Seasonal cycles of mixing ratio and ¹³C in atmospheric methane at Suva, Fiji

Abstract

[1] A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08 0 S, 178°26 0 E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid-2002, have been analyzed for methane mixing ratio and d 13 C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Niña conditions. A modeling study was carried out using a modified version of the UK Meteorological Office's Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its d 13 C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Niña conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in d 13 C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…The δ 13 C-CH 4 measurements at NIWA are ultimately calibrated against CO 2 produced from NBS-19, IAEA-CO-9 and LSVEC. The long-term δ 13 C-CH 4 records have been presented since then (Lowe et al, 1994(Lowe et al, , 1997(Lowe et al, , 2004Bergamaschi et al, 2001;Schaefer et al, 2016). reported that repeated measurements of the two working reference gases and archived air indicated no detectable drift over 16 years since 1992.…”

Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?

“…2b. Available data, all from the Southern Hemisphere (SH), are also shown: air trapped in polar firn or ice as indicated; air archived at Cape Grim Observatory (Francey et al, 1999); contemporary time series of annual means from Baring Head (updated from Lowe et al, 2004). The KIE value, ε, optimized separately for each source history S&K, E-H v1.3 and E-H v1.4, is −8.1‰, −6.9‰ and −7.4‰ respectively; to these values should be added an adjustment of ∼0.2‰ to "globalise" the SH datasets (see text, Sect.…”

Real-time analysis of <i>δ</i><sup>13</sup>C- and <i>δ</i>D-CH<sub>4</sub> in ambient air with laser spectroscopy: method development and first intercomparison results

“…Repeatability based on 10 consecutive analyses of standard air is ±0.05 ‰ or better. δ 13 C-CH 4 values of RHUL are offset corrected by −0.3 ‰ based on intercomparison measurements with NIWA (Lowe et al, 2004).…”