This proposal is to assess the feasibility of adapting technology currently used for leak detection in commercial and domestic boiler systems to provide a low cost (50-100 euro) disposable sonde based instrument for measuring vertical profiles of CO2. With suitable measurement characteristics, a small network of such instruments could contribute significantly to global atmospheric CO2 flux monitoring, adding to the information obtainable from the current ground-based and satellite based approaches, with potentially significant implications for understanding and predicting future climate,. The principal deliverable of the project will be a prototype instrument optimised for CO2 profile measurements, and the design and performance specification for a production CO2 sonde instrument. The novelty of this approach is the adaptation of technology proven at concentrations well above background ambient conditions to the untried environment of the upper atmosphere where far higher precision is required. Technological approach: non dispersive infra-red (NDIR) absorption spectroscopy The basis of the technology is absorption spectroscopy, as expressed by the Beer-Lambert law. The advantage of this technique is that it has a fundamental calibration, based on known or calculable absorption-cross-sections. In this case a spectral filter rather than a dispersing element is used to define the spectral passband, hence the term 'non-dispersive'. CO2 is measured using its fundamental vibration-rotation absorption band centred at 2349 cm-1. The research program will take place in four phases: 1) Optimisation of sensor design 2) Construction of optimised prototype instrument 3) Laboratory and profile simulation instrument characterisation 4) Design and performance specification for a production CO2 sonde instrument.