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) in methane (CH) may provide a method for identifying regional CH emissions from fossil versus biogenic sources because adding C-free fossil carbon reduces the C/C ratio (ΔCH) in atmospheric CH much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH at regional scales using atmospheric ΔCH observations. As a case study to demonstrate expected ΔCH and ΔCH-CH relationships, we simulate and compare ΔCH at a network of sites in California using two gridded CH 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 CH (F) is closely linked with the simulated ΔCH-CH 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 CH produce differences of >10‰ in ΔCH, which are likely detectable from regular observations. Nuclear power plant CH emissions generally have small simulated median influences on ΔCH (0-7‰), but under certain atmospheric conditions they can be much stronger (>30‰) suggesting they must be considered in applications of ΔCH in California. This study suggests that atmospheric ΔCH measurements could provide powerful constraints on regional CH emissions, complementary to other monitoring techniques.