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

Abstract

Regional emissions of methane and their attribution to a variety of sources presently have large uncertainties. Measurements of radiocarbon ( 14 C) in methane (CH 4 ) may provide a method for identifying regional CH 4 emissions from fossil versus biogenic sources because adding 14 C‐free fossil carbon reduces the 14 C/C ratio (Δ 14 CH 4 ) in atmospheric CH 4 much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH 4 at regional scales using atmospheric Δ 14 CH 4 observations. As a case study to demonstrate expected Δ 14 CH 4 and Δ 14 CH 4 ‐CH 4 relationships, we simulate and compare Δ 14 CH 4 at a network of sites in California using two gridded CH 4 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 4 (F) is closely linked with the simulated Δ 14 CH 4 ‐CH 4 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 4 produce differences of >10‰ in Δ 14 CH 4 , which are likely detectable from regular observations. Nuclear power plant 14 CH 4 emissions generally have small simulated median influences on Δ 14 CH 4 (0–7‰), but under certain atmospheric conditions they can be much stronger (>30‰) suggesting they must be considered in applications of Δ 14 CH 4 in California. This study suggests that atmospheric Δ 14 CH 4 measurements could provide powerful constraints on regional CH 4 emissions, complementary to other monitoring techniques.

Radiocarbon dating

“…Combined atmospheric and ocean observations of the 14 C bomb peak evolution provide information on the uptake of anthropogenic CO2 emissions by the ocean 196 and modeling of future atmospheric 14 C content 197 . Radiocarbon analysis is a crucial tool for qualitative estimations of fossil fuel emissions in urban regions [198][199][200][201] .…”

Radiocarbon in Marine Methane Reveals Patchy Impact of Seeps on Surface Waters

“…Fossil CH 4 is devoid of measurable 14 C and can be older than approximately 57,300 years (i.e., ten 14 C half‐lives), which is equivalent to 0% Modern Carbon (pMC) (Stuiver & Polach, 1977). While modern levels of 14 C in CO 2 are approximately 100 pMC and decreasing yearly, atmospheric CH 4 has a 14 C signature of approximately 135 pMC due to the influence of 14 C‐enriched effluent from nuclear reactors (Eisma et al, 1995; Graven et al, 2019; Kessler et al, 2008; Lassey et al, 2007; Townsend‐Small et al, 2012). Thus, measurements of the natural 14 C content of marine CH 4 , together with conventional concentration and stable isotope measurements of CH 4 , can provide reliable information to determine the extent of mixing between different CH 4 sources in surface waters (e.g., Kessler et al, 2008; Sparrow et al, 2018).…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…The global 14 CH 4 budget is balanced by three flux factors: (i) emissions of 14 CH 4 from nuclear power plants and military use of nuclear propulsion, (ii) biogenic emissions whose carbon has ultimately been derived from atmospheric CO 2 (which contains 14 CO 2 ), and (iii) fossil CH 4 which is devoid of 14 CH 4 . As with the stable isotope ratios, a more immediate benefit of Δ 14 CH 4 could come at the regional scale; however, in regions where nuclear sources exist, careful consideration needs to be paid to properly assess and quantify this interfering component (Graven et al, ; Townsend‐Small et al, ). Coupling regional transport models to measurements of Δ 14 CH 4 would allow quantification of the fossil component of CH 4 ; however, the sampling methods to make this possible are only starting to be developed (Espic et al, ).…”