Atmospheric methane and its carbon isotopes in the southern hemisphere: Their time series and an instructive model (original) (raw)
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Journal of Geophysical Research, 1994
Measurements of •3C in atmospheric methane made at Baring Head, New Zealand (41 øS), over the 4-year period, 1989-1993, display a persistent but highly variable seasonal cycle. Values for •j•3C peak in summer at about -46.9%o and drop to around -47.5%o in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78øS. While the phase of the •jl3C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in •3C in the atmosphere after burden of methane has been attributed to growth in various agricultural and industrial sources [Ehhalt and Schmidt, 1978]. Recent reviews of budgets estimating methane source strengths have been published by Khalil and Rasmussen [1990] and by Lassey et al. [1992]. Isotopic determinations of methane in the atmosphere in conjunction with concentration measurements provide important constraints on estimates of the sources and sinks of the gas. For example, •4C determinations provide information on the role of fossil methane sources to the atmosphere [Lowe et at., 1988; Wahten et at., 1989; Manning et at., 1990; Quay et at., 1991], because such sources from coal mines and gas wells are devoid of •4C. Also, because biogenic methanogenesis under anaerobic conditions produces methane depleted in •3C, this isotope has been used to trace such sources [Tyler, 1989; Quay et at., 1991 ].
A source inventory for atmospheric methane in New Zealand and its global perspective
Journal of Geophysical Research, 1992
Methane is an important greenhouse constituent of the atmosphere. Its global source inventory is briefly reviewed, and the aggregated source in the range 270-760 Tg CHn/yr is compatible with independent measurement. The constraints imposed by isotopic information are also discussed. We then exploit this global data base to provide a first detailed estimate of New Zealand sources of methane and their uncertainties, placing these in global perspective. The aggregate New Zealand methane source is estimated to lie in the range 1.3 -2.2 Tg/yr. Nearly all of this emission is caused by human modification of the environment and is dominated ( -75 %) by ' enterically fermented' methane from farmed livestock, predominantly by sheep and lambs (58%) and by cattle 40%). At about 0.3 % of global emissions, New Zealand is a disproportionately large methane source on either a per capita or land area basis, or when compared with the New Zealand share (0.13%)of global carbon dioxide emissions from industry and from fossil fuel consumption. 3751 3752 LASSEY ET AL.: ATMOSPHERIC METHANE IN NEW ZEALAND T g/yr to the stratosphere and to removal processes there, and
Journal of Geophysical Research: Atmospheres, 2001
The atmospheric trend of methane isotopic ratios since the mid‐20th century has been reconstructed from Antarctic firn air. High volume air samples were extracted at several depth levels at two sites in East Antarctica. Methane concentration and its 13C/12C and D/H ratios were determined by gas chromatography, mass spectrometry, and infrared spectroscopy. A firn air transport model was applied to reconstruct past atmospheric trends in methane and its isotopic composition. By subsequent application of an atmospheric model, changes in methane sources and OH sink compatible with the past atmospheric trends are explored. In step with increasing methane mixing ratios, δ13C increased by ∼1.7‰ over the last 50 years. These changes mainly reflect a shift in relative source strength toward the heavier anthropogenic methane source, such as biomass burning and methane of nonbiological origin. The δD (CH4) showed a period of decline between the 1950s and 1975, followed by a gradual increase of ...
Tropospheric methane in the mid-latitudes of the Southern Hemisphere
Journal of Atmospheric Chemistry, 1984
Results of more than 800 new measurements of methane (CH 4) concentrations in the Southern Hemisphere troposphere (34-41 ° S, 130-150°E) are reported. These were obtained between September 1980 and March 1983 from the surface at Cape Grim, Tasmania, through the middle (3.5-5.5 km) to the upper troposphere (7-10 km). The concentration of CH 4 increased throughout the entire troposphere over the measurement period, adding further support to the view that CH4 concentrations are currently increasing on a global scale. For data averaged vertically through the troposphere the rate of increase found was 20 ppbv/yr or 1.3%/yr at December 1981. In the surface CH4 data a seasonal cycle with a peak to peak amplitude of approximately 28 ppbv is seen, with the minimum concentration occurring in March and the maximum in September-October. A cycle with the same phase as that seen at the surface, but with a significantly decreased amplitude, is apparent in the mid troposphere but no cycle is detected in the upper tropospheric data. The phase and amplitude of the cycle are qualitatively in agreement with the concept that the major sink for methane is oxidation by hydroxyl radicals. Also presented is evidence of a positive vertical gradient in methane, with a suggestion that the magnitude of this gradient has changed over the period of measurements.
