Supplementary material to "Atmospheric radiocarbon measurements to quantify CO<sub>2</sub> emissions in the UK from 2014 to 2015"

Novel quantification of regional fossil fuel CO2reductions during COVID-19 lockdowns using atmospheric oxygen measurements

“…14 CO 2 , the current “gold-standard” ground-based ffCO 2 tracer method, is a high-precision measurement that has been recently used to successfully provide a top-down assessment of ffCO 2 emissions in the United States ( 11 ). The main limitations of using 14 CO 2 are twofold: First, it is currently only possible to measure atmospheric 14 CO 2 with high accuracy from discrete samples (i.e., noncontinuously, with relatively low temporal resolution), which are moreover expensive and laborious to analyze ( 30 ); second, in some regions, such as the United Kingdom, 14 CO 2 measurements can be severely influenced by CO 2 emissions from gas-cooled nuclear power plants, which obscure ffCO 2 signals in 14 CO 2 data ( 31 , 32 ). CO, a continuous high-frequency ffCO 2 tracer that is easier to measure and is unaffected by nuclear power plant emissions, can also be used, either as an alternative to 14 CO 2 sampling or in conjunction with 14 CO 2 ; however, CO-based ffCO 2 is limited by poor precision and accuracy, mostly arising from highly variable and inaccurate CO:ffCO 2 emission ratio information, which is required for CO-based ffCO 2 quantification ( 24 , 25 ).…”