Browsing by Author "Lassey, KR"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Atmospheric cycling of radiomethane and the "fossil fraction" of the methane source
    (European Geosciences Union, 2007-05-02) Lassey, KR; Lowe, DC; Smith, AM
    The cycling of (CH4)-C-14 ("radiomethane") through the atmosphere has been strongly perturbed in the industrial era by the release of C-14-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 C-14-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. © 2007, European Geosciences Union
  • Thumbnail Image
    Item
    Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?
    (European Geosciences Union, 2007-05-02) Lassey, KR; Etheridge, DM; Lowe, DC; Smith, AM; Ferretti, DF
    Little is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition ((CH4)-C-12, (CH4)-C-13 and (CH4)-C-14) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including "EDGAR"- based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both (CH4)-C-12 and (CH4)-C-13 budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for (CH4)-C-14 exposes uncertainties in inferring the fossil-methane source from atmospheric (CH4)-C-14 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb C-14" ( originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of (CH4)-C-14 from nuclear-power facilities is influential but poorly quantified. Atmospheric (CH4)-C-14 determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source. © Author(s) 2007