Sources, sinks, and seasonal cycles of atmospheric methane
Journal of Geophysical Research: Oceans, 1983
It is shown that a long lifetime of about 8 years is most consistent with the observed latitudinal variation of atmospheric methane, requiring the current global emissions of methane to be around 550 teragrams per year (Tg = 10 •2 gm). On average there is 25-34 ppbv less methane in the atmosphere of the northern hemisphere during summer when compared with the rest of the year. Methane concentrations rise rapidly to their yearly maximums •n fall. Seasonal cycles of Cg-i 4 concentration in the southern hemisphere include lowest concentrations during the late Australian summer and fall, being about 14 ppbv less than during the rest of the year. The repeating pattern of a rapid rise of CH4 concentrations during fall in the northern hemisphere suggests a large fall source at latitudes above 30øN. The remaining observed seasonal variations are consistent with the seasonal cycle of OH, which removes methane from the atmosphere. The extensive set of self consistent measurements of methane are reported and analyzed showing that methane has increased during the last 3-4 years at rates of 1-1.9% per year all over the world at sites ranging from inside the arctic circle to the south pole. Observational results are used to estimate the sources, sinks, seasonal cycles of CH4, and the effects of human activities on its atmospheric abundance.
2014
Abstract. Little is known about how the methane source in-ventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition (12CH4, 13CH4 and 14CH4) from analyses of air trapped in polar ice and firn can enhance this knowl-edge. 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 exam-ine the constraints imposed by isotope information on those budget evolutions. Reconciling both 12CH4 and 13CH4 bud-gets with EDGAR-based source histories requires a combi-nation of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions
Growth Rate, Seasonal, Synoptic, Diurnal Variations and Budget of Methane in the Lower Atmosphere
Journal of the Meteorological Society of Japan. Ser. II, 2009
We have used an AGCM (atmospheric general circulation model)-based Chemistry Transport Model (ACTM) for the simulation of methane (CH 4) in the height range of earth's surface to about 90 km. The model simulations are compared with measurements at hourly, daily, monthly and interannual time scales by filtering or averaging all the timeseries appropriately. From this model-observation comparison, we conclude that the recent (1990-2006) trends in growth rate and seasonal cycle at most measurement sites can be fairly successfully modeled by using existing knowledge of CH 4 flux trends and seasonality. A large part of the interannual variability (IAV) in CH 4 growth rate is apparently controlled by IAV in atmospheric dynamics at the tropical sites and forest fires in the high latitude sites. The flux amplitudes are optimized with respect to the available hydroxyl radical (OH) distribution and model transport for successful reproduction of latitudinal and longitudinal distribution of observed CH 4 mixing ratio at the earth's surface. Estimated atmospheric CH 4 lifetime in this setup is 8.6 years. We found a small impact (less than 0.5 ppb integrated over 1 year) of OH diurnal variation, due to temperature dependence of reaction rate coe‰cient, on CH 4 simulation compared to the transport related variability (order of G15 ppb at interannual timescales). Model-observation comparisons of seasonal cycles, synoptic variations and diurnal cycles are shown to be useful for validating regional flux distribution patterns and strengths. Our results, based on two emission scenarios, suggest reduced emissions from temperate and tropical Asia region (by 13, 5, 3 Tg-CH 4 for India, China and Indonesia, respectively), and compensating increase (by 9, 9, 3 Tg-CH 4 for Russia, United States and Canada, respectively) in the boreal Northern Hemisphere (NH) are required for improved model-observation agreement.
Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?
Atmospheric Chemistry and Physics, 2007
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 (12 CH 4 , 13 CH 4 and 14 CH 4) 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 12 CH 4 and 13 CH 4 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 14 CH 4 exposes uncertainties in inferring the fossil-methane source from atmospheric 14 CH 4 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb 14 C" (originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of 14 CH 4 from nuclear-power facilities is influential but poorly quantified. Atmospheric 14 CH 4 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.
Recent changes in methane mixing ratio and its13C content observed in the southwest Pacific region
Journal of Integrative Environmental Sciences, 2010
After nearly a decade without growth in atmospheric methane, there are indications of renewed growth from 2007. Reports of this renewal portray it as global in extent, and due wholly or largely to growth in emissions. Surface methane mixing ratios and constituent d 13 C values have been measured approximately twice monthly at Baring Head, New Zealand (418S, 1758E) since 1989. Surface mixing ratios have been measured continuously at Lauder, New Zealand (458S, 1708E) since 2007. Also at Lauder, tropospheric-mean mole fractions of methane have been retrieved from ground-based near-infrared solar spectra since 2004. These mixing ratio datasets are consistent with growth rates of about 7.5 and 4.9 ppb year 71 during 2007 and 2008. We consider the possible origins of this growth based on their imprint on d 13 C values.