Detection of Fossil and Biogenic Methane at Regional Scales Using Atmospheric Radiocarbon

Regional emissions of methane and their attribution to a variety of sources presently have large uncertainties. Measurements of radiocarbon (C-14) in methane (CH4) may provide a method for identifying regional CH4 emissions from fossil versus biogenic sources because adding C-14-free fossil carbon reduces the C-14/C ratio (Delta(CH4)-C-14) in atmospheric CH4 much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH4 at regional scales using atmospheric Delta(CH4)-C-14 observations. As a case study to demonstrate expected Delta(CH4)-C-14 and Delta(CH4)-C-14-CH4 relationships, we simulate and compare Delta(CH4)-C-14 at a network of sites in California using two gridded CH4 emissions estimates (Emissions Database for Global Atmospheric Research, EDGAR, and Gridded Environmental Protection Agency, GEPA) and the CarbonTracker-Lagrange model for 2014, and for 2030 under business-as-usual and mitigation scenarios. The fossil fraction of CH4 (F) is closely linked with the simulated Delta(CH4)-C-14-CH4 slope and differences of 2-21% in median F are found for EDGAR versus GEPA in 2014, and 7-10% for business-as-usual and mitigation scenarios in 2030. Differences of 10% in F for >200 ppb of added CH4 produce differences of >10 in Delta(CH4)-C-14, which are likely detectable from regular observations. Nuclear power plant (CH4)-C-14 emissions generally have small simulated median influences on Delta(CH4)-C-14 (0-7), but under certain atmospheric conditions they can be much stronger (>30) suggesting they must be considered in applications of Delta(CH4)-C-14 in California. This study suggests that atmospheric Delta(CH4)-C-14 measurements could provide powerful constraints on regional CH4 emissions, complementary to other monitoring techniques.