High Precision Stable Isotope Analysis of Methane and Carbon Dioxide using Trace Gas CF-IRMS: Instrumentation Development, London Diurnal Studies and Measurements of Emissions from Irish Wetlands

Continuous flow isotope ratio mass spectrometry coupled with the Trace Gas preparation system (GV Instruments) has been used to analyse air samples with a precision of better than 0.1 ‰ for δ ¹³C of CH₄ and δ ¹³C and δ ¹⁸O of CO₂. 75cm³ of air is required, with a 16-minute analysis time. This precision makes the technique ideal for source studies, although it is also approaching the precision required to identify annual variations in measurements at some background stations. Diurnal studies have been carried out from the sampling site at Royal Holloway, west of London, measuring either δ ¹³C of CH₄ or δ ¹³C and δ ¹⁸O of CO₂ at 30-minute intervals. Source calculations for methane showed the local sources had δ ¹³C values of between -37.9‰ (typical of gas leaks) and -54.0‰ (landfill emissions). The methane was most enriched in ¹³C in the early evening, during the rush hour, suggesting that at this time vehicle exhaust emissions are a measurable local source of methane. Methane and carbon dioxide mixing ratios and isotopes were measured on samples collected from Irish wetlands. Samples were collected in early summer and autumn to investigate variations in emissions from blanket bogs, raised bogs and fens. Air samples were collected upwind, downwind and in the centre of the bogs and were supplemented with soil air samples and chamber collection above waterlogged areas. Results were compared with the Atlantic background record of methane δ ¹³C at Mace Head for which measurements have been carried out using a conventional cryogenic extraction line since 1995, to investigate whether Irish wetland emissions affect the Mace Head record for trajectories passing over the southwestern corner of Ireland. Preliminary results from the summer campaign indicate that blanket bogs, which are the prevalent type of peatland along the western coast of Ireland, did not emit significant amounts of methane. Hence Atlantic blanket bog is unlikely to affect the methane isotopes measured at Mace Head. However raised bogs and fens were found to have much larger methane emissions. Source δ ¹³C isotope values of emitted methane ranged from -55 ‰ at a fen (Pollardstown fen) to -80 ‰ at a raised bog (Clara bog).