CARBON ISOTOPIC COMPOSITION OF ATMOSPHERIC CH4: FOSSIL AND BIOMASS BURNING SOURCE STRENGTHS

The C-13/C-12 of atmospherimc ethane (CH4) was measured at Point Barrow (71 degrees N, 156 degrees W), Olympic Peninsula(48 degrees N, 126 degrees W), Mauna Loa (19 degrees N, 155 degrees W), and Cape Grim (41 degrees S, 144 degrees E) between 1987 and 1989. The global average 1513C(PDB) from these measurement (n = 208) was -47.20 +/- 0.13 parts per thousand. The lowest mean annual delta C-13 value of -47.61 +/- 0.14 parts per thousand was measured at Point Barrow with values increasing to -47.03 +/- 0.14 parts per thousand at Cape Grim. The seasonal cycle in the delta C-13 of CH4 was greatest at Point Barrow, with an amplitude of 0.5 parts per thousand, and varied inversely with concentration. The isotopic fractionation during CH4 oxidation is calculated to be 0.993 +/- 0.002 based on the measured CH4 concentration and delta C-13 values. The C-14 content of atmospheric CH4, 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 (CH4)-C-14 release from nuclear reactors. The global average C-14 content of 122 pM for CH4 implies a fossil methane source strength that is 16% of the total source. The global mean delta C-13 of -47.2 parts per thousand, when coupled with the C-14 results, implies that similar to 11% of the total CH4 release rate is derived from biomass burning. These results indicate for a total CH4 source of similar to 550 Tg yr-1 that natural gas release accounts for similar to 90 Tg yr(-1) and biomass burning yields similar to 60 Tg yr(-1). Preliminary analyses of the delta C-13 data using a three-dimensional chemical tracer model indicate that the observed meridional gradients in the annual average delta C-13 and concentration of CH4 are most closely matched with a CH4 source scenario in which 11% of the CH4 is derived from biomass burning.