Methane Emissions Deduced from a Two-Dimensional Atmospheric Transport Model and Surface Measurements

Abstract

Latitudinal and temporal distributions of CH₄ emission were estimated by an iterative inverse method using a two-dimensional atmospheric transport model and the 1983-1994 CH₄ concentration data from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Sampling Network. The atmospheric transport of the model was validated by simulating the concentrations of ⁸⁵Kr, CFC-11, CFC-12 and CO₂ observed at various locations world wide. A zonally averaged OH field for the destruction of CH₄ in the atmosphere, originally derived from a three-dimensional photochemical transport model, was adjusted to simulate the observed concentration of atmospheric CH₃CCl₃. A calculated average latitudinal distribution of CH₄ emission showed a large north-south difference, with about 75% of the total global emission residing in the northern hemisphere. The CH₄ emission varied seasonally in high latitudes of the northern hemisphere, with a maximum in the summer season, while no seasonality of the CH₄ release was found in the southern hemisphere. Averaged global emission of the natural and anthropogenic CH₄ for the period 1984-1994 was estimated to be 559±9Tg/yr, with chemical loss of 528±10Tg/yr and the atmospheric increase of 31Tg/yr. In sensitivity experiments of the model results, the global emission of CH₄ was found to be sensitive to the OH concentration and the atmospheric temperature but less to the atmospheric transport coefficients and the CH₄ concentration data used. The latitudinal CH₄ emission distribution was dependent largely on the specification of the horizontal transport coefficients. It was also found that the δ¹³C value of a bacterial source associated with a large amount of CH₄ emission, as well as the soil absorption process of CH₄ with a large kinetic isotopic fractionation, significantly impacts the determination of δ¹³C in atmospheric CH₄.