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

Abstract

Sources and sinks of methane and methyl chloroform are investigated using a global three‐dimensional Lagrangian tropospheric tracer transport model with parameterized hydroxyl and temperature fields. By comparison with methyl chloroform observations a global average tropospheric hydroyl 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. A large decrease in hydroxyl radical concentrations could explain the disagreement between the emission data and atmospheric methyl chloroform concentrations, but this is unlikely. Using the hydroxyl radical field calibrated to the methyl chloroform observations, the globally averaged release of methane and its spatial and temporal distribution were investigated. 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. The most significant difference between the two models were predictions of methane fluxes over China and South East Asia, the location of most of the world's rice paddies, indicating that either the assumption that a uniform fraction of NPP is converted to methane is not valid for rice paddies, or that NPP is underestimated for rice paddies, or that present methane emission estimates from rice paddies are too high. Using a recent measurement of the reaction rate of hydroxyl radical and methane by Vaghjiani and Ravishankara (G. L. Vaghjiani and A. R. Ravishankara, Rate coefficient for the reaction of OH and CH4: Implications to the atmospheric lifetime and budget of methane, submitted to Nature, 1990) (hereinafter referred to as Vaghjiani and Ravishankara, 1990) leads to estimates of the global total methane source for SF1 of ∼524 Tg CH4 giving an atmospheric lifetime of ∼10.0 years and for SF2 ∼514 Tg CH4 yielding a lifetime of ∼10.2 years. These results are provisional pending any revision of the reaction rate for hydroxyl radical and methyl chloroform.