Methane is one of the three most important greenhouse gases and is considered responsible for 20% of the observed enhanced greenhouse effect. As one of six gases targeted under the Kyoto protocol, EU member states require accurate knowledge of their methane emissions to meet their annual reporting requirements. Whilst total global emissions are well constrained, emissions by source sector are less well known and current observation networks are unable to verify emission reductions at a national level. A valuable tool for distinguishing amongst emission sources is isotopic measurement. The acquisition of real-time, high-precision isotopic methane data, when coupled with back-trajectory analysis, provides a measureable discriminant which can identify methane emissions by geographic location, time and source type. Such information will be invaluable in providing a quantitative understanding of methane sources and sinks, which in turn will be critical for the implementation of methane emission mitigation strategies and enforcement of regulatory legislation. The development and application of a transportable, ultra-sensitive optical spectrometer is thus proposed, to allow, for the first time, real-time measurement of the ratios of stable methane isotopologues in atmospheric samples. This will permit measurements on a timescale sufficient for back-trajectory analysis. The project focuses on developing a new analytical technique capable of detecting 12CH4, 13CH4 and CH3D in atmospheric samples at sub parts-per-billion sensitivity, and providing information on the isotopic fractionation within these samples on a 1 Hz timescale, with measurements that can be traceable to the SI system. The applicant will incorporate his chemical physics experience in gas phase molecular spectroscopy with the optical detection expertise of the host institute on a project which can provide significant contributions to the fields of atmospheric research and environmental monitoring.