The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source K. R. Lassey1, D. C. Lowe1, and A. M. Smith2 1National Institute of Water and Atmospheric Research, P.O. Box 14-901, Wellington, New Zealand 2Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW 2234, Australia
Abstract. The cycling of 14CH4 ("radiomethane") through the atmosphere has
been strongly perturbed in the industrial era by the release of 14C-free
methane from geologic reservoirs ("fossil methane" emissions), and in the
nuclear era, especially since ca 1970, by the direct release of nucleogenic
radiomethane from nuclear power facilities. Contemporary measurements of
atmospheric radiomethane have been used to estimate the proportion of fossil
methane in the global methane source (the "fossil fraction"), but such
estimates carry high uncertainty due to the ill-determined nuclear-power
source. Guided by a mass-balance formulation in a companion paper, we apply a
contemporary time series of atmospheric radiomethane to quantify both the
fossil fraction and the strength of the nuclear power source. We deduce that
30.0±2.3% (1 s.d.) of the global methane source for 1986–2000 has
fossil origin, a fraction which may include some 14C-depleted refractory
carbon such as from aged peat deposits. Since this estimate depends upon the
validity of assumptions underlying a linear regression model, it should be
seen as providing a plausible re-estimate rather than a definitive revision.
Such a fossil fraction would be much larger (by 50%) than is commonly
accepted, with implications for inventory compilation. The co-estimated
strength of the global nuclear-power source of radiomethane is consistent
with values inferred independently from local nuclear facilities.
Citation: Lassey, K. R., Lowe, D. C., and Smith, A. M.: The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source, Atmos. Chem. Phys., 7, 2141-2149, doi:10.5194/acp-7-2141-2007, 2007.