Shipboard determinations of the distribution of 13 C in atmospheric methane in the Pacific (original) (raw)
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The isotopic composition of atmospheric methane
Global Biogeochemical Cycles, 1999
Measurements of the 13C/12C, D/H and 14C composition of atmospheric methane (CH4) between 1988 and 1995 are presented. The 13C/12C measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71 øN), Olympic Peninsula, Washington (48øN), Mauna Loa, Hawaii (20øN), American Samoa (14øS), Cape Grim, Australia (41 øS), and Baring Head, New Zealand (41 øS). North-south trends of the 13C/12C and D/H of atmospheric CH4 from air samples collectei:l during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual õ13C increased southward from about-47.7 %0 at 71 øN to-47.2 %0 at 41 øS. The amplitude of the seasonal cycle in õ13C ranged from about 0.4 %o at 71øN to 0.1%0 at 14øS. The seasonal õ13C cycle at sites in tropical latitudes could be explained by CH4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the õ13C of the CH4 source were needed to explain the seasonal cycle. The higher õ13C value in the southern (-47.2 %0) versus northern (-47.4 %0) hemisphere was a result of interhemispheric transport of CH4. A slight interannual õ13C increase of 0.02+_0.005 %o yr-1 was measured at all sites between 1990 and 1995. The mean õD of atmospheric CH4 was-86+_3 %o between 1989 and 1995 with a 10 %0 depletion in the northern versus southern hemisphere. The 14C content of CH4 measured at 48øN increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH4 released from fossil sources was 18+_9% in the early 1990s as derived from the 14C content of CH4.
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 ].
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.
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.
Chemosphere, 1993
We report data from the clean air monitoring station at Baring Head, New Zealand on concentrations of atmospheric methane and its 13C/~2C and ~4C/12C ratios. The record for methane concentration features a recent (post-July 1991) and persistent elevation above expected levels. The 2-year 8~3C record shows a surprisingly large seasonal cycle (ca 0.3%o peak to peak) about a mean value of -47.14 + 0.03%o (2 standard deviations) with no discernible trend. The ~4C/'2C record is relatively featureless, corresponding to 119.7+0.7 percent modern carbon (pMC) in January 1990 with a small upward trend of 1.3 + 0.8 pMC/yr over the past three years. A simple model depicting the southern hemisphere atmosphere as a single methane reservoir with seasonally-modulated sources and sinks is used to examine methane dynamics in this hemisphere. According to this model, approximately half the methane in the southern hemisphere atmosphere is delivered from the northern hemisphere. The model can interpret the 8~3C seasonal cycle only as a large injection of isotopically heavy methane during the austral spring, such as might result from biomass burning.
Journal of Geophysical Research: Atmospheres, 1999
We report CH 4 mixing ratios and •513C of CH4 values for remote air at two groundbased atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)-Tropics A. Surface values of 1513C-CH4 ranged from-47.02 to-47.52%0 at Niwot Ridge, Colorado (40øN, 105øW), and from-46.81 to-47.64%0 at Montafia de Oro, California (35øN, 121øW). Samples for isotopic analysis were taken from 2 ø to 27øS latitude and 81 o to 158øW longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At-11 km, •513C-CH4 was-1%o greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in 1513C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH 4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH 4 Model-calculated values for •513C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups. 0.5 W m-2, whereas for CO 2, radiative forcing has increased 1.56 W m '2 [Shine et al., 1995]. Furthermore, an increase in
A history of δ 13 C in atmospheric CH 4 from the Cape Grim Air Archive and Antarctic firn air
Journal of Geophysical Research, 1999
Marine (baseline) air from Cape Grim,(41 øS), collected and archived in highpressure metal containers, provides a history of b'oC in atmospheric methane from 1978. A similar history is obtained from air pumped from different layers of the tim at Law Dome, Antarctica, after correction for diffusion and gravitational settling effects in the tim. The archive records are linked to measurements since 1992 using 5-L glass flasks filled at Cape Grim, and compared to data since 1989 from a comparable site at Baring Head, New Zealand. Over 18 years the/5•3C of atmospheric methane in the extratropical Southern Hemisphere has increased by •0.6%o while the methane mixing ratio increased by •200 ppb. The bn3C growth rate decreases over the 18-year period, but by relatively less than the simultaneous decrease in mixing ratio growth rate. The overall increase in/5•JC is significantly smaller than, and the recent slowing is in conflict with, previous estimates [Stevens and Engelkerneir, 1989]. The long-term trend in b•3C, and the different shape to the trend in mixing ratio, are shown to be consistent with constan3t global methane sources and sinks since 1982. The slower equilibration of observed/5• C, compared to that of the mixing ratio, is an example of an effect pointed out recently by Tans [ 1997]. The data presented here constrain changes in the relative mix of isotopically heavy and light sources to be small and suggest that there was little change in the ratio of anthropogenic to natural sources in the 1978 to 1995 period. 