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 (¹⁴C) in methane (CH₄) may provide a method for identifying regional CH₄ emissions from fossil versus biogenic sources because adding ¹⁴C-free fossil carbon reduces the ¹⁴C/C ratio (Δ¹⁴CH₄) in atmospheric CH₄ much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH₄ at regional scales using atmospheric Δ¹⁴CH₄ observations. As a case study to demonstrate expected Δ¹⁴CH₄ and Δ¹⁴CH₄-CH₄ relationships, we simulate and compare Δ¹⁴CH₄ 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 Δ¹⁴CH₄-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 Δ¹⁴CH₄, which are likely detectable from regular observations. Nuclear power plant ¹⁴CH₄ emissions generally have small simulated median influences on Δ¹⁴CH₄ (0-7‰), but under certain atmospheric conditions they can be much stronger (>30‰) suggesting they must be considered in applications of Δ¹⁴CH₄ in California. This study suggests that atmospheric Δ¹⁴CH₄ measurements could provide powerful constraints on regional CH₄ emissions, complementary to other monitoring techniques.