Anomalies in Tropical Methane Emissions: Using Isotopes to Characterise Source Variability

In 2007 a sustained atmospheric global growth of methane began and was particularly strong in the tropics. The tropics are poorly understood with few measurements, despite being an important part of the global methane budget. Sources include natural wetlands, rice agriculture, biomass burning, ruminants, fossil fuels and waste. 
This thesis focuses on sources sampled across the tropics using Tedlar bags, with Bachok, Malaysia and Ascension Island selected for more detailed sampling and studies. Samples were analysed for methane mole fractions using cavity ring-down spectroscopy and for isotopic source signatures, δ¹³C, using gas chromatography/isotope-ratio mass spectrometry. Using δ¹³C and mole fraction measurements means both global and regional emissions can be constrained in models with better source apportionment improving the understanding of the sustained tropical methane growth. Keeling plots have been used to calculate the δ¹³C source signature of each source type. Tropical wetlands have a signature between -61.5 ± 2.9 ‰  and -53.0 ± 0.4 ‰ and are generally more enriched than temperate and boreal wetlands.
Continuous monitoring of greenhouse gases and collection of air samples at Bachok research station, Malaysia showed the background CH₄ mole fraction is influenced by air masses arriving from over China and Southeast Asia. Local CH₄peaks were not predominantly biomass burning as expected, but instead gave a signature of -54.9 ± 1.2 ‰, suggesting mixing with biogenic sources, showing the importance of monitoring CH₄ isotopes.
Ascension Island is a remote South Atlantic equatorial site, ideal for monitoring tropical background CH₄. Octocopters were used to collect air samples from different heights above and below the Trade Wind Inversion (TWI). This work has shown it is possible to retrieve air samples up to an altitude of 2700m above mean sea level. Both campaigns show a consistently higher mole fraction above the TWI, representative of air masses arriving from over Africa compared to below the TWI where air masses arrive from the remote south Atlantic.