Temporal variations of the carbon isotopic ratio of atmospheric methane observed at Ny Ålesund, Svalbard from 1996 to 2004

Authors


Abstract

[1] Systematic observations of the atmospheric CH4 mole fraction and its carbon isotope ratio δ13CH4 have been carried out at Ny Ålesund, Svalbard (78°55′N, 11°56′E) since 1991 and 1996, respectively. The CH4 and δ13CH4 showed clear seasonal cycles with respective peak-to-peak amplitudes of 48 ppb and 0.42‰. By comparing the anomalies in the increase rate of the CH4 with those of δ13CH4, it is suggested that the cause of the rapid increase in the CH4 mole fraction observed at Ny Ålesund in 1998 could be attributable to an enhanced CH4 release from wetland and biomass burning.

1. Introduction

[2] To obtain a better understanding of the global CH4 cycle, systematic observations of atmospheric CH4 have been made around the world since the 1980's [Cunnold et al., 2002; Dlugokencky et al., 2001]. However, our knowledge of the contributions of individual CH4 sources and sinks, and their variations, to atmospheric CH4 is still limited. The carbon isotope ratio of atmospheric CH4, δ13CH4, defined as;

equation image

where V-PDB denotes the international reference for carbon isotope ratio, provides an additional constraint on the CH4 cycle studies, since each source has its own characteristic δ13CH4 value, e. g. CH4 from wetland (microbial source including rice paddies), fossil fuel and biomass burning have δ13CH4 values of around −60, −40 and −25‰, respectively, relative to V-PDB [cf. Quay et al., 1999]. We have maintained a systematic CH4 observation at Ny Ålesund, Svalbard, since 1991. In 1996, we started to measure δ13CH4 using a gas chromatograph-combustion-isotope ratio mass spectrometer (GCC-IRMS). In this paper, we present temporal variations of the CH4 mole fraction and δ13CH4 observed at Ny Ålesund and discuss the relative contributions of various CH4 sources to the atmospheric CH4 variations.

2. Experimental Procedures

[3] Whole air samples have been collected once a week into 800 ml stainless steel flasks at a pressure of about 0.9MPa at the Japanese Observatory at Ny Ålesund (78°55′N, 11°56′E), Svalbard (Figure 1). Collected flask samples were shipped to Japan and the CH4 mole fractions were determined by using a gas chromatograph (Shimadzu, GC-8A) and a set of standard gases calibrated against the Tohoku University (TU) scale with a reproducibility of 1.0 ppb [Aoki et al., 1992; Cunnold et al., 2002]. The TU scale is based on gravimetrically prepared standards, of which the CH4 mole fraction is determined against dry air. In this paper, ‘ppb’ is used as effectively identical to the unit, ‘nmol/mol’. A part of the air samples collected between June 1996 and June 2000 had been stored in 200 ml Pyrex glass bottles until the GCC-IRMS system was developed for the δ13CH4 analysis.

Figure 1.

Location of Ny Ålesund, Svalbard. Also shown are the locations of Barrow and Alert.

[4] Our GCC-IRMS system was developed using Thermo Electron (Finnigan) MAT252 following Merritt et al. [1995]. Each air sample of 50–100 mlSTP was flushed by pure helium gas into a CH4 extraction trap containing HayeSep-D kept at −120°C. The CH4 released from the HayeSep-D at room temperature was transferred into a cryo-focusing trap CP-PoraBOND with a column diameter of 0.32 mm kept at −197°C. Then, concentrated CH4 was further purified through PoraPLOT capillary column and combusted into CO2 at 940°C for subsequent continuous flow mass spectrometer analysis. Reproducibility for our δ13CH4 analysis was determined to be within 0.13‰ (one standard deviation) by replicate analyses of our CH4-in-air ‘test gas’ between June 2000 and September 2001; it was then improved to 0.08 and 0.06‰ after September 2001 and August 2002, respectively. The stored air samples collected before June 2000 were analyzed when the reproducibility was 0.08‰. The δ13C scale used in our study was developed at Tohoku University based on NBS-18 and 19 [Nakazawa et al., 1993]. Intercomparison of the δ13CH4 scales with the National Institute of Water and Atmospheric Research (NIWA), New Zealand, was carried out in 2004. The preliminary results showed our scale was 0.37 ± 0.04‰ heavier than the NIWA scale at the atmospheric δ13CH4 levels. The source of the difference is still unclear, however, our δ13CH4 data are found to be internally consistent by the ‘test gas’ analyses during the period of this study.