1986; Blake and Rowland, 1988; Steele et al., 1987; Hirota et al., 1989; Brunke et al., 1990; Khalil and Rasmussen, 1990; Aoki et al., 1992]. The overall increase in methane is generally associated with a range of anthropogenic activities, including rice production, livestock, landfill, biomass burning, and coal mining. Steele et al. [1992] and Dlugokencky et al. mospheric Administration (NOAA) global flask network to show that the atmospheric growth rate of methane generally slowed from the beginning of their global record in 1983, with a dramatic drop to zero growth in 1992. The reasons for short-term growth rate variations (in particular the 1991-1992 anomaly) are the subject of ongoing debate [Bekki et al., 1994; Dlugokencky et al., 1996; Lowe et al., 1997] but are viewed of limited relevance to this paper. The more recent analyses also suggest that total methane sources have not increased significantly since about 1980; however, the allocation between different source types is not well understood [Fung et al., 1991;Hein et al., 1997]. Methane isotopic determinations in conjunction with mixing ratio measurements can provide constraints on estimates of the sources and sinks of the gas. For example, 14C measurements in atmospheric methane have been used to provide information on fossil sources of methane [Lowe et al., 1988; Wahlen et al., 1989; Manning et al., 1990; Quay et al., 1991] because such sources, for example methane from coal mines and gas wells, are 14C free. The stable 13C isotope in atmospheric methane, of main interest here, generally distinguishes bacterial and non-bacterial sources [Stevens and Rust, 1982; Tyler, 1986; Quay et al., 1991; Lowe et al., 1991, 1994; Levin et al., 1993]. The ratio of the 13C to 12C isotopic species is close to 1% in terrestrial carbon. Small variations in the ratio are expressed using the delta-notation: [13C•]--0 -1'1513C)RsTD [12C] where [•3C], [12C] represents a concentration of species 13C 13 12 and I2C respectively, RST D is the C/ C ratio in a standard material and 1513C values are normally written in per mil (1960 23,631 23,632 FRANCEY ET AL.' b•3C IN ATMOSPHERIC CH4 FROM ARCHIVED AIR
Interannual variability and trends in atmospheric methane over the western Pacific from 1994 to 2010
Journal of Geophysical Research, 2011
We present an analysis of interannual variability (IAV) and trends in atmospheric methane (CH 4) mixing ratios over the western Pacific between 55°N and 35°S from 1994 to 2010. Observations were made by the Center for Global Environmental Research (CGER) of the National Institute for Environmental Studies (NIES), using voluntary observation ships sailing between Japan and Australia/New Zealand and between Japan and North America, sampling background maritime air quasi-monthly (∼10 times per year) with high latitudinal resolution. In addition, simulations of CH 4 were performed using NIES atmospheric transport model. A large CH 4 increase was observed in the tropics (10°N-5°S) during 1997 (between 15 ± 3 and 19 ± 3 ppb yr −1) and during 1998 for other regions (40°N-50°N: 10 ± 2-16 ± 1 ppb yr −1 ; 10°S-25°S: 12 ± 2-22 ± 4 ppb yr −1). The CH 4 increase leveled off from 1999 to 2006 at all latitudes. The CH 4 growth rate was enhanced in 2007 (25°N-50°N: 10 ± 1-12 ± 3 ppb yr −1 ; 15°S-35°S: 7 ± 1-8 ± 1 ppb yr −1) but diminished thereafter; however, a large CH 4 growth (10 ± 1-17 ± 1 ppb yr −1) was observed in 2009 over the northern tropics (0°-15°N). These observations, combined with the simulation results, suggest that to explain the CH 4 increase in 2007 would require an increase in surface emissions of ∼20 ± 3 Tg-CH 4 yr −1 globally and an increase in the Northern Hemisphere (NH) of 4-7 ± 3 Tg-CH 4 yr −1 more than that in the Southern Hemisphere (SH), assuming no change in OH concentrations; alternatively, a decrease in OH concentrations of 4.5 ± 0.6%-5.5 ± 0.5% yr −1 globally would be required if we assume no change in surface emissions. Over the western Pacific, the IAV in CH 4 within the northern tropics was characterized by a high growth rate in mid-1997 and a reduced growth in 2007. The present data indicate that these events were strongly influenced by the IAV in atmospheric circulation associated with El Niño and La Niña events. Our observations captured the CH 4 anomaly in 1997 associated with forest fires in Indonesia. The IAV and trends in CH 4 as seen by our data sets capture the global features of background CH 4 levels in the northern midlatitudes and the SH, and regional features of CH 4 variations in the western tropical Pacific.