Use of 14C for quantification of fossil methane in central London and evaluation of 14C emissions from nuclear power plants

Measurements of radiocarbon (14C) in atmospheric methane provide a very powerful tool to distinguish fossil from biogenic methane emissions, because fossil methane is completely devoid of 14C. However, these measurements are particularly challenging, due to difficulties in the sampling procedure and in assessing the influence of 14C emissions from nuclear power plants. At Imperial College London we developed a sampling system that facilitates collection of methane samples for 14C analysis (Zazzeri et al. 2021), enabling routine measurements of 14CH4 in central London. While in Europe it is particularly difficult to discern the influence of nuclear power plants on 14CH4 measurements due to the large number of pressurise water reactors operative in most of European countries, in London it is possible to ignore this bias by collecting samples when air is coming from the Atlantic or north of the UK. Samples collected when air was originating from Europe show values more than two times higher than the expected background (350 ) (Sparrow et al. 2018, Hmiel et al. 2020). In this study we use samples collected in central London in summer 2020 to quantify the proportion of regional fossil CH4 emissions and show that emission inventories largely underestimate the fossil fraction of CH4. We also assess the influence of European nuclear power plants in London by simulating 14CH4 values using the Met Office atmospheric dispersion model NAME coupled with annual 14C emissions provided by the European Commission RAdioactive Discharges Database (RADD) and with emission factor based 14C estimates. We demonstrate that simulated 14CH4 emissions from nuclear power plants are too low to match the 14CH4 observations, perhaps due to variable 14C emission rates from the reactors and venting procedures that are not included in the reported values. Hmiel, B et al. Nature (2020), 578, 409412 Sparrow, K. J et al. Sci. Adv. (2018), 4, No. eaao4842 Zazzeri, G et al. Environ Sci Technol (2021)