3. Results and Discussion

[5] Figures 2a and 2b show temporal variations of the CH4 mole fraction and δ13CH4, respectively, observed at Ny Ålesund, together with their best-fit curves and trends calculated using the curve-fitting procedure developed by Nakazawa et al. [1997]. The average increase rates of the CH4 mole fraction and δ13CH4 were calculated to be 3.9 ± 0.2 ppb/yr and 0.010 ± 0.004‰/yr for 1991–2003 and 1996–2003, respectively.

Figure 2.

(a) CH4 mole fraction observed at Ny Ålesund (open circles), together with a best fit curve (solid line) and long-term trend (dotted line). (b) Same as (a), but for δ13CH4. (c) Seasonally adjusted CH4 mole fraction (closed circles), long-term trend (solid line), increase rate (dotted line) and the uncertainly of the increase rate (broken line; one standard deviation). (d) Same as (c), but for δ13CH4.

[6] The peak-to-peak amplitudes of the CH4 and δ13CH4 seasonal cycles averaged 48 ppb and 0.42‰ for 1991–2003 and 1996–2003, respectively. The amplitude for the CH4 mole fraction is comparable to NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) data obtained at Point Barrow (BRW), Alaska (http://www.cmdl.noaa.gov), however, the amplitude for δ13CH4 is smaller than 0.65‰ observed at BRW for 1998–1999 [Miller et al., 2002]. Consistent with other Northern Hemisphere sites [Bergamaschi et al., 2000; Miller et al., 2002], the seasonal cycles of δ13CH4 and CH4 at Ny Ålesund are out of phase, with the maximum and minimum values of δ13CH4 occurring in June and October, respectively, compared with the minimum and maximum CH4 in July and January–February, respectively. This phase difference could be caused by the fact that CH4 emissions from wetlands and biomass burning and the CH4 destruction rate via OH show relative seasonal variations, with marked different influences on atmospheric δ13CH4 [Allan et al., 2001].

[7] Seasonally adjusted values of the CH4 mole fraction and δ13CH4 observed at Ny Ålesund are plotted in Figures 2c and 2d, respectively, together with the fitted curves, the increase rates and the uncertainties (one standard deviation) of the increase rates. There are large interannual fluctuations in the CH4 and δ13CH4 long-term trends, but these fluctuations are not correlated. For example, annually-averaged increase rate of the CH4 mole fraction was 25.9 ppb/yr in 1998, and then, the rate reduced to −9.0 ppb/yr in 1999. However, δ13CH4 showed almost a constant trend in 1998 and large fluctuations in 2000–2001. The rapid increase of the CH4 concentration in 1998 was already reported by Dlugokencky et al. [2001]; they analyzed the NOAA/CMDL global network data and found that the enhanced CH4 increase was most prominent in the northern high latitudes (53–90°N) and the southern low latitudes (0–30°S).

