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Atmospheric Methane Record from Alert, NWT, Canada
L.P. Steele, P.B. Krummel and R.L. Langenfelds
Commonwealth Scientific and Industrial Research Organization (CSIRO),
Aspendale, Victoria, Australia, 3195
Period of Record
May 1991 - December 2001
The listed data have been obtained from flask air samples returned to the CSIRO GASLAB for analysis. Typical sample storage times range from days to weeks for some sites (e.g., Cape Grim) to as much as one year for Macquarie Island and the Antarctic sites. Experiments carried out to test for any change in sample CH4 mixing ratio during storage have shown no drift to within detection limits over test periods of several months to years (Cooper et al., 1999).
Samples were analysed by gas chromatography with flame ionization detection (FID). Three individual but similarly configured Carle gas chromatographs were used over the length of the record. They are labeled "Carle-1" (C1), "Carle-2" (C2) and "Carle-3" (C3). Further details of CSIRO's global sampling network, sampling and analytical techniques are provided by Francey et al. (1996); measurement uncertainty is discussed by Langenfelds et al. (2001).
Data are reported in the CSIRO94 CH4 scale (Steele et al., 1996), which is derived from the CH4 scale maintained at NOAA/CMDL (Dlugokencky et al., 1994). The link to this scale was established with two high pressure cylinders containing dry, natural air that were calibrated by NOAA/CMDL between 1987 and 1990. Results from later exchanges of high-pressure cylinders indicate a small, but measurable difference (0.36 ±0.17 ppbv). Stability of the CSIRO scale is monitored with approximately 25 assorted long-lived standards. More detailed calibration information is given by Langenfelds et al. (2001). Comparison of CSIRO and NOAA flask measurements indicates agreement to within 1 ppbv (Masarie et al., 2001).
These data represent monthly means, calculated as the mean of daily values from a smooth curve fit to the data using the curve-fitting routines described by Thoning et al. (1989).
Alert, NWT, Canada
82°27' N, 62°31' W,
6 m above MSL
An annual pattern in atmospheric CH4 is evident, largely due to an increase in its destruction by the OH radical during the summer months. Additional influences on CH4 concentrations at Alert include spatial and seasonal variations in source strength associated with the varying trajectories of arriving air at different times of the year. The overall trend since 1992 is a slow and unsteady increase, averaging about 3 ppbv/year. A decrease of almost 6 ppbv occurred from 1996 to 1997. In the two most recent years, the general increase has leveled off to about 0.55 ppbv/year.
- Cooper, L.N., L.P. Steele, R.L. Langenfelds, D.A. Spencer and M.P. Lucarelli. 1999. Atmospheric methane, carbon dioxide, hydrogen, carbon monoxide and nitrous oxide from Cape Grim flask air samples analysed by gas chromatography. Baseline Atmospheric Program (Australia) 1996, edited by J.L. Gras, N. Derek, N.W. Tindale and A.L. Dick, pp 98 - 102, Bureau of Meteorology and CSIRO Atmospheric Research, Melbourne, Australia.
- Dlugokencky. E.J., L.P. Steele, P. M. Lang and K.A. Masarie. 1994. The growth rate and distribution of atmospheric methane. J. Geophys. Res., 99, 17021-17043.
- Francey, R.J., L.P. Steele, R.L. Langenfelds, M.P. Lucarelli, C.E. Allison, D.J. Beardsmore, S.A. Coram, N. Derek, F.R. de Silva, D.M. Etheridge, P.J. Fraser, R.J. Henry, B. Turner, E.D. Welch, D.A. Spencer and L.N. Cooper. 1996. Global Atmospheric Sampling Laboratory (GASLAB): supporting and extending the Cape Grim trace gas programs. Baseline Atmospheric Program (Australia) 1993, edited by R.J. Francey, A.L. Dick and N. Derek, pp 8 - 29, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia.
- Langenfelds, R.L., L.P. Steele, C.E. Allison and R.J. Francey. 2001. GASLAB Calibration Information, 2001. Internal Report, CSIRO Atmospheric Research, Aspendale, Australia.
- Masarie, K.A., R.L. Langenfelds, C.E. Allison, T.J. Conway, E.J. Dlugokencky, R.J. Francey, P.C. Novelli, L.P. Steele, P.P. Tans, B. Vaughn and J.W.C. White. 2001. NOAA/CSIRO Flask Air Intercomparison Experiment: A strategy for directly assessing consistency among atmospheric measurements made by independent laboratories, J. Geophys. Res., 106, 20445-20464.
- Steele, L.P., R.L. Langenfelds, M.P. Lucarelli, P.J. Fraser, L.N. Cooper, D.A. Spencer, S. Chea and K. Broadhurst. 1996. Atmospheric methane, carbon dioxide, carbon monoxide, hydrogen and nitrous oxide from Cape Grim flask air samples analysed by gas chromatography. Baseline Atmospheric Program (Australia) 1994-95, edited by R.J. Francey, A.L. Dick and N. Derek, pp 107 - 110, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia.
- Thoning, K.W., P.P. Tans and W.D. Komhyr. 1989. Atmospheric carbon dioxide at Mauna Loa Observatory, 2, Analysis of the NOAA/GMCC data, 1974 - 1985, J. Geophys. Res., 94, 8549-8565.
CITE AS: Steele, L. P., P. B. Krummel and R. L. Langenfelds. 2002. Atmospheric CH4 concentrations from sites in the CSIRO Atmospheric Research GASLAB air sampling network (October 2002 version). In Trends: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, U.S.A.