A study of the sources and sinks of methane and methyl chloroform using a global three-dimensional Lagrangian tropospheric tracer transport model

By comparison with methyl chloroform observations a global average tropospheric hydroxyl radical concentration of 6.4 × 10⁵ cm^-3 was found to be consistent with published methyl chloroform emission data for the year 1980. Published methyl chloroform emissions data for 1981-1984 were found to be inconsistent with the observed methyl chloroform concentration increases. Using the hydroxyl radical field calibrated to the methyl chloroform observations, two source function models of the spatial and temporal distribution of the flux of methane to the atmosphere were developed. The first model was based on the assumption that methane is emitted as a proportion of net primary productivity (NPP). With the average hydroxyl radical concentration fixed, the methane source term was computed as ~623 Tg CH₄, giving an atmospheric lifetime for methane ~8.3 years. The second model identified source regions for methane from rice paddies, wetlands, enteric fermentation, termites, and biomass burning based on high-resolution land use data. This methane source distribution resulted in an estimate of the global total methane source of ~611 Tg CH₄, giving an atmospheric lifetime for methane ~8.5 years. -from Authors