[8] To examine the cause of the CH4 fluctuations observed at Ny Ålesund, we subtracted the 1996–2003 average annual increase rates of the CH4 concentration and δ13CH4 from the annually-averaged increase rates obtained from Figures 2c and 2d, respectively. The resulting annual anomalies of CH4 and δ13CH4 are shown in Figures 3a and 3b, respectively. Furthermore, based on the distinct δ13CH4 signatures of wetland, fossil fuel and biomass burning sources, we calculated the relative contributions of these three sources to the annual CH4 anomalies. In the calculation, the expected δ13CH4 anomaly from a particular source in a particular year was obtained by multiplying the anomaly of the CH4 increase rate for that year by the following sensitivity factors: −0.007, +0.004 or +0.012‰/ppb for wetland, fossil fuel and biomass burning, respectively. These factors were derived from the formulae published by Lassey et al. [2000], using the average CH4 mole fraction and δ13CH4 observed at Ny Ålesund for 1996–2003 and the characteristic isotope ratios of −60, −40 and −25‰ for wetland, fossil fuel and biomass burning, respectively. The results of the calculation are also shown in Figure 3b. It is evident that no single source category could be identified to account for the entire fluctuations of the CH4 mole fraction and δ13CH4. However, for the rapid CH4 increase in 1998, it is suggested that an anomalous CH4 release from both isotopically heavier (biomass burning and fossil fuel) and lighter (wetland) sources could have occurred, since the observed anomaly of the δ13CH4 increase rate was small for that year. In contrast, relatively small changes in the CH4 mole fraction in 2000–2001 were accompanied by large fluctuations of the δ13CH4 increase rate. The observed changes could result from an enhanced CH4 release from an isotopically light source in 2000, followed by a reversed situation in 2001.

Figure 3.

(a) Anomalies of the annually-averaged increase rate of the CH4 mole fraction. (b) Same as (a), but for δ13CH4 (gray bars). Also shown are the expected relative changes in δ13CH4 when wetland (blue), fossil fuel (red) or biomass burning (green) sources contribute independently to the mole fraction anomalies. (c) Contributions from wetland (blue) and biomass burning (green) to the CH4 anomalies observed at Ny Ålesund. (d) Temperature (open circles) and precipitation (closed circles) anomalies (right axis) over wetland regions of 44°–90°N for May–October, as well as burnt areas by forest fires in the circumpolar countries (left axis). “fSU” denotes former Soviet Union.

[9] At this point in the investigation, we made an assumption that year-to-year fluctuations of the CH4 emission from fossil fuel and the OH destruction rate were relatively small for 1996–2003 and ignored these effects in our analysis. If we can assume that the CH4/CO2 ratio, used by Stern and Kaufmann [1998] as a factor representing the CH4 emission from fossil fuel, has been constant after 1995, fluctuations of the fossil fuel CH4 emission could be evaluated using the fossil fuel CO2 data [Marland et al., 2005] to be within 2% (one standard deviation) between 1996 and 2003. On the other hand, it is possible that the CH4 destruction rate varies from year to year, reflecting the temperature-dependent reaction of CH4 with OH and changeable content of atmospheric OH. In fact, Manning et al. [2005] recently suggested from their 14CO measurements that atmospheric OH was reduced by the order of 10% in the extra tropical Southern Hemisphere in 1997/98, responding to Indonesian fires. Therefore, the effect of the OH fluctuation on atmospheric CH4 should be taken into account in the future when quantitative knowledge of global OH variation become available.

[10] We then partitioned the observed CH4 anomalies into the wetland and biomass burning sources by using the sensitivity factors given in the previous paragraph and the observed anomalies shown in Figures 3a and 3b. The results are shown in Figure 3c, together with uncertainties. The dominant source of the uncertainties shown comes from the respective uncertainties of ±8 and ±3‰ in the δ13CH4 values for the wetland and biomass burning [Quay et al., 1999]. As shown in Figure 3c, a large positive anomaly in the wetland CH4 emission was estimated for 1998, along with significant negative anomalies for 1997, 1999 and 2001, in phase with the anomaly of the CH4 increase rate. In addition, CH4 emission from biomass burning was estimated to be also variable from year to year, with a significant positive anomaly estimated for 1998 and negative anomalies for 1999 and 2000.

