Atmospheric radiocarbon measurements to quantify CO2 emissions in the UK from 2014 to 2015

We present Delta(CO2)-C-14 observations and related greenhouse gas measurements at a background site in Ireland (Mace Head, MHD) and a tall tower site in the east of the UK (Tacolneston, TAC) that is more strongly influenced by fossil fuel sources. These observations have been used to calculate the contribution of fossil fuel sources to the atmospheric CO2 mole fractions; this can be done, as emissions from fossil fuels do not contain (CO2)-C-14 and cause a depletion in the observed Delta(CO2)-C-14 value. The observations are compared to simulated values. Two corrections need to be applied to radiocarbon-derived fossil fuel CO2 (ffCO(2)): one for pure (CO2)-C-14 emissions from nuclear industry sites and one for a disequilibrium in the isotopic signature of older biospheric emissions (heterotrophic respiration) and CO2 in the atmosphere. Measurements at both sites were found to only be marginally affected by (CO2)-C-14 emissions from nuclear sites. Over the study period of 2014-2015, the biospheric correction and the correction for nuclear (CO2)-C-14 emissions were similar at 0.34 and 0.25 ppm ffCO(2) equivalent, respectively. The observed ffCO(2) at the TAC tall tower site was not significantly different from simulated values based on the EDGAR 2010 bottom-up inventory. We explored the use of high-frequency CO observations as a tracer of ffCO(2) by deriving a constant ratio of CO enhancements to ffCO(2) ratio for the mix of UK fossil fuel sources. This ratio was found to be 5.7 ppb ppm(-1), close to the value predicted using inventories and the atmospheric model of 5.1 ppb ppm(-1). The TAC site, in the east of the UK, was strategically chosen to be some distance from pollution sources so as to allow for the observation of well-integrated air masses. However, this distance from pollution sources and the large measurement uncertainty in (CO2)-C-14 lead to a large overall uncertainty in the ffCO(2), being around 1.8 ppm compared to typical enhancements of 2 ppm.