observation estimates and allows reliable independent evaluation of emissions.