[11] Figure 3d shows temperature and precipitation anomalies for May–October for wetland regions in the northern high latitudes (44°–90°N) calculated using NOAA/Climate Diagnostics Center archived climate data and the wetland map compiled by Matthews and Fung [1987]. The CH4 emission from wetlands is accelerated by higher surface temperature and more precipitation [Dlugokencky et al., 2001]. As can be seen in Figures 3c and 3d, the wetland CH4 anomaly estimated in this study is qualitatively consistent with the precipitation anomaly, however, the correspondence to the temperature anomaly can be found only in 1997–1999. It might be possible that interannual variations in air mass transport to Ny Ålesund was partly responsible for the lack of relationship between the climate variables and the CH4 emission change, since temperature and precipitation anomalies show large spatial variations. Recently, Zhuang et al. [2004] suggested, using a process-based biogeochemistry model, that the CH4 release rate from wetlands north of 45°N showed large interannual variations, which is consistent with the δ13CH4-based estimate shown in Figure 3c.

[12] Also shown in Figure 3d are the statistical data of burnt areas by forest fires in the circumpolar countries including U.S.A., Canada, Europe and former Soviet Union [Food and Agriculture Organization/United Nations Economic Commission for Europe, 2002]. There is a considerable interannual variability in the areas burnt during the period 1998 to 2000, showing a qualitative correspondence to the year-to-year variation in the calculated amount of CH4 released from biomass burning shown in Figure 3c. It was also reported that large forest fires occurred in Southeast Asia and South America in 1997/1998 ENSO years [Langenfelds et al., 2002].

[13] Figure 3c suggests that the rapid increase of CH4 observed at Ny Ålesund in 1998 could have been caused by the equivalent of 7.4 and 14.9 ppb/yr of CH4 emitted from biomass burning and wetlands, respectively. This result indicates that about one third of the CH4 fluctuation observed at Ny Ålesund in 1998 could be explained by the CH4 release from biomass burning. Our result differs from those obtained by Dlugokencky et al. [2001] and van der Werf et al. [2004]. Dlugokencky et al. [2001] concluded, using a wetland model, that a primary reason for the rapid CH4 increase in 1998 could be attributable to enhanced CH4 release from wetland, and that the biomass burning made a minor contribution. On the other hand, van der Werf et al. [2004] suggested that nearly all CH4 anomalies in 1997–98 could be ascribed to fires, based on satellite data, atmospheric CH4 data and inverse modeling. The relative contributions of individual CH4 sources to the global CH4 budget are still unclear, since direct observations of the extensive burnt area and wetland area are very difficult and scaling up calculations of the CH4 flux from regional to global scale may introduce large uncertainties. This study provides further evidence that precise measurements of the atmospheric CH4 mole fraction and δ13CH4 could provide valuable constraints in separating the contribution of biomass burning from that of wetland to the CH4 variations in the atmosphere.

4. Concluding Remarks

[14] The atmospheric CH4 mole fraction and δ13CH4 observed at Ny Ålesund, Svalbard, showed clear seasonal cycles with peak-to-peak amplitudes of 48 ppb and 0.42‰, respectively. The phases of the seasonal cycles were significantly different; maximum and minimum values of δ13CH4 appeared in June and October, respectively, however, the corresponding minimum and maximum of the CH4 mole fraction were found in July and January–February, respectively. Rapid increase of the CH4 at a rate of 25.9 ppb/yr was observed in 1998, but very little change in δ13CH4. Comparison between the increase rate anomalies of the CH4 and δ13CH4 indicated that the CH4 release from biomass burning and wetlands could account for about one third and two thirds, respectively, of the anomaly of the CH4 increase rate observed at Ny Ålesund in 1998.

Acknowledgments

[15] We are grateful to Dominic Ferretti and David Lowe, NIWA, who carefully analyzed the intercomparison samples. This work was supported by Grants-in-aid for Scientific Research Program (11208201, 13740283 and 15710016) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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