Carbon isotopic composition of atmospheric CH4: Fossil and biomass burning source strengths

The 13 C/ 12 C of atmospheric methane (CH 4 ) was measured at Point Barrow (71°N, 156°W), Olympic Peninsula (48°N, 126°W), Mauna Loa (19°N, 155°W), and Cape Grim (41°S, 144°E) between 1987 and 1989. The global average δ 13 C PDB from these measurements (n = 208) was −47.20 ± 0.13%o. The lowest mean annual δ 13 C value of‐47.61 ± 0.14‰ was measured at Point Barrow with values increasing to ‐47.03 ± 0.14‰ at Cape Grim. The seasonal cycle in the δ 13 C of CH 4 was greatest at Point Barrow, with an amplitude of 0.5‰, and varied inversely with concentration. The isotopic fractionation during CH 4 oxidation is calculated to be 0.993 ± 0.002 based on the measured CH 4 concentration and δ 13 C values. The 14 C content of atmospheric CH 4 , measured at monthly intervals at the Olympic Peninsula site between 1987 and 1989, is increasing at 1.4 ± 0.5 pM yr −1 , primarily owing to 14 CH 4 release from nuclear reactors. The global average 14 C content of 122 pM for CH 4 implies a fossil methane source strength that is 16% of the total source. The global mean δ 13 C of −47.2‰, when coupled with the 14 C results, implies that ∼11% of the total CH 4 release rate is derived from biomass burning. These results indicate for a total CH 4 source of ∼550 Tg yr −1 that natural gas release accounts for ∼90 Tg yr −1 and biomass burning yields ∼60 Tg yr −1 . Preliminary analyses of the δ 13 C data using a three‐dimensional chemical tracer model indicate that the observed meridional gradients in the annual average δ 13 C and concentration of CH 4 are most closely matched with a CH 4 source scenario in which 11% of the CH 4 is derived from biomass burning.