Constraining Changes in the Global CH4 Budget Based on Global Ethane Changes: Insights From 30 Years of Global Trace Gas Monitoring
Abstract
UC-Irvine has directly monitored global CH4 mixing ratios for 30 years, since 1978, and it is the longest- running of the continuous global monitoring networks. Every three months 80 ground-level whole air samples are collected in the remote Pacific Basin (71 °N-46 °S) and analyzed by gas chromatography for many dozens of trace gases including hydrocarbons, halocarbons, organic nitrates and sulfur species. For example our network is unique in that, in addition to CH4, ethane has been retrieved from each air sample since 1984. Tetrachloroethene (C2Cl4) has been retrieved since 1988. Methane, ethane, and C2Cl4 are all OH-controlled species, but only CH4 and ethane share common anthropogenic sources (fossil fuel and biomass burning). Therefore CH4, ethane and C2Cl4 are a powerful combination to help us determine which source and sink variations are consistent with the observed CH4 trends. In the long-term, methane's annual growth rate has slowed from 15.2 to 18.9 ppbv yr-1 in the early-to- mid 1980s, to -3.8 to 6.6 ppbv yr-1 since 2000. For the first time, our network has simultaneously shown a long-term decline in the global ethane mixing ratio, by 150 pptv (20%) in the past two decades. In the short-term, CH4 has shown positive growth rate anomalies every 3½--4½ years since 1991, the fifth and most recent of which peaked at 6.6 ± 0.9 ppbv yr-1 in 2007. Interestingly, each short-term CH4 growth rate peak has been matched by a peak in ethane's mixing ratio. Coincident CH4 and ethane fluctuations that are not matched by C2Cl4---for example the 1998, 2002-2003, and 2007 CH4 and ethane fluctuations---point to anomalous biomass burning emissions at those times as an influencing source. Methane and ethane are emitted from biomass burning in quantitative ratios relative to each other and to CO, and our results suggest that roughly one-third of the 1998 and 2007 CH4 growth rate anomalies were attributed to biomass burning, with the remainder from anomalous wetlands emissions. By comparison, we found that, to within the uncertainties, almost all of the 2002-2003 CH4 growth rate anomaly was explained by biomass burning. We also show an apparent link between strong El Niño events and strong biomass burning emissions in the northern hemisphere during the following growing season.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B23E..02S
- Keywords:
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- 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 1600 GLOBAL CHANGE