Atmospheric Methane Record from Cape Grim, Australia

Graphics    Digital Data

Investigators

L.P. Steele, P.B. Krummel and R.L. Langenfelds
Commonwealth Scientific and Industrial Research Organization (CSIRO),
Atmospheric Research,
Aspendale, Victoria, Australia, 3195

Period of Record

September 1984 - December 2001

Methods

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 analyzed 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).

Cape Grim, Australia
40°41' S, 144°41' E,
94 m above MSL

Trends

A well-defined annual cycle in atmospheric CH₄ is evident, this is largely due to an increase in its destruction by the OH radical during the summer months. Air at Cape Grim is well mixed, having recently traveled over the Southern Ocean, and is far removed from variations in source strength characteristic of land areas. An average increase of around 12 ppbv/year from 1985 through 1991 was followed by a reduced rate of increase, averaging around 6.5 ppbv/year, until 1999. Since 1999, the annual average CH₄ concentration has been almost constant, even exhibiting a slight decrease from 2000 to 2001. This pattern is remarkably similar to that for Mawson, Antarctica.

References

• 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.

January 2003